physics exam 4
which of the following statements do NOT contribute to the formation of a mirage?
light is reflected from the asphalt
In a specific atom as electron undergoes quantum jump from n=3 to n=1 orbit, green light is emitted. Which of the following is the color that could correspond to photon that is emitted if the jump occurred from n=2 to n=1 instead?
red
unlike mechanical waves, the speed of light waves move in matter depends on their wavelengths so each wavelength of light is refracted at
slightly different angle (blue light bends more)
is diffraction more pronounced through a small opening or through a large opening?
small
the ejection of electrons could be accounted for by classical physics, which pictures the incident light waves building an electron's vibration up to greater and greater amplitudes until it finally breaks loose from the metal surface
, waves of light make the atoms vibrate until it knocks out an electron
What principal advantage does an electron microscope have over an optical microscope?
Electron microscopes give higher resolution due to smaller diffraction.
The frequency of violet light is about twice that of red light. How does the energy of a violet photon compare with the energy of a red photon?
Energy of violet light is 2 times bigger than energy of red light.
In the Bohr model of the atom, what do Bohr orbits represent?
The binding energy of electrons with nucleus.
Red light has a longer wavelength than violet light Which has the higher frequency?
Violet light
transverse wave
a wave for which the medium vibrates perpendicular to the direction of energy transfer left end is moved up and down not everything can create transverse waves e.g., interior of water
In the diagram, the energy difference between states E and F is half the energy difference between states F and G. In a transition (quantum jump) from E to F, an electron emits a photon of wavelength 900 nanometers (nm).(a) What is the wavelength of the photon emitted when the electron jumps from F to G? (b) When it jumps from E to G?Hint: bigger energy means bigger frequency but smaller wavelength.(homework 27 question 1)
a) 450 nm b) 300 nm
the speed of light increases with the temperature of
air
sound travels faster in water than in
air (about 330 m/s vs 440 m/s)
angle of incidence=
angle of reflection lecture 21 slide 6
poisson
believed light was a particle. day to day life we dont see that bc we rarely have monochromatic and coherent (in phase) light source or perfect circles in nature diffraction huygens principle (lecture 24 slide 5 and 6)
bell at one end wold act as _____ finger acts as ____
bell: energy receiver finger: energy source
white color is a
combination of different colors and red filter absorbs all the other colors and only lets red light through
here constructive interference would occur on a line AB because both waves travel the same distance. But to get to point D the two waves would have to travel the distance that would make them exactly out of phase. At C we set
constructive interference and at D destructive interference lecture 23 slide 5
pitch
describes how we perceive frequency or wavelength
Interference is one phenomena that shows that light has a wave nature, what is the second phenomena that reveals wave nature of light?
diffraction
Had young removed the second barrier and kept only the first one with single slit, he would have seen a second evidence for wave nature of light
diffraction. the spreading of waves after passing through narrow slits
mechanical waves
ex: sound produced when chunks of matter (vocal chords, rock, water) vibrates which collide with other matter and disturbance is transmitted through surrounding material. these waves are produced and transmitted through solids, liquids, and gases require a medium and can be longitudinal (sound) or transverse (water surface waves) ex: seismic waves, ropes and springs, ultrasounds, tsunami waves some are longitudinal (along) and some are transverse (across)
we can form standing waves by oscillating the source at specific
frequencies
Consider just four of the energy levels in a certain atom, as shown in the diagram. Which transition corresponds to the highest-frequency light emitted? To the lowest- frequency light emitted? (homework 27 question 3)
from n=4 to n=1; from n=4 to n=3
frequency (f)
number of cycles per second (Hertz) number of oscillations/time 1/period
amplitude
the maximum distance the parts of the medium move from their natural position, or maximum displacement from equilibrium
If the phase shift between two monochromatic waves is 3.5 wavelengths, then:
they wold interfere destructively
rank the speeds of sound through these materials, from greatest to least: a) air, b) steel, c) water
v(b)>v(c)>v(a)
lecture 21 slide 13 speed of sound waves (has picture of material and speed of sound) waves travel at different speeds in different media but
v=wavelength * f
Constructive vs. destructive interference
• Constructive Interference - waves add together • Destructive Interference - the waves subtract form each other as they overlap create straight line Lecture 21 slide 3
What two physics mistakes occur in a science fiction movie when you see and hear at the same time a distant explosion in outer space?
1-sound waves can not propagate through vacuum 2- sound waves travel slower than light
The wavelength is 10 m and the time between crests at a point on the surface is 2.5 s, find the speed of propagation for this wave
4 m/s
Which diagram represents correctly the reflected and refracted waves as sound hits the concrete wall from air. some of the wave is reflected back to the air and the rest is refracted through the wall. (hint: it moves into a medium where sound moves at HIGHER speed)
E Homework 21 question 5 picture
compared to photon of orange light, how much more energy is scarried by a photon of green light (the freq of green light is 5.5*10^14 Hz and the of orange light is 5.0*10^14 Hz and plancks constant is h= 6.63*10^-34Js)
Egreen= hf= 6.63*10^-34 Js *5.5*10^14 Hz Eorange =hf=6.63*10^-34 Js*5.0*10^14 Hz so difference in energies is 3.3 10^-20 J
Which of the following statements is FALSE about diffraction?
Longitudinal waves do not undergo diffraction.
fluorescent tubes is an example of excitation and de-excitation. they have a thin layer of phosphorus on its inner surface. light is produced by phosphorus by transforming ultraviolet light generated by ionization of mercury.
UV light is emitted by gas in the tube excited by an alternating electric current. the UV light, in turn, excites phosphors on the inner surface of the glass tube, which emit white light
Does the light bend away or toward the normal to the surface as it moves from air to ice? Would is bend by bigger or smaller angle if it moved from air to diamond instead?
a) it bends toward the normal b) it would bend more
lecture 21 slide 14 echoes and sound insulators sound waves can be reflected back to your ears from the walls of the large space or canyon. your ear can distinguish the echoes and the original sound as separate sounds only if they are at least 0.1 seconds
apart
directional microphones
are designed to reflect the waves and focus them in one point ears are evolved to have a shape that helps at capture sound efficiently
because of diffraction, there is a gradual increase in the light intensity rather than an abrupt change from dark to light. slight fringes of intensity on either side of the main pattern
are evidence of interference that is more pronounced with a double slit or multiple slits. with single slit, light still interferes with itself.
When two waves interfere (happen to be at the same place at the same time) they can either partially or completely
cancel each other or reinforce each other
if the frequency of sound is doubled, what change will occur in its speed? in its wavelength?
changing the frequency of the source does not affect wave propagation speed, only the wavelength would be halved
wave speed depends only on the medium characteristics like
density, material, type of wave
water, fat, and other substances in the food absorb energy from the microwaves in a process called
dielectric heating.
sound wave propagates in all
directions in 3D space
wavelength changes with speed but frequency remains the same
frequency of the sound only depends on its source
electrons don't stay on higher energy levels for long and soon it will come down to lower energy level and emit photon in this process. During de-excitation only certain amounts of energies can be emitted and the same
is true for excitation only certain amounts of energies can be at certain energy levels, it will absorb only specific wavelength of light
(refraction) if the car is driving on a road and moves on to the mud,
it will slow down
speeds of electromagnetic waves in various materials
lecture 22 slide 4
electromagnetic spectrum pic
lecture 22 slide 5
use of photoelectric effect
lecture 25 slide 11
light and sound
like radio signals, transfer messages in the form of waves
if you want to get a message across a pond you can
make a wave by dropping a stone at your end that would cause ripples, that would reach the other end. friend across the lake could detect the message. (throwing a stone would be the alternative way to transfer the messgae)
Electric charges that oscillate produce electromagnetic waves. They dont depend on matter for their existence and are transmitted through empty space as well as
matter and are always transverse waves
there are two models for two types of phenomena: wave and particle model. when energy is transmitted from source to receiver along with the matter, the particle model is used and when energy is transmitted without the matter then the wave model applies.
neat and clean separation between the two models was short lived and investigation into the structure of the atoms blurred the distinction and renewed the wave particle controversy
infrared:
night vision goggles pick up the infrared light emitted by our skin and objects with heat. in space, infrared light helps us map the distance between the stars
Will high frequency light eject a greater number of electrons than low-frequency light?
not necessarily. the energy (not the #) of ejected electrons depends on the freq of the illuminating photons. a bright source of blue light, for ex, may eject more electrons a lower energy than a dim violet source
careful examination of the photoelectric effect led to several observations that were quite contrary to the classical wave picture and found big contradiction. photoelectric effect
only happened for light of some wavelength while for others no electrons jumped
visible light occupies only
small part of electromagnetic spectrum
why is an echo weaker than the original sound?
some of the sound waves get absorbed and transmitted by the wall
why is it a sensible procedure for soldiers to break step when marching over a bridge?
that way they could avoid resonance, in case they all march at a frequency that coincides with fundamental frequency of the bridge
if the surfaces are not perfectly flat, the bands are distorted (newtons rings)
this is a useful testing technique in polishing precision lenses
Period (T)
time required for one full oscillation (seconds) time/number of oscillations 1/frequency
speed of sound depends on medium only, if it depended on the frequency, than you would be hearing different pitch at different
times if you are at a concert (e.g., violin before trumpet)
why does the plate inside the microwave turn
to avoid any parts of food being located at nodes
bats and dolphins send out
ultrasound waves (ultrasonic frequencies) and use their echoes, or reflected waves, to identify the locations of objects they cannot see doctors use ultrasound to examine development and create pictures of unborn babies
for the hubble telescope, which light- red, green, blue, or ultraviolet- is better for seeing fine details of distant astronomical objects?
ultraviolet light would give us better resolution because there is less diffraction with smaller wavelengths
electrons were used to build more powerful microscopes.
wavelength associated with electrons is around the xray of electromagnetic spectrum so much smaller than visible light wavelengths. because of that electrons would undergo less diffraction. using electron microscopes we were able to resolve objects that are much smaller
(wave speed) velocity=
wavelength x frequency
Consider the de Broglie wavelength of an electron that strikes the back face of one of the early models of a TV screen at 1/10 the speed of light. Find the electron wavelength.
wavelength=h/p=6.63*10^-34 Js/ (9.1x 10^-31 kg *0.1*3*10^8 m/s)=2.4*10^-11 m
interference tends to occur even with a single slit/opening.
where the wave seem to fade away represent destructive interference
the common incandescent lamp consists of glass enclosure with a filament of tungsten wire inside, through
which an electric current is passed
interference makes it possible to measure extremely small distances with great accuracy. instruments called interferometers
which use the principle of interference, are very accurate instruments for measuring small distances
sound waves can be absorbed when energy stored in the disturbance is dissipated by the thermal energy of molecules of medium. pockets of air within solids are very good sound absorbers. sound reflects
within the air pocket many times before dissipating
Which are more successful in dislodging electrons from a metal surface: photons of blue light, photons with green light, photons with yellow light or photons of red light?
blue light
diffraction (along with interference) provides the explanation for the existence of a
bright spot in the middle of the shade of a coin, aka Poisson spot
photoelectric effect in two dif ways: in terms of our models-particle model and wave model
but wave model was not able to explain this effect. treating light as waves could not provide explanation but if we treat light as photons then we could explain the result
scale of excitation and de-excitation of electrons
lecture 26 slide 5
bats can detect frequencies as high as
100,000 Hz
human ear can easily detect frequencies between
20 Hz and 20 KHz
the pattern young observed is similar to what we would have observed if we had two oscillating objects on the surface of water. they would produce circular waves that would overlap and create
2D interference patterns that is similar to what young observed with alternating nodes and antinodes
What would be the de Broglie wavelength associated with an electron moving at half the speed of light? (You will need to look up some quantities online, or in lecture notes.)
4.86*10^-12 m
You have four different sound waves going through a narrow door. The wavelengths of the sound waves are 5m, 3m, 2m, and 2.5m. Which of them would diffract more as it passes through the door?
5m wave
dogs can hear sounds up to about
60,000 Hz
which one is more easily diffracted around buildings: AM or FM waves?
AM is smaller freq or bigger wavelength, so they bend more around buildings and bigger obstacles. this is one of the reasons FM reception is often poor in localities where AM comes in loud and clear
which of the graphs represents correctly the resulting wave as the given two waves interfere?
D (homework 21, question 4 picture)
What did Young's double slit experiment show.
It showed that light behaves as a wave.
Would a beam of protons in a "proton microscope" exhibit the same diffraction as electrons of the same speed in an electron microscope? Why?
No, because the proton's wavelength is smaller.
The occasional large-angle scattering of alpha particles from gold atoms in Rutherford's experiment indicated that:
Particles more massive than alpha particles are present
If the double slits were illuminated with monochromatic (single- frequency) yellow light, would the bright fringes be more widely or more closely spaced than if they were illuminated with monochromatic red light?
The bright fringes would be closer to the main bright fringe.
poisson's spot is
The bright spot in the middle of a shadow of a sphere or a circle
hologram
a 2D photographic plate produced with laser light that allows you to see a reproduction of a scene in 3D. whole message or entire pic. no image-forming lens is used. each point of the object being photographed reflects light to the entire photographic plate, so every part of the plate is exposed with light reflected from every part of the object. light of a single frequency and all parts exactly in phase(coherent). white light would wash fringes for a frequency out by other freq. only a laser can easily produce such light
An electron orbit has a circumference of 0.3 nm. Which of the following wavelength electron could be on this orbit. a) 0.1 nm. b) 0.2 nm. c) 0.03 nm.
a and c
X-ray:
a dentist uses x-rays to image your teeth, and airport security uses them to see through your bag. hot gases in the universe also emit x-rays
All the waves shown have the same speed in the same medium, rank these waves from most to least for a) amplitude b) wavelength c)frequency d) period
amplitude: B>C wavelength: B>C frequency: C>B period: B>C **picture is on homework 20 question 5
rutherfords experiment, a beam of positively charged particles (alpha particles) from a radioactive source was directed through a sheet of extremely thin gold foil. because alpha particles are thousands of times more massive than electrons, it was expected that stream of alpha particles would not be impeded as it passed through the sea of electrons. indeed, most alpha particles passed through the gold foil with little or no deflection and produced a spot of light when they hit a fluorescent screen beyond the foil.
a few alpha particles were deflected, and a small number were even scattered backward. these alpha particles must have hit something relatively massive. rutherford reasoned that the undeflected particles traveled through empty space in regions of the gold foil, while the small # of deflected particles were repelled from extremely dense, positively charged central cores. each atom, must contain one of these cores., which he named the atomic nucleus. unlike plum-pudding model, rutherford's planetary model has a nucleus in the middle and electrons are orbiting them.
thomas young placed two barriers between light source (candle) and the screen. One with a single opening and the other with doble slit.
a light passing through these two barriers produced a surprising pattern on the screen. instead of two bright spots, he saw series of dark and bright spots on the screen. young's experiment showed the wave nature of light.
a) Can electron In the ground state absorb infrared and emit red photon? b) Can electron in the ground state absorb infrared and emit green photon?
a) yes b)no
Electron gains 2 eV of energy as it jumps from ground state to higher energy level. Did it absorb or emit a photon? What was the color of this photon? (1 eV = 1.6 10 ^ -19 Joules)
absorb orange
lecture 21 slide 12 loudness and pitch when the bell rings, its motion alternatively compresses and rarefies the surrounding air. it's a longitudinal wave so each molecule of air vibrates in the direction of propagation. as the disturbances reach the air, they produce vibrations in the air drum which are eventually converted to nerve impulses and
are sent on to the brain
photoelectric effect showed that electron can only absorb 1 photon. we know that photon's energy E=hf. with photoelectric effect electrons in metals were absorbing photons, but it does not have to be a metal for this absorption to occur. let's say we have 1 electron in a shell around the nucleus. When photon reaches it, its absorbed by electron, then electron get excited and either jumps to higher level of energy, to a shell that is further
away from a nucleus or it can get enough energy to completely escape, in which case we will be left with a free electrons and an ion (ionization)
A cat can hear sound frequencies up to 70,000 Hz. Bats send and receive ultra-high-frequency squeaks up to 120,000 Hz. Which hears sound of shorter wavelengths, cats or bats? If the speed of sound in air is 330 m/s what is the smallest wavelength the CATs can hear?
bats. about 0.0047 m
holography allows us to record information about the depth of the object
because of the addition of the reference beam against which we measure the distance travelled by the reflected light.
Why do radio waves diffract around buildings, while light waves do not?
because radio waves have wavelength that are comparable to those big distances ass w building width or height
if the moon blew up, why wouldn't we hear it? would we still see it? what differences in the properties of sound and light does this indicate?
because sound waves cannot travel through empty space, mechanical waves need medium to propagate light waves are electromagnetic waves that can travel through empty space (e.g., sunlight reaches us through vacuum)
energy can be transferred in 2 different ways
by a wave (spring), or by matter (car) . if phenomena is visible its easy to distinguish between the two models. with sound or light we can see neither particle or wave. wave model of light met with formidable opposition during 17th and 18th centuries. if we would find a phenomena that would successfully be described by one model but totally unexplained by another model we would be able to claim that light for example is either a wave or a particle.
frequencies lower than red (infrared) or higher than blue (ultraviolet)
cannot be perceived by our eyes as visible light
waves tend to spread into the shadow region. when the wavelength is about the size of the object, the shadow is soon filled in. when the wavelength is short relative to the objects size, a sharper shadow is
cast
Doppler effect is evident when you hear the
changing pitch of an ambulance siren as it passes you. when the vehicle approaches, the pitch is higher than normal (like a higher note on a musical scale) This is because of the crests of the sound waves encounter your ear more frequently. when the vehicle passes and moves away, you hear a drop in pitch because the crest of waves hit you ear less frequently
many living creature-from bacteria to fireflies and larger animals, such as jellyfish
chemically excite molecules in their bodies that emit light. we say that such living things are bioluminescent
when red light is subtracted from white light, the mixture left will appear as the
complementary color of red, which is cyan. at another place, where the film is thinner, a different color may be cancelled by interference. the different colors, then, correspond to different thicknesses of the thin film. the colors in soap bubbles are predominantly cyan, magenta, and yellow due to the subtraction of primary red, green, and blue, or other colors of a single wavelength (newton's rings)
total internal reflection
complete reflection of a ray of light within a medium such as water or glass from the surrounding surfaces back into the medium. the phenomenon occurs if the angle of incidence is greater than a certain limiting angle, called the critical angle
If red wavelength gets cancelled by destructive interference within white beam of light, what would the remaining color of light seem to be.
cyan
the principle of least time
demands for the light to travel the path indicated above (lecture 22 slide 6), it is the optimal path for light. all of the other possible paths would take longer to cover due to light slowing down in water.
lecture 21 slide 11 resonance in bridges or houses you would want to avoid resonance from occurring while in musical instruments resonance is
desirable
Superposition principle
destructive interference occurs where a crest of one wave overlaps the trough of another to produce regions of zero amplitude. It's easier to visualize wave interference with water waves noise cancelling headphones can effectively lock out the noise in some circumstances
fluorescent dyes are used in paints and fabrics to make them glow when they are bombarded with ultraviolet photons in sunlight. they can be spectacular when illuminated with an ultraviolet lamp. to make counterfeiting more difficult, many govs, including US, use fluorescent (use uv light). near one end, a line will appear that cannot be seen with visible light. front and back of the bill
detergents that make the claim of cleaning your clothes "whiter than white" use the principle of fluorescence. contain a fluorescent dye that converts the ultraviolet light in sunlight to blue visible light, so clothes washed in this way appear to relfect more blue light than they otherwise would. whiter
speed
distance/time
frequency and wavelength
distinguishes sound from earthquakes, and sunlight from x-rays and radio waves how well a wave can be detected by receiver is also determined by this mechanical waves: boat in the sea. if the wave is too big, the boat and your fishing would not be interrupted. the same is true if the wavelength of the incoming waves is too small. however, if we have the right frequency wave that has wavelength comparable to the wavelength of the boat itself (about 2 meters) then the boat would oscillate most lecture 21 slide 10
gamma ray
doctors use gamma-ray imaging to see inside your body. the biggest gamma-ray generator of all is the universe
lecture 22 slide 3 has examples of
electromagnetic waves and their sources
plum pudding model *b4 rutherford*
electrons are like plums in a sea of positively charged pudding. wrong
absorption spectrum is unique, characteristics of the atom, and the black lines on the absorption spectrum show all the wavelength that electrons can absorb.
emission spectrum shows which colors can be emitted by a specific atom
What happens to the energy of a photon if its wavelength is halved?
energy is doubled
If the ray is reflected from a surface of a different medium, reflection occurs in such a way that incident and reflected angles are always
equal
huygen's principle
every point on a wavefront may be considered the source of secondary wavelets that spread out in all directions with a speed equal to the speed of propagation of the waves can also explain reflection and refraction
einstein formulated new hypothesis combining the 2 previous ones. light is neither wave or particle but both, it consists of wave packets. concept of photons allows him to explain the photoelectric effect.
explaining photoelectric effect lecture 25 slide 7
constructive interference
extra distance= n wavelengths where n= 0,1,2,3...
destructive interference
extra distance= n/2 wavelengths where n= 1, 3, 5, 7...
the two sources of light are resolved when we see them as separate objects. because of diffraction of light as it goes through the circular aperture e.g., lens, opening or eye pupil, image from one light source overlaps with the second one, that's when we can not resolve them any longer.
eyesight is diffraction (seeing far away) lecture 24 slide 9
Rank the pitches heard form the siren of a fire engine, from highest to lowest, when the fire engine is traveling a) toward the listener at 30 km/h b) toward the listener at 50 km/h c) away from the listener at 20 km/h
f(b)> f(a)> f(c) For a) and b) apparent frequency is increased but for c) apparent frequency decreases. The greater the speed of the source (or observer), the greater the Doppler effect. So b) has the biggest increase in (apparent) frequency a) increases frequency but by less amount c) corresponds to decrease in frequency
rutherford: the occasional large-angel scattering of alpha particles from gold atoms led rutherford to the discovery of the very massive small nuclei at their centers.
finding that most alpha particles were undeflected indicated much empty space. finding that some alpha particles bounced backward indicated the presence of particles more massive than the alpha particles
if a child is on a swing, pushing them at random rate would not help them swing. you need to apply the
force at certain times in order to achieve resonance
bohr reasoned that electrons occupy stationary states (of fixed energy, not fixed position) at different distances from the nucleus and that the electrons can make quantum jumps from one energy state to another.
he reasoned that light is emitted when such a quantum jump occurs.
blue has a
higher frequency but smaller wavelength (carries more energy)
most of the phenomena observed with light can either be described with wave or with particle nature. a marble and a water wave behaves the same way when they reflect from the wall, angle of incidence and reflection is equal in each situation
however, diffraction and interference could not be explained with the particle motion and is only a characteristic of a wave
amplitude is meaningless with photons, increasing amount of light means more photons. energy of each photon is given by E=hf. so if you need to increase number of photons not amplitude, each having enough energy/freq to knock out electron.
if it has even bigger freq then the ejected electron will have some leftover KE, or it would move faster. each photon interacts with one electron only
single color thin-film interference a simple demonstration of light interference can be set up with a monochromatic light source and a couple pieces of glass. The two pieces of glass are placed one atop the other. A very thin piece of paper is placed between the plates at one edge to provide a very thin wedge-shaped film of air between the plates.
if the eye is in a position to see the reflected image of teh lamp, the image will not be continous but will be made up of dark and bright bands.
holographic films capture light as a series of bright and dark bands. the info needed to reconstruct the 3D image is stored in this interference pattern.
if we hold holographic film in front of the same monochromatic light source, light is transmitted by bright bands and is blocked by dark bands and makes a 3D image.
patterns produced due to diffraction and interference of light put limitations to the optical instruments that we create (where light has to go through some narrow opening)
in microscopes, telescopes, cameras and even human eyes, diffraction puts a fundamental limit to the max resolution we can get
incident, reflected and refracted waves will always lie
in the same plane
is interference restricted to only some types of waves, or does it occur for all types of waves?
interference is a characteristic of any type of wave- mechanical and electromagnetic waves and longitudinal and transverse
if the ray is transmitted through the medium, it will change its speed as well as direction of motion. it will bend towards the normal (or perpendicular) to the surface if it moves to a medium where it has higher speed and it will bend away from the normal if it moves
into the medium in which it slows down e.g., light waves refract through the glass or sound waves refract through the wall. sometimes the incident wave is neither reflected or refracted but absorbed by surface
longitudinal wave
is a wave for which the medium vibrates parallel to the direction of energy transfer is produced by right and left motion of the source ex: sound wave, can be transmitted through water and fluids rope cannot create longitudinal waves
rainbow
is another example of dispersion. sunlight undergoes total internal reflection twice from the inner part of the raindrop and each color in the white sun ray leaves raindrop at different angles. red waves reach my eye from the upper drops while i see blue light at lower angles or slightly lower layer of drops.
photoelectric effect
is the ability of light to knock electrons out of the surface of a metal
When light passes through an opening that is very narrow compared with the wavelength of light,
it casts a shadow
why does wax make a table look more like a mirror?
it reduces diffuse reflection and makes the surface flat
A mirage is a naturally occurring optical phenomenon in which
light rays bend via refraction to produce a displaced image of distant objects or the sky. Fermat's principle of least time is one way to explain this phenomenon.
dishes washed in soapy water and poorly rinsed have a thin film of soap on them. hold such a dish up to a light source so that interference colors can be seen. then turn the dish to a new position, keeping your eye on the same part of the dish, and the color will change.
light reflecting from the bottom surface of the transparent soap film is cancelling light reflecting from the top surface of the soap film. light waves of different wavelengths are cancelled for different angles
The Doppler effect also occurs for
light. When a light source approaches, there is an increase in its measured frequency; when it recedes, there is a decrease in its frequency. An increase in frequency is called a blue shift because the increase is toward the high-frequency (or blue) end of the color spectrum. A decrease in frequency is called a red shift, referring to a shift toward the lower-frequency (or red) end of the color spectrum. Ex: distant galaxies show a redshift in the light they emit. A measurement of this shift permits a calculation of their speeds of recession.
electrical charge sets up electric field around it. (columbs law) as it oscillates, it produces disturbance that moves through this electric field. when disturbance reaches another charged particle it can induce its oscillation too. Any acceleration charge can set up magnetic field as well (changing electric field created magnetic field). Energy is propagated in the direction that is perpendicular to both,
magnetic and electric fields which oscillate perpendicular to each other.
microwave:
microwave radiation will cook your popcorn in just a few minutes, but is also used by astronomers to learn about the structure of nearby galaxies
What kind of light do lasers produce?
monochromatic, in phase, coherent
wave particle duality. wave and particle models offer complementary views of the microscopic world.
neither by itself is sufficient
the microwave radiation raises the kinetic energy of the molecules, which results in a temperature increase. the formation of standing waves results in
nodes and antinodes within the cooking chamber
the extent to which waves are diffracted depends on the width of the opening compared to the wavelength of the wave when the width of the opening is much larger than the wavelength very little diffraction occurs but becomes noticeable
only when the opening is the same size as the wavelength
waves
oscillation propagating in space and transferring energy
electromagnetic waves in microwave ovens can be described as standing waves, a waveform is reflected back on itself. this means that instead of the peaks of the wave moving (like waves at the beach move towards the shore), parts of the wave move up or down, or not at all. the microwaves are reflected by the metal on the
other side of the oven from the transmitter. This creates nodes and antinodes
visible:
our eyes detect visible light emitted by light bulbs and stars. on each end of visible light spectrum we have ultraviolet and infrared frequencies (just like we had ultrasonic and infrasonic frequencies for sound waves on each end of audibe spectrum)
loudness
our perception of the energy carried by the wave and is related to the amplitude of the sound wave
waves that strike the boundary between to media can also be
partially or fully transmitted through the boundary
when two sets of waves of equal amplitude and wavelength pass through each other in opposite directions, the waves are steadily in and out of phase with each other.
pictures on lecture 21 slide 5
speed of sound also depends on temperature, if the air is hot, sound travels faster because faster moving molecules transfer energy to neighboring molecules more quickly. sound travels faster in solids because restoring forces help ratifications and compressions
propagate faster
huygens studied wave behavior
proposed that the wavefronts of light waves spreading out from a point source may be regarded as the overlapped crests of tiny secondary waves that wavefronts are made up of tinier wavefronts.
the magenta seen in a soap bubble is due to the cancellation of green light. when red light is subtracted from white light, the mixture left will appear as the complementary color of red, which is cyan.
red green and blue are primary colors. if we remove blue from white light, we will be left with yellow. on the other hand, the reason we see a red object is because all the other colors are absorbed by it and only red remains. our eyes have 3 types of color receptors and combinations of these let's see all other colors.
when white light diffracts upon passing through a thin slit, dif color components diffract by dif amounts so that a rainbow of colors appears at the edge of the pattern. Which color is diffracted through the greatest angle? which color through the smallest angle?
red would diffract most since it has greatest wavelength but blue/violet would diffract the least
the rope on the left has the end that is flexible and can move up and down, but the rope on the right has a fixed end that can not slide up and down
reflection at the interface of two media (lecture 23 slide 9)
diamonds get their brilliance from three things:
reflection, refraction and dispersion. it is one of the hardest material, therefore light bends in diamond more than it would bend in water or in air. Light that hits the diamond and is immediately bounced back up is giving it an instantaneous shine. also light undergoes total internal reflection several times before it emerges from the diamond. the sparkling effect of the diamond is due to its shape. the way its cut and its shape, creates kaleidoscope effect while the light incident on top is reflected back out the top of the stone, so that to the viewer it seems light is coming from the inside of the diamond. the light that strikes the diamond at an angle is dispersed and also undergoes total internal reflection several times before leaving the diamond.
so classical or wave theory of light predicted
regardless of freq of incident of light, electrons should still be emitted if the amplitude (intensity) of light is big enough. max amount of kinetic energy of the ejected electrons should increase as the amplitude increases (if wave is bigger you will be able to emit faster electrons)
diffraction is the reason we can still hear radio waves behind big obstacles and why we can not see or
resolve two distinct objects as separate sources of light.
the fishing boat is energy receiver, its behavior can be described in terms of
resonance. boat oscillates most when the water waves push in time with natural frequency (that depends in boat's length). in order for the wave detector to be effective at detecting waves, it needs to be about the same size as wavelength of the waves it detects. so the closer the length of the boat is to the wavelength of the wave, the better it detects the wave
diffraction and interference were
significant in temporarily resolving the wave-particle duality of light
red has a
smaller frequency (or higher wavelength)
why is sound so quiet after a snowfall?
snow is made of snowflakes and pockets of air that are good sound absorbers
electromagnetic waves are absorbed when the energy stored in the electromagnetic disturbance is dissipated into thermal energy. infrared radiation and the visible light have the wavelengths that roughly correspond to the sizes of the atoms and molecules (resonance analogy).
so these electromagnetic waves easily transformed into the thermal energy associated with the motions of atoms and molecules
many materials that are excited by ultraviolet light emit visible light upon de-excitation. The action of these materials is called fluorescence. in these materials, a photon of ultraviolet light excites the atom, boosting an electron to a higher energy state. in this upward quantum jump, the atom is likely to leapfrog over several intermediate energy states. so when the atom de-excites, it may make smaller jumps, emitting photons with less energy.
some rocks, when illuminated with ultraviolet light, are seen as different colors because they contain fluorescent materials, e.g., calcite and willemite
does the speed of sound wave increase or decrease as it enters glass from air? is the same true when light enters glass from air?
sound wave speed increase but light wave speed decreases
if the wave is transmitted through the medium, it will change its
speed as well as direction of motion but frequency remains the same
if we oscillate one end of the string that is attached to the wall, for specific frequencies we will set up a
standing wave, with the incident and reflected waves, where parts of the rope, called the nodes, appear to be standing still.
you would expect more line on the emission spectrum, because all electrons, when they absorb, move from ground level to a specific energy level but ont eh way back, they can
take any route they like. (chart of spectrum on lecture 26 slide 6)
the planetary model of the atom begged a major question. accelerated electrons, according to maxwell's theory, radiate energy in the form of electromagnetic waves. so an electron accelerating around a nucles should radiate energy continuously. this radiating away of energy should cause the electron to spiral into the nucleus. bohr boldly deviated from classical physics by stating that the electron doesnt radiate light continuously while it accelerates around the nucleus in a single orbit, but
that radiation of light occurs only when the electron makes a transition from a higher energy level to a lower energy level
so the level of refraction of light changes at the surface of the hot road or sand. light travelling down form the sky is refracted as it encounters warmer layers of air until the path is actually bent back upward. when we see the image of the sky we automatically assume that it is a reflection from the surface of the lake. the most common cause of varying air density is varying air temperature, since hot air is less dense than cold air.
the change in density between the hot air near the road and the cooler air higher in the atmosphere causes light rays to bend as they move between the mediums
uncertainty principle. the wave-particle duality of quanta has inspired interesting discussion about the limits of our ability to accurately measure the properties of small objects.
the discussions center on the idea that the act of measuring something affects the quantity being measured
wavelength
the distance from the top of one crest to the top of the next crest. the wavelength is the distance between any successive identical parts of the wave
typically, argon is the gas inside the enclosure (incandescent). if a small amount of halogen element such as iodine is added to the interior, the evaporation of the tungsten is slowed and the bulb lasts longer
the efficiency of incandescent light bulbs as a visible light emitters is typically less than 10%. hence, they are gradually being replaced by lamps that convert percentage of electric energy into visible light. (spectrum on lecture 26, slide 10)
fluorescent materials emit light immediately after being excited. for phosphorescent materials there is a time delay between excitation and de-excitation.
the element phosphorus is used in a variety of luminous materials, even tooth-brushes, that are made to glow in the dark. atoms or molecules in phosphorescent material are excited by incident visible light. rather than de-exciting immediately, as fluorescent materials do, many of the atoms remain in a metastable state (prolonged state of excitation) sometimes for as long as several hours, although most de-excite rather quickly.
light shining on the neg charged, photosensitive metal surface liberates electrons.
the liberated electrons are attracted to the pos plate and produce a measurable current (motion of charge)
no matter how big the intensity of light was, the electrons would not be ejected unless the incident light had the right freq.
the max KE of the ejected electrons was unaffected by the brightness of the light. However, there were indications that the electron's energy did depend on the freq of the light
uncertainty princ. for ex: we know that, if we place a cool thermometer in a cup of hot coffee, the temp of the coffee is altered as it gives heat to the thermometer
the measuring device laters the quantity being measured.
water dominoes and rope act as
the medium through which energy is transferred
for the central bright fringe, the paths from the two slits are the same length and so the waves arrive in phase and reinforce each other. the dark fringes on either side of the central fringe result from one path being longer (or shorter) by 1/2 wavelength, so that the waves arrive half a wavelength out of phase.
the other sets of dark fringes occur where the paths differ by odd multiples of 1/2 wavelength: 3/2, 5/2 and so on (young)
why would it be impossible for a fluorescent material to emit ultraviolet light when illuminated by infrared light?
the photon energy output would be greater than the photon energy input, which would violate the law of conservation of enegy
reflection from the upper and lower surfaces of a "thin film of air" could be in phase or out of phase. the dark portions occur where the air thickness is just right to produce destructive interference, and the bright portions occur where the air wedge is just
the proper amount thinner or thicker to result in the reinforcement of light
diffuse reflection is
the reflection of light or other waves or particles from a surface such that a ray incident on the surface is scattered at many angles rather than just one angle as in the case of specular reflection lecture 22 slide 7
you can have more than one wave at
the same location at the same time, in which case they would overlap and produce wave interference
how would the spacing between newton's rings differ when illuminated by red light and by blue light?
the spectrum of colors reflected from a soap bubble or from gasoline on a wet street are the result of interference as well. when illuminated by white light, the film may be just the right thickness at one place to cause the destructive interference of, red light for ex.
Why is lightning seen before thunder is heard?
the speed of sound wave is lower than the speed of light
quantum uncertainties stem from the wave nature of matter. a wave, by its very nature, occupies some space and lasts for some time. it cannot be squeezed to a point in space or limited to a single instant of time because then it would not be a wave. this inherent fuzziness of a wave give a fuzziness or uncertainty of measurement at the quantum level
the uncertainty of measurement in the atomic domain, is the uncertainty principle. when the ucnertainties in teh measurements of the momentum and position of a particle are multiplied, the product must be equal or greater than planck's constant.
we see that the superposition of a pair of identical waves in phase with each other produces a wave of the same frequency but twice the amplitude. if the waves are exactly one-half wavelength out of phase,
their superposition results in complete cancellation (lecture 23 slide 4)
before bohr model for hydrogen atom was put forward people were discovering spectra as they were splitting the lights into all of its wavelenght.
there were seeing the discrete units of light emitted by sun, which could now be explained. bohr model predicted them
diffraction fringes are evident in the shadows of single-frequency laser light
these fringes would be filled in by multitudes of other fringes if the source were white light from multiple slits
for water and sound wave diffraction is more noticeable due to bigger wavelength. sound waves range from few cm to several m.
this is why if you have an open door and stand next to it, you can still hear the sound coming from the other room
the wavelengths and separation of slits hugely determines the outcome of this experiment. with water we had no difficulty observing the pattern because they had much bigger wavelengths than light. for light you need to place the slits really close together to spread out the pattern to be able to observe it (otherwise they are as close together as the thickness of a hair) because the visible light has a small wavelength, about 5*10^-7
this is why scientists did not observe interference pattern for light before young, they just did not look close enough. lecture 23 slide 7
lecture 21 slide 7 pic in a) both of the front wheels slow down at the same rat, while in part b) wheels on the left slow down sooner than wheels on the right.
this will have a turning elect on the car
unlike traditional ovens the microwave oven can heat the food from the inside. If the food happens to be located at antinodes than it will not absorb as much energy
to avoid uneven heating of the food microwaves have a plate that rotates.
ultraviolet:
ultraviolet radiation is emitted by the sun and are the reason skin tans and burns. "hot" objects in space emit UV radiation as well
In order to increase the resolution of a microscope I should:
use a shorter wavelength of light
the wave particle controversy
was temporarily resolved in the beg. of the 19th century after young's and arago's experiments, which showed that light undergoes interference and diffraction
the wavelength is 1.5m and the time between crests at a point on the surface is 0.5 s, find the speed of propagation for this wave
wave moves 1.5 m in 0.5 s, 1.5/.5= 3 m/s
light has particle and wave nature but so do electrons. supposedly particles behave like waves (double slit experiment) and waves behave like particles (photoelectric effect) then maybe everything in the universe can behave as a particle or a wave depending on the experiment that is being conducted. de broglie postulated this associated wavelength to be
wavelength= h/p (where h=6.63*10^-34 J s is a Planck's constant and p is momentum of the particle p=mv) wavelength= h/mv
You decide to roll a 0.1-kg ball across the floor so slowly that it will have a small momentum and a large de Broglie wavelength. If you roll it at .001 m/s , what is its wavelength? How does this compare with the de Broglie wavelength of the high-speed electron that strikes the back face of one of the early models of a TV screen at 1/10 the speed of light (2.4×10^−11m)?
wavelength=6.63*10^-34 Js/(0.1*0.001 m/s)= 6.63*10^-30m. smaller than TV problem
what is the wavelength of a 340 Hz tone in room-temperature air? What is the wavelength of a 34,000-Hz ultrasonic wave in the same air? speed of sound at room temperature is 340 m/s
wavelength=v/f= 340 m/s / 340 Hz= 1 m. ultrasonic wave travels at the same speed (speed of the wave does not depend on frequency) so 340 m/s/340,000Hz=0.001m
photoelectric cells can be found in a variety of everyday devices. elevator doors and street lamps we have two separate metal plated inside a flass tube
when light strikes the plate the ejected electrons create current acting as a switch, turning the device on or off. (more example on lecture 25 slide 12)
light behaves like water waves, when both waves have a crest at the same point they constructively interfere (bright spot) and
when one has a crest and second has a trough as they reach a point, they interfere destructively (dark spot) lecture 23 slide 5
the hot filament (incandescent) emits a continuous spectrum, mostly in the infrared, with visible light as the smaller and useful part. the glass enclosure prevents oxygen in air from reaching the hot filament, which otherwise would be destroyed by rapid oxidation. eventually, the filament fails anyway because of its gradual evaporation,
which leads finally to a break in the filament and to the bulb "burning out"
a mixture of all colors is perceived by our brain as
white dispersion of light: the splitting of white light when it passes through a glass prism into its constituent spectrum of colors (violet, indigo, blue, green, yellow, orange and red)
optical fibers are
widely used in medicine or communications for fast and cheap method of information transfer
If light is reflected from the surface of plastic back to the air, would the reflected wave be inverted (undergo a phase change)?
yes
will brighter light eject more electrons from a photosensitive surface than dimmer light of the same freq?
yes the number of ejected electrons depends on the # of incident photons
the amount of information conveyed increases with the number of photons.
you can see the light even if 2-5 photons strke the retina but you need much more photons to reflect from the image in order to recognize the image (lecture 25 slide 14)
you see lightning sooner than
you hear it (speed of light is much higher than speed of sound)
radio:
your radio captures radio waves emitted by radio stations, bringing your favorite tunes. Radio waves are also emitted by stars and gases in space
