Module 7: Physics The Nature of Light
What were the problems in terms of explaining the black body radiation graph?
(Wien's Law) - account for peak wavelength, (Stefan-Boltzmann law) - account for the amount of energy, neither theory could account for the shape of the curve. One of the theories predicted that an infinite amount of energy would be produced (Rayleigh-Jean law), (UV catastrophe), which was not known to occur Max Planck - successful theory to explain blackbody radiation which had as its core assumption that energy is quantised, oscillators oscillate at specific energies, radiation is quantized where each quanta of energy was related to the frequency of the radiation by E = hf. Where h is Planck's constant, frequency f, they could only give out bursts of energy given by the equation E = nhf, where n is an integer (Only radiate quanta of energy which were integral multiples of hf) Planck's hypothesis formed the cornerstone for the the theory developed by Einstein, to explain the photoelectric effect
How does diffraction grating impact laser pattern on a wall?
0.1mm -> small gap 0.003mm -> medium gap 0.0016 mm -> large gap
What was Thomas Young's experiment? What did it show?
1801, Thomas Young provided strong evidence to support wave model of light Placed a screen with 2 slits cut in front of a monochromatic (single colour) light. Different results for particle vs waves Particle → 2 defined bright lines Wave → Areas of constructive, destructive interference Only 2 lines pass through, but start to diffract out, then using constructive interference, bright lines will occur, and destructive interference where dark areas occur Some points will meet crest to crest → constructive interference (antinode), become a bright spot Some points will meet crest to trough → destructive interference (node) black areas Thus light is a wave According to d sinø = mλ Different colours will have different interference patterns due to varying wavelengths
What is a blackbody model? What objects fit the blackbody model?
A blackbody is a theoretical or model body which absorbs all radiation falling on it, reflecting or transmitting none. Hypothetical, "perfect" absorber, "perfect" emitter of radiation over all wavelengths All objects above 0K (-273.15ºC) emit energy in form of black body radiation In astronomy stars are often modelled as blackbodies, although not always a good approximation
Faraday's law
A changing magnetic field produces an emf E = -N∆ø/∆t
How can you model a blackbody physically?
A good model for black body radiation is a cavity with a small hole in it. If we heat this to 1000ºC it will emit all types of radiation. This radiation can't escape except through the hole. The radiation will reflect around the cavity until it has enough energy to be absorbed As the walls (containing oscillators) absorb this energy and oscillate more, the EM radiation then emitted will have more energy and therefore a different wavelength. This will eventually reach a state of equilibrium (ie absorption and emission are the same) If the radiation that is being emitted from the hole is measured, it will demonstrate the range of radiation that is characteristic of this particular temperature
What did Huygens/wave model say about reflection?
A straight wavefront striking a mirror The wavelets shown were emitted as each point on the wavefront stuck the missor. The tangent to these wavelets shows that the new wavefront has been reflected at an angle equal to the incident angle . The direction of propagation is perpendicular to the wavefront
What did Huygens/wave model say about refraction?
A straight wavefront travelling from one medium to another where it slows down. Ray bends towards perpendicular since wavelets have a lower speed in the second medium
Blackbody
A theoretical object that absorbs 100% of radiation that hits it. Therefore it doesn't reflect any radiation and appears black Blackbodies are a perfect emitter of radiation
How did accuracy improve during the evolution of the measured speed of light?
Accuracy improved as methods changed from observations of physical phenomena to calculate speed of light, to measuring the fixed values of natural phenomena on small scale. Romer observed how the moon eclipse changed to derive the speed of light which involved utilising a distance not known accurately. Michelson improved by using a measurable distance, however it still relied on observing a phenomenon then deriving the speed of light. Essen measured fixed value of natural phenomena to find the exact frequency and wavelength of a microwave to find the speed of light. The fixed dimensions could be studied and accurately measured, reducing error and increasing accuracy.
What experiment was done in class to show the propagation of EM waves
Aim: produce and detect radio waves Method: Radio waves produced by induction coil, and wire with AC current flowing Radio waves received by cathode ray oscilloscope and stereo radio Induction coil, radio receiver and speaker Induction coil, Cathode ray oscilloscope Appliance cable, Cathode ray oscilloscope
How did Maxwell relate Ampere's law
Ampere-Maxwell Law → Electric fields induce a magnetic field If a changing electric field is generated, then a magnetic field is induced → results in changing magnetic field that induces a changing electric field that continues back and forth. These are electromagnetic waves
How does the experiment detecting radio waves from an induction coil apply to Maxwell's theory of EM waves?
As electrons were accelerated in the induction coil, radio waves were produced and detected by oscilloscope An oscillating electric field is formed by a vibrating charge, forming a MF fromeing an EF forming an EM wave. Maxwell's theory applies not only to light but wavelengths outside visible spectra → experimental evidence
How did Max Planck explain the nature of radiation emitted in experiments?
Assumed radiant energy, although exchanged between the particles of the black body and the radiant energy field in continuous amounts, may be treated statistically as if it was exchanged in multiples of packs of energy or quanta. Each pack is characteristic of each frequency of radiation emitted
What other conditions did Hertz try in his spark gap experiment? What did these show?
Blocking between receiver and producer reduced spark --> Diffraction Illuminating the spark gap in the receiving loop with UV light from the transmitting gap gave bigger sparks in the receiving loop. → energised Using glass as a shield between the two loops reduced the intensity of the sparking in the receiving. → lattice hard to transfer through Quartz used instead of glass didn't do this. Hertz had discovered the photoelectric effect (release of electrons from the surface of a metal exposed to EM radiation). But he didn't investigate it any further.
What are the similarities and differences of electric fields and magnetic fields?
Both are a consequence of a charge EF → charge can be stationary or moving MF → only produced when charge is moving
Rayleigh-Jeans law/Ultraviolet catastrophe
Calculated average energy of the oscillators in relation to their frequency (and λ). Issue trying to explain results of experiments using classical theory. R-J law accurately predicts experimental results at radiative frequencies below 105 GHz Calculation was based on classical wave theory (where radiation in the black body was a series of standing waves) and equipartition (equipartition theorem is a general formula that relates the temp of a system with its average energies) But above this frequency the theory does not match the result Classical theory: Energy emitted by an oscillator is equal at all frequencies (proportional only to temperature). Particles are more likely to oscillate at a frequency as that frequency increases (infact, probability was proportional to f2)
Who proposed the wave model of light in the 17th century? How did it suggest light propogated?
Christian Huygens Proposed that every point that a luminous disturbance touches, itself becomes a source of a spherical wave (wavefront) The sum of the secondary waves (wavelets) determines the form of the new wave. He was able to come up with an explanation of the linear and spherical wave propagation and derive the laws of reflection Each point on the wavefront emits a wave at speed v. The emitted waves are semicircular, and occur at time t, the new wavefront is tangent to the wavelets. This works for all wave types, not just light wave
How does light intensity impact photoelectric effect?
Controlled: surface (sodium), Wavelength 472nm, voltage Results The greater the light intensity, the greater the number of electrons released, the greater the current. Electrons are released at the greatest kinetic energy.
How does light frequency impact photoelectric effect?
Controlled: surface (sodium), intensity 100% Results Wavelength/frequency of incident light affects stopping voltage Above a certain wavelength no electrons are emitted even as intensity increases, below this electrons begin to release. Kinetic energy increases as stopping voltage increases and wavelength decreases
What was Newton's theory of light
Corpuscular/particle model
Oersted
Current flowing produces a magnetic field B = µ0I/2πr
What information can astronomical spectra provide
Details about the characteristics, motion and composition of objects eg. Surface temperature, translational motion, rotational motion, stellar density, chemical composition
What is the doppler effect?
Doppler effect combined with star spectra allows us to determine information about velocity
What was Hertz's reasoning as to why a spark jumped across the gap in his experiment?
Hertz reasoned that as spark jumps back and forth across the gap of the induction coil, it must set up rapidly changing electric and magnetic fields. Upon arrival at the loop of wire, the changing electric and magnetic fields produce a potential difference across the ends of the wire loop. If the potential is large enough, a spark jumps across the gap. The wave produced had the same frequency as the sparks (about 108 Hz).
What is Malus's law?
I = IMax(cosØ)2 Light through a polariser will halve intensity initially
How is translational velocity determined from spectra? What does translational velocity say about the universe? What is Hubbles law?
If spectral lines in a star's spectrum are uniformly redshifted this would indicate relative recessional motion between the observer and star → redshift Moving towards us → blue shift Edwin Hubble showed all galaxies (with some exceptions) are moving away from earth and away from each other because absorption spectra were found to be red shifted Further he found the further away the galaxy was, the faster it was receding Expressed using hubble's law (v = H0D (v = speed moving away, H is hubbles constant, D is distance from earth)
What is cosmic microwave background radiation? How was it discovered?
In 1965, the cosmic microwave background radiation (CMBR), was discovered by Penzias and Wilson who later won the Nobel Prize for their work Radiation spectrum was measured by COBE satellite, found to be a remarkable fit to the blackbody radiation curve with a temp of 2.725K → evidence that universe has been expanding and cooling for the past 13.7 billion years
Compare the vibration of transverse and longitudinal waves?
In a transverse wave, the direction of the oscillation is perpendicular to the direction of motion of the wave In longitudinal → Vibrates parallel to propagation
How does water work as a polariser?
In water, ions are free to oscillate in the parallel direction but not in other directions. Thus unpolarised sunlight causes parallel ions to oscillate and emit polarised light at the angle of reflection. The remaining orientations are either absorbed scattered or transmitted → polarised sunglasses, removes this polarised reflected light, so you can see under the water
When a spectroscope is pointed at an incandescent bulb
Incandescent produced light at all wavelengths
What can be observed from a black body radiation graph?
Intensity of emitted radiation is finite regardless of the temperature of the black body There is a peak intensity of radiation emitted The peak rises as temperature of the black body increases The peak shifts towards lower wavelength as temperature increases
How do you convert from J to eV or from eV to J
J/1.602*10-19 = eV eV*1.602*10-19 = J
Why can't LED and fluorescent lights produce incandescent tone light?
LED and fluorescent produce light at specific, few wavelengths, but incandescent produces light at all wavelengths therefore changing the tone
What were Maxwell's predictions from his equations?
Light and EM waves must be transverse All EM waves must travel at the speed of light v = 1/√(E0µ0), v = 1/√(8.854*10-12*4π*10-7), v = 2.998 *10^8 v = 3 * 10^8 m/s They must exist at a wide range of frequencies, predicting the existence of a spectrum
How did Albert Michelson measure the speed of light?
Light was projected through a slit on to a revolving mirror, and using mirrors, was projected down a 1.6km pressurised pipe 5x before returning along the path to reflect off the revolving mirror, into eyepiece. Through knowing the angle of reflection, speed of mirror, and length travelled by light, the speed of light could be accurately determined. Value was 299,774 km/s (+- 11km), error of 0.006%. Sources of error: Pipe did not create a perfect vacuum, variation in the speed of light. High geological activity which meant that instruments moved slightly during earthquakes and tremors, impacting the measurements.
What did Maxwell do? What could we conclude?
Linked electricity and magnetism equations A time varying electric field in one region produces a time and space varying magnetic field at all points around it This varying MF then similarly produces a varying electric field Thus if an EM disturbance is started at one location (eg radio antenna) the disturbance can travel out to distant points through the mutual generation of electric and magnetic fields The electric and magnetic fields propagate through space in the form of EM waves
What experiment was done to investigate Malus' law?
Materials: 2 mounted polaroid filters, 2 boss/clamp/stand, lamp, powerpack, 2 wires, light meter, protractor Method: 1. Measure the background light level without lamp on. 2. Set up polaroids in parallel i.e at Ø=0º 3. Measure light intensity and subtract background reading 4. Rotate polaroid on top 10º 5. Measure light intensity and subtract background reading 6. Repeat 4 and 10 every 10º to 90º 7. Plot I vs cos2Ø Results Linear graph of I vs cos2Ø Gradient of 367 → original intensity y = mx → I = m cos2Ø, m = original light = Imax Due to background light, y =mx +c, where I = 366.6 cos2Ø + 1.1, Errors Background light not well controlled → changing light → nonlinear relationship
What were Maxwell's 4 relationships?
Maxwell's form of Gauss' law → electric fields Maxwell's form Gauss' law → magnetic fields Faraday's law linked to electric fields → Magnetic fields induce an electric field Ampere-Maxwell Law → Electric fields induce a magnetic field
What are the types of spectroscopy
Nuclear magnetic resonance (NMR) → flip spin of atoms to see what they;re made of Infrared spec → Firing infrared, molecules absorb particular parts UV spec → organic molecules absorbing light X Ray crystallography →(Bragg) X-ray into molecule, bounces x ray off to find shape and design of molecule
How did Ole Römer measure the speed of light?
Observed time of orbit of Jupiter's moon Io, to be eclipsed by Jupiter. He was restricted to measuring in 4 month periods, noting that the time between eclipses grew shorter as the Earth moved towards Jupiter and longer as the Earth moved away. Hypothesized this was due to the finite speed of light taking longer to reach the Earth as the distance between Jupiter and Earth varied through Earth's orbit. Using Kepler's research into planetary orbits, he estimated that light takes 22 mins to cross the diameter of Earth's orbit, giving light a velocity of 220,000km/s, error of approximately 26.6%. Sources of error: Inaccuracy of the orbital diameter Inaccuracy in measurement of exact times of eclipses, inaccuracy in hypothesised time for light to cross Earth's diameter (22 minutes vs. true value of approximately 17 minutes).
What are the names of the people on the timeline in measuring the speed of light?
Ole Römer Albert Michelson Louis Essen
What light do LED's produce?
Only produces particular wavelengths of light (straight up lines on black body radiation graph)
What were the 2 prevailing ideas of light/EM radiation at the time?
Particle → Newton Wave → Huygens
How is Chemical Composition found from spectra?
Presence of spectral line corresponding to a specific energy transition for an ion, element or molecule in the spectrum of a star indicates that a specific ion, atom or molecule is present in that star Absence of a spectral line does not also necessarily mean element is not present in star. Conditions in the layers of the star responsible for absorption and emission lines may not be conducive to the formation of that specific transition or line (some more luminous than others)
How can spectra give information on a stars lifecycle?
Primarily hydrogen and helium -> red giant or main sequence, young Carbon, oxygen nitrogen --> hot main sequence
What was Huygens principle?
Proposed that every point on a wavefront acts as a source of secondary spherical wavefront
What experiment was done in class to show the propagation of EM waves - Induction coil and Cathode ray oscilloscope What were the results?
Radio waves produced by induction coil and received by cathode ray oscilloscope No clear amplitude, uneven waveform, therefore indicating each spark produces individual radio waves at different frequencies
What experiment was done in class to show the propagation of EM waves - Induction coil and speakers
Radio waves produced by induction coil and received by stereo radio Source: Induction coil Receiver: Radio receiver and speakers When coil is on, the radio detects radio waves at 522kHz When the coil is on and the radio is moved away from the sparking, the stereo volume of radio increases as you get closer, and radiowaves reduce in density as you move away, so volume also reduces As the frequency band is scanned, the radio detects waves at multiple frequencies, so radio waves are produced at multiple frequencies Changes only due to sparking, as although background noise exists, the change of sound only occurred when the spark was turned on and off
What experiment was done in class to show the propagation of EM waves - appliance cable and Cathode ray oscilloscope
Radio waves produced by wire with AC current flowing and received by and stereo radio Trace is uniform due to uniform AC current When AC turned off, no trace made When cable moved closer to CRO, amplitude increases, waveform doesn't change When length of probe cable is increased, amplitude increases, waveform doesn't change Trace only changed when cable turned on and off
What properties of light do the particle and wave model have in common
Reflection→ Particle and Wave hold this in common Refraction → Particle and Wave hold this in common Diffraction Particles - will go straight on, does not hold Waves - spread out
Stefan-Boltzmann law
Relates how much energy a blackbody would have inside a container (fixed volume) Stefan concluded that total energy density inside a container is proportional to the fourth power of temperature. This law worked with the experimental evidence E = aT4
How is rotational velocity determined from spectra?
Stars are hot, spherical balls of gas spinning on an axis If we obtain a spectrum of a distant star that is rotating in the same plane as us, then the light gathered is a combination of light from across the disc of the star As part of the star appears to rotate towards us, its light will be blue-shifted As part of star rotates away from us, its light appears to red-shift Middle will not shift. Effect of stars spectra → smears each line out (broadening), can be used to measure rotation rate of stars
What were the proposed properties of the aether?
Stationary, filled all of space, low density, perfectly transparent Permeated all matter and completely permeable to material object Great elasticity to support and propagate EM wave
How does an induction coil work? What are safety concerns?
Steps 6V DC up to 20 000V AC Interruptor turns DC into AC by turning DC on and off Produces x rays - turn on for small amount of time High voltage - don't put any metal close to it
What are the factors affecting resistance
Surface area, length, temperature, vibration
Why did Heinrich Hertz experiment with radio waves? What did he set out to find?
Taking 2 of maxwell's predictions, in 1887, Heinrich Hertz experimented to confirm these predictions Believed he could produce EM waves with frequencies that were not in the range of light High voltage through induction coil to create sparking
How can temperature be deduced from a blackbody curve?
Temp can be deduced from wavelength of peak of radiation curve
Magnetic flux
The amount of magnetic field passing through a given area according to øB = BAcosø
How does the classical and quantum theory treat energy in a black body?
The classical view treats all electromagnetic modes of the cavity as equally likely because you can add an infinitesimal amount of energy to any mode The quantum view expressed in the planck's hypothesis is that you either add the energy of a whole quanta or you don't add any energy ALL OR NOTHING PRINCIPLE Since the excitation of a high frequency EM wave takes an energy high above the average thermal energy it is therefore less likely. Thus the radiation curve falls progressively further below the classical expectation
What properties of light did Hertz try and show in EM waves?
The intensity of the generated wave decreases with distance. It could be reflected by metal plates. It could be refracted by paraffin blocks. It could be diffracted around obstructions. It could be polarised (when he rotated the receiving coil he found that the sparks were stronger at certain angles compared to others).
How can a spectra be produced from a gas?
The interaction of gas and EMR may produce electronic excitations, molecular vibrations or nuclear spin orientations to energise the atoms
Diffraction
The way in which waves bend around a corner or spread out in a circular pattern after passing through an object of similar width to their wavelength Wave property
What is threshold frequency on a photoelectric graph?
The x-intercept is the threshold frequency of the material. The electron will have zero kinetic energy when it has received a photon at the threshold frequency. The electron has just enough energy to be knocked off the surface, but that's it!
What is work function on a photoelectric graph?
The y-intercept is the work function of the material.
Electric flux
Total electrical field through a given area øelectric = q/E0
What type of wave is an EM wave? Draw a diagram What are its properties?
Transverse Transmit energy through vibration, without moving matter. They have frequency, wavelength, period (1/T), amplitude (amount of energy) and velocity.
How do polarisers work?
Transverse waves that exhibit polarisation include EM waves like light, radio and gravity waves A polarising material absorbs or reflects particular orientations of the electric field and allows only a particular orientation to pass through or be reflected This polarisation provides more evidence to the model of light being a wave
What is Maxwell's form of Gauss' law in terms of magnetic fields
Using magnetic flux instead of electric flux → magnetic flux will curve around to a magnet ∇B = 0
What are the Visible spectrum values?
Wavelength between 400-700nm
How does the photoelectric effect work?
When high energy light is shone onto a clean zinc surface, electrons are released (photoelectric effect). In a tube, the plate opposite the metal surface is made negative. It will then repel electrons, (Usually in a photocell it would be positive to attract electrons). Electrons may have enough KE to still reach the negative plate. However if the plate is made negative enough it will stop any electrons reaching it. This is how the Kinetic energy of the electrons is measured
Wien's displacement law
When the temperature of a blackbody radiator increases, the overall radiated energy increases and the peak of the radiation curve moves to shorter wavelengths. The producer of the peak wavelength and the temperature is found to be a constant Law useful for determining the temperature of hot radiant objects such as stars and determination of the temperature of any radiant object whose temperature is far above that of its surroundings
Polarisation
a property applying to transverse waves that specifies the geometrical orientation of the oscillation
Spectroscopy
branch of science concerned with investigation and measurement of spectra produced with matter interacts with or emits electromagnetic radiation
Brightness
how much light is falling per second per square meter
Continuous spectrum
one that has light being produced over the entire visible region Can be produced by thermal emission from a black body
Emission spectra:
produced when a gas is excited. A gas can be excited by heating it or passing an electrical discharge through it (discharge tubes) → what's being produced Emission spectrum is a series of narrow coloured lines on a dark background Each element has its own characteristic spectrum and can be used to identify the gas Inverse of absorption spectra
Absorption spectra
produced when white light is passed through a cool gas. Atoms in gas absorb energy from the white light and remit it → whats being absorbed. Black lines on coloured spectra Must be cool as if warm it ionises because excited E.g. If hydrogen, the electron absorbs light, jumps up a shell, then jumps back down, emitting same energy and wavelength as a photon in varying directions
What are the features of a photoelectric effect graph?
y int - work function x int - threshold frequency gradient - planck's constant
What is the equation for Wien's displacement constant?
λmax = b/T where λmax = wavelength peak (m), b = Wien's displacement constant (2.898*10-3 mK), T = temperature of the object → not on data sheet!
When were the 2 wave models competing?
17th century Corpuscular/particle model Wave Model
What is Maxwell's form of Gauss' law in terms of electric fields
Electric flux through a closed surface is proportional to charge enclosed by surface Electric flux = q/E0 ∇E = p/E0
What is the basic quantum explanation about energy emitted by an oscillator?
Energy emitted by an oscillator was now proportional to frequency as well as temperature High energy is required to achieve higher oscillation frequencies. Higher frequencies are less probable. He described this small, average packet as a 'quantum' of energy, that could be described as E = hf F → frequency (Hz), h → plancks constant
How did Louis Essen measure the speed of light?
Essen used a microwave cavity of fixed dimensions, to find the resonant frequency of a microwave in the cavity. A microwave travels at the speed of light, therefore since F = 1/∆T , d = λ (wavelength established by the dimensions of the box), and velocity = d/t, the speed of light is given by v = F*λ. A value of 299,792.5±1 km/s was found, with an error of only 0.000014% from the value held today. This was a very accurate value. Source of error: The dimensions of the box were not known down to nanometers, varying the decimal places of the speed of light very slightly.
Coulomb's law
Explains the magnitude of force between 2 charged particles → F = 1/4πE0 q1q2/r2
How did Maxwell relate Faraday's law to electric fields
Faraday's law linked to electric fields → Magnetic fields induce an electric field Link changing magnetic flux to electric fields E = -N∆ø/∆t, ∫E ds = -d∆øB/dt Electric field = induced by magnetic field
When a spectroscope was pointed at a fluorescent tube
Fluorescent produced light at select wavelengths
How is density determined from spectra?
For a given temp, the more luminous the star was, the narrower the spectral lines were Large stars - very low relative density in outer layers, as volume occupied by gas is greater Width of the line (and the depth for an intensity plot) provides info about outer shell (place for absorption and emission of photons) This is due to the reduction in motion of the particles in their atmosphere due to less pressure and therefore less range of velocities Low density → narrower lines High density → higher amount of collisions, greater number of collisions
What is the order of EM radiation
Gamma, Xray, UV, Visible, Infrared, Radio
How do fluorescent light bulbs produce light?
Gas in the inner tube (usually argon), electricity is passed through and gas produces UV light. Inside is painted with a fluorescent material, and the UV energises it, and it emits visible light
How is the definition of the metre based on the speed of light?
General trend of moving away from physical objects to define units as the properties of physical objects change, and therefore create error. When taking the speed of light in a vacuum which is a fixed value of 299,792,458 m/s, you can find the time it takes to travel a metre. This gives the value of a metre to be the distance travelled in 1/299,792,458 of a second in a vacuum.
What was Hertz experiment to determine the nature of light?
Method High voltage power source caused sparks to oscillate across a gap in a circuit. Each visible spark is actually a series of many small sparks, oscillating between the terminals. The size of the metal plates attached to the spheres from which the sparks are produced, controlled frequency of sparks produced. A loop of wire held near the oscillating spark, had a spark jump across the air gap between the ends of the wire whenever a spark jumped across the high voltage (induction coil) spark gap.
What was Newton's corpuscular/particle model
Newton's theory of light was that light is a small particle with mass, explains reflection and refraction using gravitation and motion laws. Light is attracted to the heavier object Explained reflection, refraction, colours and polarisation Particles so small they don't interfere with each other Light travels faster in optically dense materials. When light particle in medium, it is surrounded by equal numbers of matter particles. Suppose there is an attractive force between the light particles and matter particles. Then in a medium, these forces cancel each other out and there is no net force on the light particle. According to Newton's first law, the particle will continue in a straight line since no net force. Near an interface there are more matter particles on one side so light particle can experience a net force, a brief attractive force towards the medium with more matter particles. EVIDENCE: Explained reflection and refraction
What are the properties of incandescent filaments? How do they produce light?
Non-ohmic, really thin, current of electrons run through lattice, lattice vibrates, therefore highly resistive and heats up. Particles vibrate because of heat at frequency of visible light Produces wavelengths in all parts of spectra
How did Hertz find the velocity of EM waves?
Set out to measure wavelength of waves by setting up a standing wave using a metal plate to reflect the wave back Used a detecting loop with spark gap to locate antinodes (max sparking produced) and hence find wavelength Distance between 2 antinodes is one half wavelength. Substitution in v = fλ, gave velocity of waves close to speed of light
What were Hertz's results in terms of the velocity of light/EM waves?
Showed speed of EM waves as 3.00*10^8 m/s
Why was the aether developed?
Since waves need a medium, the 'luminiferous aether' was the hypothesised medium → Newton, Huygens and Young were the early developers With Faraday's discovery of lines of electrical and magnetic force, the need for some conducting medium was obvious, His field concept would be more meaningful if there was an appropriate medium to fill it Then, with Maxwell's electric and magnetic wave theory, there was a need for propagating medium
Give a method for measuring the emission spectra of hydrogen
Spectrometer aligned close to the tube, straight at the selected tube to restrict the measurement of spectra to the discharge tube. 300 slits/mm diffraction grating was slotted perpendicular to the collimator. Power turned on, base held down to prevent movement, and the middle line aligned in the center of the telescope cross-hairs to get the angle of the line (control). The degree and decimal was measured on the angle scale (dependent variable) and recorded. The telescope was moved anticlockwise. Brightest spectral lines were measured following the method provided above. Once the middle line colour returned on the spectra, the telescope was moved to middle line, and brightest spectral lines were measured clockwise according to above method to collect a second set of results to increase reliability. The angles found for each spectral line were calculated using the formula for diffraction (nλ=d sinθ, where n, d and θ are known) and the wavelength of each spectral line (dependent variable) was found.
