Unit 1: What is Light?

hertz =

# of cycles per second (units 1/s) frequency

speed of light

-300,000,000 m/s -3 x 10^8 m/s

laser shined on a really small pinhole

-a bulls-eye pattern because of DIFFRACTION -the laser is a single color of light, all in phase -when waves add and subtract, they make dark and light spots (ray model does not predict this, much more complex)

WAVE definition

-a periodic (recurring) disturbance -the wave is the disturbance, not the material being transported -vertical oscillation, propagating outwards

a pinhole works by....

-blocking the majority of light rays -"less is more"

if you cover an extended light source with tinfoil and poke pinholes in it...

-each pinhole provides one formed image of the bulb's filament -there is a unique ray for each unique point on the filament that passes through the pinholes

Galileo's experiment

-for light to travel from Galileo to Luigi and back (roundtrip), would've taken 0.000018 seconds -human reaction time is about 0.1 seconds

when there are multiple light sources....

-gives MULTIPLE shadows -region where they overlap is the umbra -region where the shadows don't is fuzzier

wavelength

-length of one cycle of the wave ->wave repeats its shape each time you move left or right

SI units of measure -length: -time: -velocity:

-length: meters (m) -time: seconds (s) -velocity: meters/second (m/s)

fastest to slowest: sound, light, smell

-light -sound -smell

diffraction grating example

-middle bulb has distinct bands of color -top & bottom have continuous ranges of wavelength

how does light travel from an extended source?

-multiple ray model -each point sends multiple rays

a light ray

-originates at the light source -points in the direction of travel of light -light rays allow us to draw diagrams of the motion of light

we treat the sun as a...

-point source, multiple rays coming out of it -only some of the light rays are implicated in part of the shadow -these are the rays that define the edge of the shadow

The size of the image depends on...

-the distance of the object -the distance of the screen relative to the pinhole

with the multiple-ray model, the closer an object is to a light source, the _________ (clearer/fuzzier) its shadow will be

FUZZIER -as you move object closer to light, shadow gets fuzzier

LARGER light source, (sharper/fuzzier) shadow

FUZZIER -many lights (ie in a classroom)

a fuzzy shadow has a (small/large penumbra)

LARGE

if pencil is closer to wall, (large/small) umbra & (large/small penumbra)

LARGE umbra, SMALL penumbra -sharp shadow -->sharpness depends on distance between object & screen, and object & light, and size of light source

Vision needs...

Light, eyes (sensors), brain (processing)

does the object photographed with a pinhole have to be self luminous?

NO

can you make a sharp shadow on your desk?

NO -multiple light sources -light sources are closer to us, light rays are not parallel like the sun

SMALLER light source, (sharper/fuzzier) shadow

SHARPER

if an object is farther away from the pinhole camera, the image will be smaller/larger

SMALLER

the larger the wavelength, the (smaller/larger) frequency

SMALLER larger wavelength (more spread out) = smaller frequency

You are seeing me now because:

Something is coming into your eyes

Where does moonlight come from?

Sun (moonbeams are just reflected sunbeams)

wavelengths are really (small/large) for visible light

TINY

Lo (distance between object and pinhole) Li (distance between pinhole & screen) --> if Lo = Li, what would happen?

The image would be the exact same size as the object -the rays would intersect halfway between the object and image

Testing experiment: "Light travels in straight lines"

We can sight a maglite bulb through a straw only when straw is pointing at it

what is a model?

a *representation or simplification* of an idea or process that cannot be visualized directly

Fact

a close agreement by competent observers who make a series of observations about the same phenomenon

light source is

a disturbance in the electromagnetic fields, causes an oscillation outwards that travels to your eye and you process as light

a fuzzy shadow will be produced if...

a large, extended light source or an object far from the ground

what will you observe on the screen when a bulb with a looped filament is placed in front of the pinhole?

a loop of illumination, exactly the shape of that filament but inverted vertically and horizontally

penumbra

a partial shadow (only some light is blocked, some light is not)

a sharp shadow will be produced if...

a pointlike light source or an object near the ground

white light

a prism spreads out white light into the colors of the rainbow

what is an image?

a reproduction of an object, especially a reproduction that is made optically (lenses, mirrors, pinholes)

Point source

a single dot of light (candles, mini maglite bulb)

To see something, need...

a source of light and an object off which the light bounces (reflects) and then reaches the eyes of the observer

Theory

a synthesis of a large body of information that encompasses well-tested and verified hypotheses about certain aspects of the natural world

umbra

a total shadow (all light is blocked)

Laser

all light rays are traveling in same direction, can only see when obstructed by object and light scatters into eye -obstructing laser with hand scatters light in all directions and into your eye -smoke, fog water molecules sprayed can see beam of light

oscillation of a wave quantified by

amplitude & wavelength

Hypothesis

an educated guess that is only presumed to be factual until supported by experiment -MUST BE TESTABLE

brighter light has (bigger/smaller) amplitude

brighter light = bigger AMPLITUDE = taller wave

diffraction grating

closely spaced slits, results in larger separations so we can more clearly see the pattern

what is color?

color is our brain's interpretation of light of different wavelengths/frequencies entering our eyes

dispersion

colors have different wavelengths, and those wavelengths are affected differently by going through a material, and spread out

film of gasoline

colors that are displayed depends on thickness of film of gas

extended light source

consists of multiple points, each of which emits light

two slit interference

constructive interference: bright bands destructive interference: dark spots ->make the spacings smaller, light spots appear more spread out

anti-reflective lenses do what

destructive interference

speed of light lets us measure the distance from...

earth to the moon travel distance from (earth -> moon -> earth) = light speed * transmission time distance from earth -> moon = travel distance/2 returning light is much weaker than the light they send to moon

a large extended light source

forms a fuzzy shadow (very small umbra, large penumbra)

a pointlike light source (maglite, sun)

forms a sharp shadow (larger the umbra and smaller penumbra)

what captures the time aspect of waves?

frequency & period

the nautilus eye

is nothing but a pinhole

small gap (compared to wavelength) =

large diffraction effect

what travels farther distances? larger/smaller wavelength?

larger wavelength

Light travels in straight lines, called...

light rays

flashlight

light rays traveling in multiple different directions, travels directly into eye

WAVE model

light travels as a wave from a point source, in a straight line

the type of eclipse in view of most people on earth is a

lunar eclipse (all viewers on nighttime side of Earth can view the same lunar eclipse)

Testing experiment: "Light rays point out in all directions from a point source"

observers all around the maglite can see it through a straw

ridges on peacock feather

only see some colors, can determine how the height of the ridge changes by colors that are reflected

rays at top and bottom of light that intersect with bottom and top of object, represent the (UMBRA/PENUMBRA)

penumbra

as object moves closer to the light and away from the wall

shadow becomes fuzzier and penumbra becomes bigger

lunar eclipse

shadow of earth cast on moon

solar eclipse

shadow of moon cast on earth

the triangles of two shadows from the sun are....

similar because three equal angles (Hb/Lb) = (Hh/Lh)

wide gap =

small diffraction effect

interference causes....

the bright and dark areas

ray model tells you

the direction of travel of the rays

amplitude

the height (from the midline)

frequency

the number of full wave cycles per second

if i move the sources farther away

the penumbra will get larger compared to umbra (less overlap)

diffraction depends on

the size of the gap compared to the wavelength

period (T)

the time to swing back and forth once (the amount of time per unit cycle) -->the INVERSE of frequency

if i move my hand closer to the screen in the classroom...(multiple light sources)

the umbra will get larger compared to penumbra

light beams are invisible unless...

they enter directly into our eye or are scattered by smoke, fog, or some object into your eye

rays at top and bottom of light that intersect with corresponding top and bottom of object, are the (UMBRA/PENUMBRA)

umbra

If we cover a bulb in foil and poke a small hole in the foil, we see...

uniform illumination --> light is emitted in all directions (multiple-ray model)

a pinhole diagram acts as a....

unique point -each pinhole allows one light ray to get through it -rays CROSS AT PINHOLE

speed of wavelength

wave travels at some speed -speed is called "c" for light waves in vacuum

different colors are different.....

wavelengths of light (why a prism separates colors in that order)

diffraction

waves bend around a tiny hole and create a circular pattern

because the sun is so far away....

we treat the rays as parallel to each other

painted dot on the string example

what is actually moving is the VERTICAL displacement, so point moves up and down ->THE DISPLACEMENT MOVES, NOT THE MATERIAL fixed points are: -point where string is being grasped, moving string up and down -the painted point

testing experiment: single-ray vs multiple-ray model

when moving a pencil closer to the lightbulb, single-ray model would predict a dark, sharp shadow no matter how close/far away -multiple-ray model: almost uniformly illuminated screen with fuzzy shadow MULTIPLE-RAY MODEL IS CORRECT!