Ch. 24 Special Theory of Relativity

Why do different colors appear (like a rainbow in a pool of water)?

different colors correspond to different wavelengths of light bending/overlapping because each wavelength bends/interacts differently

Is light a wave or a particle?

light has a wave-particle duality

When looking in a parking lot after a rain storm you notice swirls of color…what causes these colors?

Light waves reflected from the top and bottom of the oil constructively interfere

What is a refraction?

a wave front that bends a wave as it passes from one medium into another (ex: air into water)

What rule explains refraction?

Snell’s Law

What are wave fronts?

they are a crest of a wave as it travels (surface of the wave basically)

Every point on a wave front can be considered as a source of __________ wavelets that spread out in the forward direction at the speed of the wave itself.

Every point on a wave front can be considered as a source of TINY wavelets that spread out in the forward direction at the speed of the wave itself.

What is the Huygen’s Principle?

• every point on a wavefront acts like a new “source” of tiny waves

• wavelets combine to form next wavefront

What is diffraction?

when wave bends around/through an obstacle/obstacle

How does light move in the following scenarios:

1. Sharp Edge

2. Large Hole

3. Small Hole

1. Sharp Edge = light bends around

2. Large Hole = less bending

3. Small Hole = strong bending

Whenever light enters a new material, what happens to the light?

(speed, time, distance)

light bends because the speed is different

• time is the same

• distance changes

  • if speed decreases = shorter d

  • if speed increases = longer d

What explains why the light ray bends?

when the distance changes due to the change in speed!!!

What is Snell’s Law?

tells us exactly how much the light bends at the boundary between 2 materials

What is a mirage? Does this fall under refraction or diffraction?

Mirage = light bends due to changes in air temperature

• under refraction

How does light bend in respect to density of the air?

Warm air (near ground) = less dense

• light travels faster

Colder air (up) = more dense

• light travels slower

In terms of air, light bends gradually __________ because ________ affects air density causing a change in the light _________ leading to refraction

In terms of air, light bends gradually UPWARD because TEMPERATURE affects air density causing a change in the light SPEED leading to refraction

What is interference (waves)?

when 2 waves meet at the same time, they combine

What are the 2 types of wave interference?

1. Constructive

   • In Phase (same)

     • results get bigger

2. Destructive

   • Out of phase (opposite)

     • cancel out/smaller wave

Whenever you have 2 slits, what actually happens to the light particles?

• you get many bright but decreasing bands of light (pattern)

  • showing that light behaves like a wave NOT a particle

How in a double-slit experiment does light react the way it does?

• each slit produces a new wave front when it passes through each slit

• each slit acts like a “new source” of a wave front

• wave spreads out and interferes with each other (making them screen out)

If you have a whole wavelength, waves will arrive in phase, resulting in a ______ spot.

If you have ½ a wavelength, waves will arrive out of phase, resulting in a _______ spot

If you have a whole wavelength, waves will arrive in phase, resulting in a BRIGHT spot.

If you have ½ a wavelength, waves will arrive out of phase, resulting in a DARK spot

what does “m” represent in terms of waves?

m = integer starting at 0

• represents the distance b/w the slips

• 0 = central bright area

True or False: sin(theta) cannot exceed 1

true

As you move away from the center maximum (bright spot), what happens to the brightness/intensity?

decreases!

What happens when the wavelength increases/decreases?

wavelength INC = angle/distance INC

wavelength DEC = angle/distance DEC

What does white light mean?

• means all colors/wavelengths overlap there

• going to left/right = results in rainbow-looking interference pattern

What would happen if the 2 slits were replaced by 2 lightbulbs and why?

• no interference would happen!!

• light from 2 separate bulbs are NOT IN PHASE

  • light waves are different so interference pattern is different

What are the 2 most important characteristics of light?

• intensity (W/m²)

  • “how much energy light has”

• color

  • look at frequency/wavelength

What is the wavelength and frequency range?

Wavelength → (frequency)

• 400 nm (violet) →HIGH [uv]

• 750 nm (red) → LOW [ir]

Whenever a white light passes through a prism, why is a rainbow seen?

• each color has a different angle (n = different for each)

True or False: n depends on frequency

n depends on wavelength

If there is a shorter wavelength, the light will ben _____ and this represents the colors _______, meaning n is _________

If there is a shorter wavelength, the light will bend MORE and this represents the colors BLUE/VIOLET meaning n is HIGHER

what is dispersion?

the way a color bends because it is by a different amount and they all have different wavelength

What happens if light passes by a single slit or around a disk?

• diffraction occurs

  • bright spot/pattern appears around the shadow

What is a diffraction pattern?

• pattern of light/dark stripes

• wavelets interfere with each other after passing the obstacle

How does light travel when the angle is 0?

• light rays travel same distance = in phase

  • constructive interference → bright center

How does light travel when the angle is ½ ?

• the paths differ in lengths = out of phase

  • destructive interference → dark fringe

How does light travel when the angle is 3/2 ?

• only 1/3 of the waves constructively interfere

  • show weak bright fringe

How is the formula different for diffraction compared to the one for double-slit?

Diffraction

• m = integer (NOT 0 ~ looking for minimum)

• D = width of slit

Double-slit

• d = distance between 2 slits

If you have a rectangular slit (narrow part along vertical) that line shines through, would the light spread out more horizontally or vertically?

more vertically

what happens if light is passed through many slits?

• interference occurs b/w the light waves coming from each slit

What is diffraction grating?

device with many slits that are equally spaced and parallel to each other

• shows where bright lines appear on screen

When there are many slits, the bright spots are brighter/dimmer and narrower/wider.

When there are many slits, the bright spots are BRIGHTER and NARROWER.

• there is a lot of destructive interference

what does a spectrometer measure?

measures light

define what emission + absorption spectra means

Emission spectra

• bright lines

Absorption spectra

• dark lines (lines missing)

________ absorb and emit light when electrson change energy levels

ATOMS absorb and emit light when electrons change energy levels

Does each atom have its own set of lines?

yes!

True or False: if multiple elements are present, their lines will interfere

FALSE

• …..their lines will NOT INTERFERE

How does reflection occur?

when light hurts a surface and the waves interfere with each other to give bright/dark areas

what determines whether a wave is in phase or out of phase

the index of refraction (n)

when going from low n to high n, is the reflection in/out of phase??

• out of phase (180 degree shift)

When going from high n to low n, is the reflection in/out of phase?

• in phase (no shift)

what 3 things does interference of waves depend ong?

• index of refraction (n)

• thickness (t)

• in/out of phase

How are Newton’s Rings created when a rounded glass is placed on a flat piece of glass?

light interferes with each other when…→ results in bright/dark spots (NR)

• light reflects off bottom of curved glass

• light reflects off top of flat glass

What type of light pattern is created when 2 flat pieces of glass are placed on top of each other but separated by a small wire on one end?

• light rays will reflect and interfere with each other

  • cause alternating bright/dark fringes

What does a Michelson Interferometer do?

splits a beam of light transmitting half the light to a movable mirror and the other half to a fixed mirror

• sensitive to bright fringes

what is polarization?

shows the electric field along a wave (it is perpendicular)

What is the difference between polarized and unpolarized light?

Polarized

• oscillations (plane of electric field) are in ALL directions

Unpolarized

• oscillations (plane of electric field) are in MANY directions, but perpendicular to the direction of travel

What does a polarizer do?

material that only allows light vibrate in 1 direction to pass through

What happens when unpolarized light hits a polarizer?

• 50% of unpolarized light is transmitted

  • results in linearly polarized light

after light is polarized, what does the intensity of the light transmitted depend on? This is known under what law?

depends on angle between electric field and polarize → Malus’s Law

Why does light reflect from water become polarized?

• reflected light becomes polarized (in the direction that is parallel to the surface)

what happens when you rotate a polarizer?

• can block horizontally polarized glare (reflected off the water)

how do LCD screens work ? What happens if you turn on the voltage?

inside of them they have liquid crystals that twist light if no voltage is applied

• voltage on = crystals stop twisting light/no more light and screen dark

why is the sky blue?

• light scatters and hits tiny air molecules

  • blue scatters more than red

    • at sunset, light travels farther, so more blue scatters and red/orange seeni

is the light in the sky polarized?

scattered light in the sky is partly polarized (so mostly 1 direction)