chapter 17 physics 2

3 models of light are

1) wave

2) ray

3) photon

wave model

• Most applicable model

• Respsnbile for knwown fact that light is a wave

wave optics

study of light as a wave

ray model

• Properties of prisms, mirrons, and lenses are understood best by light rays

• Waves and rays are mostly mutually exclusive models of light

- light travels in a straight line

light rays

straight line paths

photon model

• In the quantum world light consists of phonts that have wave and particle like properties

• Photons are the quanta of light

what are the 3 theories of light

1) corpuscular

2) wave

3) quantum

corpuscular theory

- means particles of light

- early theory

wave theory

• Hooke/hygens showed wave nature of light

• Maxwell/hertz showed light was electromagnetic waves

quantum theory

• Firmly established existence of quantized packets of light, photons

• Photoelectric effect

constructive interference

you add 2 very similar waves and make a bigger version of them is the sum

destructive interference

you add 2 opposite waves and the sum is a flat line

what kind of shadow does light passing through an opening make

a sharp edged one

diffraction

- spreading of a wave

- After passing thorugh the opening, a wave spreads out to fill the space behind that opening

• Sure sign of whatever is passing thorugh is a wave

• Whether a wave spreads out or travels straight ahead depends on

the size of the objects that interact with the waves

how small is a narrow opening?

hundreds of nanometers

o What happens if we make the opening more narrow

• No change in the waves

• The waves spread out

• The wavelength changes

the waves are spread out

waves approaching wide openings

continue in straight lines

wavelength of light is about

5um

what fields does light consist of?

very rapidly oscillating electric and magnetic fields (electromagnetic)

light waves are a _ of the electromagnetic field

self sustaining oscillation

speed of light

• All electronamgentic waves such as light waves travel in a vaccum at the same time

speed of light number

c = 3.00 x 10^8 m/s

wavelengths of light range from

400-700um

visible spectrum

spread of colors seen with a prism or rainbow

• If the wavenlengths of light are incredibly small,t he oscillatin frequencies are

unbelievably high

_ does not need any medium in which to propagate

light

smallest to biggest wavelengths

1) gamma

2) x rays

3) ultra viloet

4) visible

5) infrared

6) microwave

7) fm radio/tv

8) am radio

index of refraction

the ratio of the speed in vacuum to the speed in the material

in a transparent medium, the speed of light is

v < c

index of refraction of air

1.0003

• Light waves slow down as they pass through

transparent materials

why do light waves slow down in transparent materials

because it is a consqeuwnce of interactiosn between electromagnetic field and electrons in material

• The index of refraction of a material is always greater than 2 because

the speed of light in the material is always less than the speed of light in a vacuum

• Liquids and solids have higher indices of refraction than gases because

they have a much higher density of atoms

• The frequency of a wave _ as the wave moves from one medium to another

does not change

• The wavelength in a transparent material is _ than the wavelength in a vacuum

shorter

o When light enters a medium with a higher index of refraction, its wave speed changes. How?

• Increases

• Decreases

• No change

decreases

o When light enters a medium with a higher index of refraction, its wave speed changes. How is the frequency affected?

• Increases

• Decreases

• No change

No change

• Like number of cars on a freeway won't change (as long as there are no exit ramps) even when there are less lanes

_ is a wave phenomenon

interference

to have interference you have to

have two sources of light waves with exactly the same wavelength

- waves must overlap and interfere

double slit experiment

• Two long narrow slits that are very close together

• A laser beam is aimed at an opague screen that contains these slits

• Constructive interference should occur and the intesntiy of light should be high when the distances between sources differ by

a whole number of wavelengths

interference fringes

• A series of alternating bright and dark bands of light

central maximum

• The brightest fringe at mindpoint of the viewing screen

o If path difference is a whole number of wavelengths, it is

constructive interference, aka bright fringe

o If path difference is a whole number of wavelengths plus half a wavelength, it is

destructive interference, aka dark band

in double slits the path length difference increases as

you move away from the center of the screen

• The interference pattern is a series of

equally spaced bright lines on the screen

the dark frignes are located exactly _ between the bright frignes

halfway

diffraction grating

• When illuminated from one side, each slit becomes the source of a light wave that spreads out behind the slit

o The path followed by the light from one slit to a pont on the screen is very nearly parallel to

the path follwoee by the light form neignoring slits

o N light waes from N different slits will all be in _ wheyn theye arrive at a poit on the screen with angle teta

phase with each other

o The integer m is called

the order of the diffraction

o Imprortant diference between the intensity patter of double slit interference and the intensity patter of a multiple slit diffraction grating is

• Bright fringes of a diffraction grating are much narrower

o In gnereal as the number of slits N increases, the bright fribnes get

narrower and brighter

o As the number of lines (slits) is increased, the bright fringes _, but become much more _

remain at the same position, sharply defined

spectroscopy

• The science of measuring the wavelngth sof atomic and moleculer emissions

• Difrraction gratings are an ideal tool for spectroscopy because

their bright fringes are so distinct

reflection gratings

• Narrow grooves on a reflective surface provide the spacing

what is the simplest reflection grating

• The simplest one is a mirror with hudngreds/tosuands of narrow parallel grooves cut into the surface

example of reflection grating

• The reainbow of colors seen on a cd or dvd

o In thin films, light is reflected from the _ surface, but also transmitted and reflected from the _ surface.

top, bottom

o Equal frequency light waves are rpoddced when

partial refelction at a boundary splits a light wave into a reflected wave and a trasmited wave

thin film interference

• The interference of light waves eflected from the two boundaries of a thin film, such as the film of water that maeks a soap buble

when is a reflected wave inverted

• When a light wave moves from a medium with a ghiher light speed (lower index) to a medium with a lower light speed (higher index)

phase change is the

inversion of the wave

phase change is equivalent to

adding an extra half waelength to the distance the wave travels

phase change leads to 2 situations

• If neither or both waves have a p hase change due to reflection, the net addition to the path length difference is zero

• If only one wave has a phase change due to reflection,t he effective path elgnth difference is increased by one half wavelngth

• Reflecting off a material with a lower n, there is _

no phase shift

• Reflecting off a material with a higher n, there is a _ - effective path difference of lambda/2.

half cycle phase shift

• A lgith wave undergoes a phase change if

it reflects from a boundary at which the index of refratciton inreases

• There's no phase change at a boundary where

the index of refraction decreases

• The two reflected waves will interfere constructively to casuse a strong reflection if

they are in phase

_ they will intefere destructively and casue a weak reflection or no revlection at all

• If they are out of phase,

• a thin layer of air sandwhictched between two glass surfaces exhcibits thin film nterference due to

waaves that reflect off both air glass boundaries

o A film with thickness t gives constructive interference for light with a wavelength in the film of λfilm. How much thicker would the film need to be in order to give destructive interference?

• A. 2λfilm

• B. λfilm

• C. λfilm/2

• D. λfilm/4

D. λfilm/4

o Nonreflective coatings can _ the amount of light that is reflected

reduce

• The thickness needed for non reflective coatings depends on the wavelength, so usually a color near the

middle of the visible spectrum is chosen

what is non reflective coatings useful for

camera lenses and eye glasses

reflectinve coatings can be used to

reject rransmission

single slit diffraction

• difrraction thorugh a tall narrow slit of weidth a

• light pattern on the viwing screen consists of _ flanked by a sieres of _

a centra max, weaker secondary maxima and dark firgnes

hyugen's principle

• Each point on a wave front is the soruce of a spehreical wavelet that spreads out at the wave speed

• At a alter time,t he shape of the wave front is the curve that is tangenet to all the wavelets

o •When combining these wavelets as the light passes an opaque object, we see interference pattern in the _ rather than the _expected by geometric optics

shadow, sharp edges

o As the width of the slit decreases, the diffraction pattern gets

broader

why can we easily hear someone outside a foor if we cannot see them

• The wavelength for human voices is on the order of a meter, so the sound waves have a broad diffraction maximum when passing through a doorway.

o A laboratory experiment produces a single-slit diffraction pattern on a screen. If the slit is made narrower, the bright fringes will be

• A.Closer together.

• B.In the same positions.

• C.Farther apart.

• D.There will be no fringes because the conditions for diffraction won't be satisfied.

farther apart

for single slit diffraction • When we sum the dispalcments of all n waelets,

they will pair by pair add to zero

• Diffraction pattern is dominated by the central max, which is much brougher than the _

second maxima

• A narrower slit (smaller a) causes a _ diffraction pattern

wider

• The smaller the opening a wave squeezes thorugh,

the more it spreads out on the other side

o If we have a circular aperture instead of a slit, the diffraction pattern is

circular

examples of circular aperature

• A loudspeaker cone generates oudn by the rapid oscilation of a diaphragm, but th esoudn wave must pass thorugh the circular aperature on the outer edge of the speaker before going into the room

• telecopes and microscopes are the rverse of this

• lgith waves form the outside need to enter the isntruemnt

o the diffraction pattern still ahs a central max in circular aperture, but is now circular, and

is srounded by a series of secondary bright fringes

o the diameter of the diffration pattern _with distance L, showing that light spreads out behind a circular aperature

increase

the diameter of the diffraction pattern it _ if the size D of the aperature is icnreased

decreases