A Model for Light

Chapter 18

What light is? Newton: light is a stream of tinny particles Huygens: light is a wave due to Newton’s great reputation, his particle model

accepted in the 18th century it could not be accepted that a wave can travel in

vacuum --> what is vibrating in vacuum? 19th century: wave model for light accepted 20th century: light has both particle and wave

properties In this chapter we will examine some experimental

evidences in favor of the wave properties of light

Reflection Reflection of light can be easily understood by

the particle model. A particle colliding elastically with a wall reflects

- the angle of reflection = the angle of incidence Waves also reflect (standing waves…..)

- the angle of reflection = the angle of incidence Observing how light

reflects from surfaces

gives us no clues as to

its true nature

Refraction Refraction explained by Newton: - particles of light experience a force as they

pass from air into a transparent material - this force occur at the surface, act

perpendicularly to the surface, directed into the material

- this force would cause the particles to bend towards the normal

- predicts a good relationship between the angle of refraction and incidence

Refraction explained by waves: - frequency the same in the two materials - speed of waves different in the two materials - the wave-length changes - relation between the angle of incidence and refraction: - we have that the index of refraction of a

material - if speed of light in vacuum is c, and nv=1 (u is the speed of light in the given

material)

2211 /)sin(/)sin( vv

un /1~

ucn /

Refraction a test for the models…. Both the wave and particle image explains refraction HOWEVER:

- after Newton’s theory the speed of light in a material should be bigger than in vacuum

- the wave model predicts speed of light in materials smaller than in vacuum (n>1)

Measuring the speed of light in vacuum and transparent materials--> a test for the models

19th century: measurement of the speed of light in air and water (Foucault)

speed of light in air bigger! As n increases the speed of light decreases in the

materials --> prove in favor of the wave model! Should be inversely in the Newtonian model

Interference If light is wave --> should show the

phenomenon of interference making interference with light is more

difficult (without laser….): -we need two point-like sources

(dimensions smaller than the wavelength of light,

~ 10-9m) -in order to get stationary patterns the two

sources should have constant phase-difference, and produce waves with the same wavelength!

- in order to distinguish the nodal and antinodal points the two sources should be close (separation: order of the wavelength)

First successful experiment: Thomas Young (1801) --> two slit experiment

wavelength--> determines the distance between nodal lines

distance between nodal lines different for different color light

Diffraction Young’s experiment also prove the

phenomenon of diffraction for light diffraction of light passing through

a narrow slit --> leads also to interference patterns

wider slit produce more narrow pattern (for particles would be the opposite effect)

simple diffraction experiments:

- on a pinhole

- between the fingers

- behind a penny

Diffraction Limits Diffraction limits the magnification we can get by optical

instruments

- two small objects separated by a small angular distance

- each of these produce a diffraction pattern when its light passes through small opening

- in order that the two object look separated the two diffraction pattern should not overlap

The minimum angular separation of the instrument depends on the size of the objective lens and the wavelength of light (good: to have big objective lens, and small wavelength!)

Interference in thin films thin films: thin transparent layers of any material: oil slicks, soap

bubbles, coatings, air layers etc... we observe beautiful arrays of colors --> result of interference by multiple reflections and refraction inside the film we get light

beams with different phases --> superimposing one on the other produce interference

the phase difference depends on the thickness of the layer, and the wavelength of light: if the thickness is not uniform --> patterns of nodal and antinodal curves

Polarization Phenomenon characteristic for

transverse waves polarized and non-polarized

transverse waves polarizing a transverse wave polarizing light with Polaroid

filter experiments with Polaroid filters rotating the polarization plane:

transparent adhesive planes (amount of rotation depends on thickness)

common light is unpolarized reflected light is partially polarized

Holography Making 3D pictures, conceived by: Gabor Denes (1947) 3D pictures: possible to view it from different perspectives

(looking around the object) holo--> complete; gram--> message holography --> preserving all information about an object holograms: made by using laser light on the film interference pattern of

1. Laser light coming directly from the light-source

2. Laser light reflected by the object (3D information about the object on each portion of the film)

Summary particle and wave models for light both models are able to account for the law of reflection

an refraction only the wave model can correctly predict the speed of

light in transparent materials; index of reflection n=c/u interference of light possible under special conditions diffraction of light produces amazing interference

patterns narrower the opening the wider the diffraction pattern is in thin films the light rays reflected from the two

surfaces and leads to observable interference patterns light exhibits polarization, demonstrating that it is a

transverse wave Home-work Assignment:

Part I.:466/2-4,7-12,15-18,21, 23-24; 469/1-12;

Part II: 467/26-39,41-44; 468/49-51, 54-56, 59-60;

469/13, 17,18, 21, 22