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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
In this chapter we will examine some experimental
evidences in favor of the wave properties of light
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 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:
sin( 1 ) / v1  sin(
- we have that the index of refraction of a
- if speed of light in vacuum is c, and nv=1
(u is the speed of light in the given
n  c/u
n ~ 1/ u
2 ) / v2
Refraction a test for the models….
Both the wave and particle image explains refraction
- 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
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 phasedifference, 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
 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
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
- 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
Phenomenon characteristic for
transverse waves
polarized and non-polarized
transverse waves
polarizing a transverse wave
polarizing light with Polaroid
experiments with Polaroid filters
rotating the polarization plane:
transparent adhesive planes
(amount of rotation depends on
common light is unpolarized
reflected light is partially polarized
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)
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
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