a model for light chapter 18. what light is? n newton: light is a stream of tinny particles n...
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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