# A Model for Light Chapter 18. What light is? n Newton: light is a stream of tinny particles n Huygens: light is a wave n due to Newton’s great reputation,

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A Model for Light Chapter 18

What light is? n Newton: light is a stream of tinny particles n Huygens: light is a wave n due to Newton’s great reputation, his particle model accepted in the 18 th century n it could not be accepted that a wave can travel in vacuum --> what is vibrating in vacuum? n 19 th century: wave model for light accepted n 20 th century: light has both particle and wave properties n In this chapter we will examine some experimental evidences in favor of the wave properties of light

Reflection n Reflection of light can be easily understood by the particle model. n A particle colliding elastically with a wall reflects - the angle of reflection = the angle of incidence n Waves also reflect (standing waves…..) - the angle of reflection = the angle of incidence n Observing how light reflects from surfaces gives us no clues as to its true nature

Refraction n 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 n 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 n v =1 (u is the speed of light in the given material)

Refraction a test for the models…. n Both the wave and particle image explains refraction n 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) n Measuring the speed of light in vacuum and transparent materials--> a test for the models n 19 th century: measurement of the speed of light in air and water (Foucault) n speed of light in air bigger! n As n increases the speed of light decreases in the materials --> prove in favor of the wave model! n Should be inversely in the Newtonian model

Interference n If light is wave --> should show the phenomenon of interference n making interference with light is more difficult (without laser….): -we need two point-like sources (dimensions smaller than the wavelength of light, ~ 10 -9 m) -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) n First successful experiment: Thomas Young (1801) --> two slit experiment n wavelength--> determines the distance between nodal lines n distance between nodal lines different for different color light

Diffraction n Young’s experiment also prove the phenomenon of diffraction for light n diffraction of light passing through a narrow slit --> leads also to interference patterns n wider slit produce more narrow pattern (for particles would be the opposite effect) n simple diffraction experiments: - on a pinhole - between the fingers - behind a penny

Diffraction Limits n 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 n 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 n thin films: thin transparent layers of any material: oil slicks, soap bubbles, coatings, air layers etc... n we observe beautiful arrays of colors --> result of interference n by multiple reflections and refraction inside the film we get light beams with different phases --> superimposing one on the other produce interference n 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 n Phenomenon characteristic for transverse waves n polarized and non-polarized transverse waves n polarizing a transverse wave n polarizing light with Polaroid filter n experiments with Polaroid filters n rotating the polarization plane: transparent adhesive planes (amount of rotation depends on thickness) n common light is unpolarized n reflected light is partially polarized

Holography n Making 3D pictures, conceived by: Gabor Denes (1947) n 3D pictures: possible to view it from different perspectives (looking around the object) n holo--> complete; gram--> message n holography --> preserving all information about an object n holograms: made by using laser light n on the film interference pattern of 1. Laser light coming directly from the light-source 2. Laser light reflected by the object n (3D information about the object on each portion of the film)

Summary n particle and wave models for light n both models are able to account for the law of reflection an refraction n only the wave model can correctly predict the speed of light in transparent materials; index of reflection n=c/u n interference of light possible under special conditions n diffraction of light produces amazing interference patterns n narrower the opening the wider the diffraction pattern is n in thin films the light rays reflected from the two surfaces and leads to observable interference patterns n 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

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