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Chapter 26

Geometric Optics (Lecture I)

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Outline

Reflection of light The law of reflection Specular and diffuse reflection

Refraction of light Index of refraction Snell’s law

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Rays and Wave Fronts Describe light in terms of

“wave fronts” and “rays” Wave fronts: In the case of

a water wave created by a rock dropped into a still pool of water, the wave fronts indicate the crests of water waves.

Rays: The radial motion of the waves is indicated by the outward-pointing arrows, referred to as rays.

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The Law of Reflection Reflection: Light changes

its direction as being reflected onto a new path from for example a mirror or other shiny objects.

Law of Reflection: r = i and the incident ray, the reflected ray, and the normal lie in the same plane.

i = angle of incidence r = angle of reflection

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Specular reflection: the reflection of light from a smooth surface, in which all the reflected light propagates in a single direction.

Diffuse reflection: If the surface is rough, the reflected light is sent out in a variety of directions.

Specular and Diffuse Reflection

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The Refraction of Light Refraction: When a wave

propagates from a medium in which its speed is v1 to another in which its speed is v2v1 , it will, in general, change its direction of motion. This phenomena is called refraction.

Refraction is the bending of wave fronts and change in direction of propagation due to a change in speed.

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sin1v1

sin2v2

Index of Refraction and Snell’s Law

The speed of light in a given medium, v, is determined by the medium’s index of refraction, n. v = c/n For example: Diamond, n =

2.42; Flint glass, n=1.66; Water, n = 1.33; Air, n = 1.000293 (see Table 26-2)

Snell’s Law: n1sin1 = n2sin2

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Example 26-4

A beam of light in air enters (a) water (n = 1.33) or (b) glass (n = 1.50) at an angle of 60.0° relative to the normal. Find the angle of refraction for each case.

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Homework #11

Chapter 26, P. 941-942, Problems: #50, 51, 53, 54 (Physics, Walker, 4th edition).