LECTURE 11 REFLECTION & REFRACTION, Exercises of Law

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LECTURE 11

REFLECTION & REFRACTION

Instructor: Kazumi Tolich

Lecture 11 ¨ Reading chapter 31- 3

¤ Reflection

¤ Refraction

¤ Dispersion

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Demo 1 ¨ Angle of Incidence and Reflection

¤ Demonstration of angles of incidence and reflection

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Snell’s law ¨ The angle of transmitted ray, θ 2 , is called angle of refraction. ¨ The angles of incidence and refraction, and indices of refraction of two media are related by Snell’s Law: 𝑛" sin 𝜃" = 𝑛) sin 𝜃) ¤ In a medium in which light slows down, a ray bends closer to the normal. 5

• Quiz:

Quiz: 11- 1 answer ¨ You are trying to catch fish by using a spear. You observe a large fish a couple of meters in front of you and a meter below the water surface. Assuming that the fish is stationary, in order to hit the fish with your spear you must aim slightly below the point where you see the fish. ¨ For you to see the fish, the light must travel from the fish to your eyes. ¨ The ray is refracted at the water-air boundary. ¨ The angle of incidence is smaller than the angle of refraction since light travels faster in air than in water.

Specular reflection

¨ Reflection from a smooth surface is called specular reflection. ¨ After reflection, the rays diverge exactly as if they came from a point P’ called the image point. 10

Demo 3 ¨ Location of Image (candle in glass of water)

¤ Demonstration of image point.

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Demo 4 ¨ Diffuse/Specular Reflection

¤ Comparison of diffuse reflection off a rough surface and specular

reflection.

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Intensities of reflected light ¨ The fraction of light energy reflected at a boundary depends on the angle of incidence, the orientation of the E field (polarization), and the indices of refraction of the two media. ¨ For the special case of normal incidence ( θ 1 = θ’ 1 = 0), the reflected intensity is given by 14 n 1 n 2

n 1 n 2 Total internal reflection

¨ When the incident light is in a medium with a higher index of refraction, the

angle of refraction is greater than the incident angle.

¨ For incident angles greater than the critical angle, and no refracted ray

exists. This phenomenon is called total internal reflection. The critical angle,

θ c, is given by

𝜃* = sin

+"

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Binoculars ¨ If the index of refraction of the glass is typical 1.5, the critical angle for total internal reflection is 41.8º. ¨ If the angle of incidence of the ray on the glass–air interface is 45º, the light will be totally reflected. ¨ In binoculars, two prisms are used on each side. 17 Glass prism

Demo 5 ¨ Critical Angle and Total Internal Reflection ¨ Light Pipes and Fiber Optics 19

FlipItPhysics- 8 : Swimming Pool ¨ Can the person standing on the edge of the pool be prevented from seeing the light by total internal reflection at the water-air surface? ¨ No! The ray that makes smaller incident angle still makes out of water. 20