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Refraction of Light

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**Review: Reflection Law of Reflection**

The angle of incidence equals the angle of reflection The incident and reflected rays lie in the same plane with the normal.

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**Refraction Sudden change in direction of a wave as it changes speed.**

It must enter obliquely to change direction! Which direction does it bend as it slows?

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Refraction In both cases the speed of the wave has decreased. This is indicated by the decrease in wavelength!

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Refraction In which medium does light travel faster? (glass rod appears bent)

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**Speed of light v is the speed of light in the new medium.**

c= 3.0 x 108 m/s Draw a graph for index vs. speed. What happens to speed of the refracted ray as it hits a medium with n < 1? How about n > 1? n is the absolute index of refraction. This is a measure of optical density. n is defined as the ratio of the speed of light in a vacuum to the speed of light in a new medium.

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**Relative Index of Refraction**

n is the relative index of refraction. If air is not used, then remember nrel = n2/n1 What is the relative index when going from diamond into lucite? If nrel < 1 ; speeds up If nrel > 1 ; slows down

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**Refraction n(water)=1.33; n(glass)=1.50; n(air)=1.00**

Calculate the speed of light in water and glass. Vw = 2.26 x 108m/s Vg = 2.00 x 108m/s

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**Refraction n1- from n2 - into**

When a wave slows down it bends closer to the normal. {less to more – toward} n2>n1 When a wave speed up it bends away from the normal. {BLA – Big ―› Little – Away} n2<n1

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Refraction If light rays bend closer to the normal when slowing down, why does the glass rod seem to bend away form the normal?

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**Apparent Depth Diverging rays enter your eyes.**

You “think” in Straight Lines. A virtual image appears to come from point y R – Real Depth A – Apparent Depth

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Apparent Depth If the chest is 20 m below the surface at what depth will the image appear? Assume nsea water = 1.34

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Snell’s Law n1sinθ1 = n2sin θ2 v1/v2 = λ1 / λ2

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**Example What is material X? Determine the angle of refraction.**

A monochromatic light ray f= 5.09 x 1014 Hz is incident on medium X at 55˚. The absolute index of refraction for material X is 1.66 What is material X? Determine the angle of refraction. Determine the speed of light in medium X.

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**Ex: Solution To find the angle of refraction use Snell’s Law. θ2= 30˚**

The index of 1.66 is Flint Glass To find the angle of refraction use Snell’s Law θ2= 30˚ To find the speed use n=c/v. v = 1.8 x 108 m/s

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Dispersion The breaking up of white light into its component frequencies.

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**Refractive Index & Frequency**

The refractive index depends on the medium & the frequency of light. Each frequency “color” propagates at a different speed and bends a different amount.

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Critical Angles Def: The angle of incidence when the angle of refraction is 90 degrees. When a light ray exceeds the critical angle it undergoes total internal reflection (TIR)

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Snell’s Law Lab What factors effect the index of refraction of a liquid? Write up an experiment to test this question.

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