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Waves – Topic 4 Chapters 26 Reflection & Refraction Reflection & Refraction Reflection & Refraction

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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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Image position Image position You think in straight lines! The image is located behind the mirror.

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Diffuse vs Specular Reflection Diffuse Reflection –Light incident upon a rough surface –Law of reflection still holds; Normals are not parallel. Specular Reflection –Mirror like reflection –All Normals are parallel

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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 Speed of light v is the speed of light in the new medium. c= 3.0 x 10 8 m/s As the index increases the speed decrease. Draw a graph for index vs. speed. 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 n rel = n 2 /n 1 What is the relative index when going from diamond into lucite? If n rel < 1 ; speeds up If n rel > 1 ; slows down

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Refraction V w = 2.26 x 10 8 m/s V g = 2.00 x 10 8 m/s Calculate the speed of light in water and glass. n (water) =1.33; n (glass) =1.50; n (air) =1.00

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Refraction 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

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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 Apparent Depth Diverging rays enter your eyes. 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 n sea water = 1.34

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Snell’s Law n 1 sinθ 1 = n 2 sin θ 2 v 1 /v 2 = λ 1 / λ 2

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Example A monochromatic light ray f= 5.09 x 10 14 Hz is incident on medium X at 55˚. The absolute index of refraction for material X is 1.66 1. What is material X? 2. Determine the angle of refraction. 3. Determine the speed of light in medium X.

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Ex: Solution The index of 1.66 is Flint Glass To find the angle of refraction use Snell’s Law. θ 2 = 30˚ To find the speed use n=c/v. v = 1.8 x 10 8 m/s v = 1.8 x 10 8 m/s

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

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Refractive & Frequency Refractive & 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 Critical Angles Def: The angle of incidence when the angle of refraction is 90 degrees. Only exist when you have the BLA’s –Big to Little Away 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 a planning A and B for this question. (Last IB Assessment) Research any procedures you might want to use when investigating. –I may modify your methods based on materials we have available.

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