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

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.

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

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

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?

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

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

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.

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

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

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<n1 n 1 - from n 2 - into

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

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

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

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

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.

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

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

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.

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)

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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