Waves – Topic 4

Chapters 26 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

 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

 v is the speed of light in the new medium.

n is the absolute index of refraction. This is a measure of optical density. medium.

n is defined as the ratio of the speed of light in a vacuum to the speed of light in a new   c= 3.0 x 10 8 m/s  As the index increases the speed decrease.

Draw a graph for index vs. speed.

Relative Index of Refraction

 n is the relative index of refraction. If air is not used, then remember  If n rel < 1 ; speeds up  n rel = n 2 /n 1  If n rel > 1 ; slows down  What is the relative index when going from diamond into lucite?

Refraction

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

Calculate the speed of light in water and glass.

V w = 2.26 x 10 8 m/s V g = 2.00 x 10 8 m/s

Refraction

n 1 - from n 2 - 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

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

 Diverging rays enter your eyes.

R – Real Depth A – Apparent Depth

 You “think” in Straight Lines.

 A virtual image appears to come from point y

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.

2.

3.

What is material X?

Determine the angle of refraction.

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.

n=c/v.

θ 2 = 30˚ To find the speed use v = 1.8 x 10 8 m/s

Dispersion

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

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

 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.