Refraction of Light

Review: ReflectionLaw of Reflection

The angle of incidence equals the angle of reflection

The incident and reflected rays lie in the same plane with the normal.

RefractionSudden 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?Which direction does it bend as it slows?

RefractionIn 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.

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.

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 nIf nrelrel < 1 ; < 1 ; speeds up speeds up

If nIf nrelrel > 1 ; > 1 ; slows down slows down

Refraction

Vw = 2.26 x 108m/s

Vg = 2.00 x 108m/s

Calculate the Calculate the speed of light in speed of light in water and glass.water and glass.

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

nn11- from- from

nn2 2 - into- 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

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 nsea water = 1.34

Snell’s Law

n1sinθ1 = n2sin θ2

v1/v2 = λ1 / λ2

Example

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

1.1. What is material X?What is material X?2.2. Determine the angle Determine the angle

of refraction.of refraction.3.3. Determine the Determine the

speed of light in speed of light in medium X.medium X.

Ex: Solution

The index of 1.66 is Flint Glass

To find the angle of To find the angle of refraction use refraction use Snell’s Law.Snell’s Law. θθ22= 30˚= 30˚

To find the speed use To find the speed use n=c/v.n=c/v.

v = 1.8 x 10v = 1.8 x 108 8 m/sm/s

Dispersion

The breaking up of white light into its component frequencies.

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.

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)

Snell’s Law Lab

What factors effect the index of refraction of a liquid?

Write up an experiment to test this question.