REFRACTIONPrinciples of Physics

Refraction

Change in direction of light as it moves into a new medium and speed changes Only occurs if light enters medium such that 0˚< θi<

90˚

θi

θr

θr = angle of refraction

Refraction can only occur when:• there is a medium

change• the light enters at an

angle between 0˚ and 90˚

When a toy car goes from tile to carpet such that both front wheels hit the carpet at the same time, the whole car slows down at the same time. A change in direction does not occur.

When a toy car goes from tile to carpet at an angle. Each wheel hits the carpet at a different time. The first wheel to hit the carpet, slows down before the others causing the car to turn toward it.

Measuring Angles

90

˚

1. To measure the angle place the protractor on the normal line facing the ray

2. Measure the angle from the normal line to that ray

θi

Measuring Angles9

0˚

1. To measure the angle place the protractor on the normal line facing the ray

2. Measure the angle from the normal line to that ray

θr

Measuring Angles

Determine the incident and refracted angles

Measuring Angles

Determine the incident and refracted angles

Measuring Angles

Determine the incident and refracted angles

Index of Refraction

Ratio of the speed of light in a vacuum to the speed of light in other materials

n= index of refraction (unitless)

c = speed of light in a vacuum

v = speed of light in material

Greater n →more the light slows down in the material

v

cn

ExampleDetermine the speed of light in water if the index of refraction for water is 1.33

v

cn

v

sm /000,000,30033.1

33.1

/000,000,300 smv

smv /000,000,226

Snell’s Law

Relates the index of refraction to the angle of refraction

Don’t forget ˚ is a unit!!!

2211 sinsin nn

Indices of Refraction

Material Index Vacuum 1Air at STP 1.00029Ice 1.31Water at 20 C 1.33Crown glass 1.52Salt 1.54Flint glasses 1.66Sapphire 1.77Diamond 2.417Corn oil 1.47Lucite 1.5

ExampleDetermine the angle of refraction.

n1 = nair = 1.00

n2 = nglass = 1.52

θi = 30˚

θr

crown glass

2

2211

sin52.130sin00.1

sinsin

nn

2

2

2

2

sin329.0

sin52.1

5.0

sin52.15.0

sin52.1)5.0(00.1

θ2 ≈ 19˚

Critical Angle

θc = critical angle: incident angle that causes refracted angle to be 90˚ in air

Total Internal ReflectionTransparent materials can reflect.

Total Internal Reflection: nhigh to nlow

θi > θc

i i i

θi = θr

Dispersion All wavelengths of light travel at 3x108

m/s in a vacuum In other materials each wavelength

travels at a different speed In most materials red travels fastest and

violet travels slowest Red is refracted least and violet is

refracted most therefore the colors separate