Week 2Lesson 1

Refraction

Objectives:To understand the refraction of light wavesdescribe experiments to investigate the refraction of lightknow and use the relationship: n = sin i / sin r

Choose appropriate words to fill in the gaps below:

The law of reflection states that the angle of __________ is always _______ to the angle of incidence.

Both angles are measured relative to the _________, a line that is at _______ degrees to the reflecting surface at the point of reflection.

A plane mirror forms a _______ image which is unlike a _____ image in that it cannot be cast onto a screen. The image in a plane mirror is also the same ______ and the same way up as the object.

realvirtual reflectionsizenormal equal ninety

WORD SELECTION:

realvirtual

reflection

size

normal

equal

ninety

Light Refraction

Refraction occurs when a wave changes speed as it passes from one medium into another.

The change of speed causes the wave to change direction.

Light slows down as it passes from air into glass or water.

Refraction experimentTypical results:

angle of incidence / °

angle of refraction / °

deviation / °

0 0 0

15 10 5

30 19 11

45 28 17

60 35 25

75 40 35

No deviation occurs when the angle of incidence is zero.

Increasing the angle of incidence increases the deviation.

Refraction of light at a plane surface(a) Less to more optical dense transition (e.g. air to glass)

angle of incidence

normal

AIR GLASS

angle of refraction

Light bends TOWARDS the normal.

The angle of refraction is LESS than the angle of incidence.

(b) More to less optical dense transition (e.g. water to air)

angle of refraction

angle of incidence

normal

WATER AIR

Light bends AWAY FROM the normal.

The angle of refraction is GREATER than the angle of incidence.

object at the bottom of a pool

AIR

WATER

Why a pool appears shallow

image

normals

observer

Complete the paths of the RED light rays:

A

B

C

D

E

F

The refraction equationWhen a light ray passes from one medium to another:

n = sin i sin r

where:i is the angle of incidence in the first mediumr is the angle of refraction in the second mediumn is a constant number called the refractive index.

r

i

An experiment to find the refractive index (n) of glass

1. Set up the equipment as shown in the diagram opposite

2. For an initial angle of incidence, i of 30º trace the path of the light ray.

3. Measure the angle refraction, r.4. Calculate the refractive index using

the formula: n = sin (i) / sin (r).5. Repeat for a range of angles between

10º and 80º.6. Calculate the average value of n.

Question 1Calculate the refractive index when light passes from air to glass if the angle of incidence is 30° and the angle of refraction 19º.Hint: n = sin i / sin r

Question 1Calculate the refractive index when light passes from air to glass if the angle of incidence is 30° and the angle of refraction 19º.n = sin i / sin r= sin (30º) / sin (19º) = 0.500 / 0.326refractive index , n = 1.53

Question 2Calculate the angle of refraction when light passes from air to perspex if the angle of incidence is 50° and the refractive index, n = 1.50.

Question 2Calculate the angle of refraction when light passes from air to perspex if the angle of incidence is 50° and the refractive index, n = 1.50.n = sin i / sin r1.50 = sin (50º) / sin (r ) becomes: sin (r ) = sin (50º) / 1.50= 0.766 / 1.50sin (r ) = 0.511angle of refraction = 30.7º

Question 3Calculate the angle of incidence when light passes from air to water if the angle of refraction is 20° and the refractive index, n = 1.33.

Question 3Calculate the angle of incidence when light passes from air to water if the angle of refraction is 20° and the refractive index, n = 1.33.n = sin i / sin r1.33 = sin (i) / sin 20ºbecomes: sin (i) = 1.33 x sin (20º) = 1.33 x 0.342sin (i) = 0.455angle of incidence = 27.1º

Complete:medium 1 medium 2 n i r

air water 1.33 50o 35.2o

glass air 0.67 30o 48.6o

water glass 1.13 59.8o 50o

air diamond 2.40 50o 18.6o

air unknown 1.53 50o 30o

Answers

The End