Light Review

• Physics
• Secondary Education

Light Review

what is refraction?

Incident ray

Reflected ray

Angle of incidence

Angle of reflection

Law of Reflection

Angle of incidence = Angle of reflection

i

Angle of incidence

Incident ray

Refracted ray

Normal

Angle of incidence

Angle of refraction

r

Angle of refraction

Snell's Law

n = sin i / sin r

But why does the light change direction when it goes into the glass block?

And then again when it leaves?

If the ray of light hits the block at an angle, the side that hits first is slowed down and so the ray starts to bend towards the normal. It has been refracted.

If the ray of light hits the block at an angle, the side that hits first is slowed down and so the ray starts to bend towards the normal. It has been refracted.

When it leaves the block, it speeds back up and so bends away from the normal.

Refractive Index

• Refractive index measures the effect a medium has on light.
• The greater the refractive index, the greater the change in speed and direction.
• The refractive index is always a value greater than, or equal to, 1.
• The refractive indices of a vacuum and air are 1.

Absolute Refractive Indices

Material	Refractive Index, n
glass	1.5
perspex	1.47
water	1.33
diamond	2.4

Refractive Index

We calculate refractive index using Snell's Law:

where n = refractive index

i = angle of incidence

r = angle of refraction

Calculating n

Use the information in the diagram to calculate the refractive index of paraffin wax.

Solution

n = sin i / sin r = sin(34) / sin(23)

n = 1.43

Calculating an angle

Use the information in the diagram to calculate angle X.

Solution

n = sin i / sin r

We want r, so rearrange:

sin r = sin i / n = sin(20)/1.47

r = sin^-1(0.23) = 13.5 degrees

A ray of red light is incident on one side of a glass prism in air.
The angle of incidence is 53° and the angle of refraction in the glass is 30°.
Calculate the refractive index.
Take a picture of your workings and answer then upload it to this question.

Solution taken from a mark scheme

The diagram shows the wavefronts and the direction of travel of light in air.
The light travels more slowly in the transparent solid.
The refractive index of the transparent solid is 1.3.
The light is incident on the surface of the solid at an angle of incidence of 67°.
Calculate the angle of refraction of the light in the solid.

The background, through the glass of water, appears distorted which means changed to be _______

1. clearer
2. less clear
3. transparent

The background through the glass of water appears distorted because of ____________.

1. reflection
2. refraction

The light ___________when it passes from air into glass or water.

1. slows down
2. speeds up
3. remain constant

The material that light passes through is called a __________.

1. medium
2. opaque
3. ray

Light passing from one medium to another causes it to change ___________.

1. direction
2. force
3. distance
4. energy

Which is NOT an example of a medium?

1. air
2. glass
3. water
4. wood

You will remember that light travels 'very fast'. The speed of light in air is __________ kilometres per second.

1. 30
2. 30 000
3. 300 000
4. 300 000 000

When light passes from air into water or glass, the light travels ________.

1. slower
2. faster
3. at the same speed
4. at 0 m/s

The light passing from air into glass or water is bent _________ the normal.

1. away
2. under
3. across
4. towards

When light passes from air into glass or water, the angle of incidence is ________ than the angle of refraction.

1. lesser
2. the same
3. greater
4. more constant

Both of incident and refracted angles are measured from the __________.

1. normal
2. boundary
3. air
4. water

The light _________ when it passes from glass or water into air.

1. slows down
2. speeds up
3. remains the same
4. disappears

We say that the light passing from glass or water into air is bent _______ from the normal.

1. towards
2. across
3. away
4. the same

When light passes from glass or water into air, the angle of incidence is ________ than the angle of refraction.

1. lesser
2. the same
3. greater
4. more constant

The change of speed can cause the light to change ___________.

1. its speed
2. its direction
3. distance

____________ of light is defined as the change in direction of light on passing from one medium to another because of change in speed.

1. Reflection
2. Diffraction
3. Refraction