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Refraction

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Refraction : Bending light

The speed of light waves depends on the material they are travelling through.

If the light waves enter a different material [e.g. from glass into air] the speed changes.

This causes the light to bend [or refract].

Air = Fastest Diamond = slowestGlass = slower

Glass

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Refraction - at the air-glass boundary

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Refraction :Investigating Refraction

1. Place a rectangular glass block on a sheet of paper and draw around it.

2. Draw a normal line [90º] along the top surface of the block.

3. Shine rays of light with incident [i] angles of 30º, 60º and 0º into the block, making sure they all hit where the normal line crosses the glass surface]. Measure angle “r” each time and record your answers .

Angle i

Angle r

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Refraction : What happened?

Put your answers into the table below:

Angle ofI ncidence [i]

Angle ofRefraction [r]

30º

60º

0º

Write 2 ‘rules’ to describe:

a) what happens to the ray as it enters the glass.

b) what happens to the ray as it re-enters the air.

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Air to Glass:angle of incidence > angle of refraction

I > r

As the light ray moved from air into glass it moved towards the normal.

If light rays move from a less dense medium (air) to a more dense medium (glass) they ‘bend’ towards the normal.

i > r

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Glass to Airangle of incidence < angle of refraction

i < r

As the light ray moved from glass into air it moved away from the normal.

If light rays move from a more dense medium (glass) to a less dense medium (air) they ‘bend’ away from the normal.

i < r

If the surface the light ray is incident upon is parallel to the surface it emerges from than the emergent ray makes the same angle with the normal as the incident ray.

(If the two surfaces of the block are parallel, then the ray at the start is parallel to the ray at the end).

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Angle of incidence = 0°

When the angle of incidence is 0 the light ray is not deviated from its path.

Un-deviated light ray

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Refraction in a rectangular block

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Revision tip

Remember the word:TAGAGA

Towards (normal) Air Glass Away (from normal) Glass Air

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Fast and slow

If you were running along a beach and then ran into the water when would you be moving slower, in the water or on the beach?

In a similar way as light moves from one medium to another of different density the speed of light changes.

Do you think light moves faster or slower in a more dense medium?In the water.

Light moves slower through a more dense medium.

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The speed of light

Light travels at 300 000 km/s in a vacuum.As it enters denser media the speed of light decreases.

0306090

120150180210240270300

Vac

uu

m

Wat

er

Per

spex

Speed oflight(thousandskm/s)

Looking at the chart, which do you think is denser, Perspex or water?

Perspex must be denser because light travels more slowly through Perspex than water.

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Why does light change direction as it enters a material?

Imagine a car driving from the road into a muddy field.

In the muddy field it slows down as there is more friction.

If it enters the field at an angle then the front tyres hit the mud at different times.

Tyre one hits the mud first and will move more slowly than tyre two.This causes the car to turn towards the normal.

When the car leaves the mud for the road, tyre one hits the road before tyre two and this causes the car to turn away from the normal.

Tyre 1 Tyre2

Mud

Road

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No change in direction

If the car approached the muddy field at an angle of incidence of 0° then both front tyres would hit the mud at the same time.

The tyres would have the same speed relative to each other so the direction of the car would not change, it would just slow down.

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Same for light

When light hits a medium at an angle to the n_____ the light ‘bends’ in a similar way to that described for the car in a muddy field. Part of the light ray s____ d___ before the rest and this causes the change of d______.

If the light enters a new medium along the normal (i = 0) then it does not ‘bend’ because all of the light ray slows down at the s___ t___.

ormal

lows own

irection

ame ime

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Refraction : Effects of Refraction

Many visual effects are caused by refraction.

This ruler appears bent because the light from one end of the ruler has been diffracted, but light from the other end has travelled in a straight line.

Would the ruler appear more or less bent if the water was replaced with glass?

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Refraction : Apparent Depth

The rays of light from the stone get bent [refracted] as they leave the water.

Your brain assumes they have travelled in straight lines.

Your brain forms an image at the place where it thinks the rays have come from - the stone appears to be higher than it really is.

Actual location

Image

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The Archer fish

The Archer fish is a predator that shoots jets of water at insects near the surface of the water, say on a leaf.

The Archer fish allows for the refraction of light at the surface of the water when aiming at the prey.

image of prey

prey location

The fish does not aim at the refracted image it sees but at a location where it knows the prey to be.

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Refraction : Magic coins

Place a coin in the bottom of a bowl and clamp an empty cardboard tube so that it points above the coin.

Gradually add water to the bowl and watch the coin through the tube float up - can you explain this?

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Refraction : Summary

When light bends this is called refraction.

Refraction happens because the light changes speed [or velocity].

When light enters a more dense medium [e.g. glass], it bends towards the normal.

When light enters a less dense medium [e.g. air], it bends away from the normal.

If the incident ray hits a surface at 0º, no refraction occurs.

Remember that the angle of reflection [r] and angle of refraction [r] have the same symbol.

In reflection, I = r

In refraction, I r