Refraction

D. Crowley, 2008

Refraction

To know what refraction is, and why it happens

Light

• Look at the answers drawn by pupils to an exam, which asked the following question: a man looked into a mirror, and saw a lamp behind him

• Analyse the diagrams, and explain say which one you think is correct (and why the others are wrong – look carefully)!

Light

• Nita’s diagram is correct – ruler drawn line from lamp to mirror surface to the eye (at the correct angle)

The rest are incorrect: -

• Brent’s diagram – ray line goes behind mirror

• Sara’s diagram – ray line was drawn freehand

• Sanjay’s diagram – ray line did not reach the mirror

• Niki’s diagram – ray line does not enter the eye

Refraction

• Light travels in straight lines

• Light can bend at the boundary between two materials with different densities - this is called refraction

• The light ray bends towards the normal as it enters

• The light ray bends away from the normal as it leaves

• The ray entering the block is parallel to the ray leaving the block, if the block has parallel faces

• A ray entering the block at 90° is not refracted

Speed

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

• If light waves enter a different material (e.g. travel from glass into air) the speed changes, causing the light to bend or refract

glass

air

air = fastest diamond = slowestglass = slower

Experiment

• Using the ray box and glass blocks test for yourself the properties of refraction.

• As you carry out the experiment, complete the worksheet summary for your findings…

Experiment

Refraction

Refraction

• Why does the light ray bend towards the normal when it enters the glass block, and then bend away from normal as it leaves?

• 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

• First to hit the mud is tyre 1, and will move more slowly than tyre 2. This causes the car to turn towards the normal

• When the car leaves the mud for the road, tyre 1 hits the road before tyre 2 and this causes the car to turn away from the normal

Tyre 1 Tyre 2

Refraction

• 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

Water

• Water is denser than air, so light is refracted when it travels through the surface of the water.

• This is why sticks seem to be bent if they are partly underwater, and why swimming pools look shallower than they really are

• Refraction lets you see objects that are normally hidden…

Archer Fish

• The Archer fish is a predator that shoots jets of water at insects near the surface of the water, e.g. 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