•Refraction of light including identification of the normal, angle of incidence and angle of refraction.

•Description of refraction in terms of change of wave speed.

Light

Refraction:

Waves change speed when they travel from on medium to another.

This means they sometimes change direction.

An analogy to explain refraction.

Soldiers marching on concrete and sand,

concrete

sand

When they reach the sand the soldiers slow up.

The soldiers at the left of each row reach the sand first so slow before the ones at the right.

The column changes direction.

Shine a single ray of light through a rectangular block. Draw where it goes on a diagram in your jotter.

You should draw an arrow each time the line changes direction.

air

water

Normal – at right angles to the boundaryi

i – incident angle

r – refracted angle

r

Now use 3 rays of light to shine on the two lenses. The middle ray should go straight through.

Convex lensConcave lens

Total internal reflection

demo

refraction

Critical angle – when refracted angle is 90o

Smaller than critical angle

Larger than critical angle

When the angle in the air gets to 90o it can no longer be refracted and is then reflected. This is called total internal reflection.

Use of total internal reflection - Optical fibre demo

Endoscopes – to look inside people

Hyperopia (farsightedness)

Normal eye

Myopia (nearsightedness) Corrected with concave lens

Corrected with convex lens

No correction needed

Uses of refraction - Glasses

No one’s perfect

Demo of eyesight correction