HOW DO LENS’S HELP US SEE?

Aims

Revisit the idea that light can be bent (refracted).

Know how light bends in a convex (converging lens) and a concave lens.

Explain how a lens forms an image. Understand how Gallileo was able to use

a refracting telescope to observe jupiters moons.

Refraction Practical

Add a coin to a metal beaker.

Fill up the water slowly and watch the coin appear.

Shine a ray of light through a glass block at an angle and draw the results in your exercise book.

Refraction

Air

Water

The Kingfisher• Refraction of light is the bending of light rays at the interface of dissimilar substances

Refraction

What is refraction?

In each of the two diagrams the light was travelling between two different mediums.

This causes the light to partially or wholly slow down.

As it does so the light starts to bend.

How does this tank move?

ConclusionComplete the conclusion by choosing the correct words. When light rays pass through a lens they are ___________.

A __________ lens is fat in the middle. It brings the rays of light closer together. The light rays are _____________. The rays meet at a point called the ____________.

A _________ lens is thin in the middle. It makes the light rays spread out. The rays of light are ______________. diverging converging convex

focus refracted concave

Practical - finding the focal point Cut an arrow out of card approx 10cm long. Stick to the window using blu tac. Get a convex lens (20cm and 10cm) and a

mini whiteboard with graph paper on it. Hold the lens infront of the graph paper

and observe the image on the graph paper. Move the lens back and forth unilt the image is clear.

Measure the distance between the lens and the screen (the focal length).

Practical 2 – investigating converging lenses

Get a ray box, white board and graph paper, lens kit, meter ruler.

Use a 20cm convex lens at 50cm on the ruler.

Move the ray box 20cm from the lens. Now move the screen away from the

lens until you get a clear image. Repeat with a 10cm convex lens.

Exam past paper

Complete the questions

http://www.bbc.co.uk/science/space/universe/exploration/hubble_space_telescope#p00ftsdh

What telescope took the ultra deep field image.

How long was the picture exposure?

How far away is the most distant galaxy?

Gallileos telescope.

Plenary

Lens Converge Diverge Convex Concave Refraction Focus

Splat – place these words on the board and ask pupils to hit the board when you give them a clue.

Glossary - pupils to find the definition of these key words.

Key words Activities