Lenses

Question 0

Describe the relationship between the focal length of a lens and how fat (curved) the lens is.

Answer 0

The fatter the lens the closer the focal point is.

Question 1

Describe a converging lens.

Answer 1

A converging lens is thinner on the outer edges and thicker in the centre. It causes parallel rays of light to move together to a focal point. It focusses light.

Question 2

Describe how to measure the focal length of a converging lens using a distant object and write a step by step method.

Answer 2

A simple experiment can be used to work out the focal length of a converging lens:
1. Clamp the lens at one end of a track. Then clamp a piece of white card further down the track

2. Set up the equipment near a window directed at a distant object, e.g. a building- you should be able to see an image of the object on the piece of card. To produce a more visible image turn off the lights in the room.
3. Move the card along the track until the image is focussed (this is where the image looks the clearest). When the image is the clearest clamp the image in place securely.
4. Use a ruer to measure the distance between between the centre of the lens and the card- this is the focal length.