Lenses

Question 1

List the two types of lens.

Answer 1

1. Converging/convex

2. Diverging/concave

Question 2

Appearance of a converging lens

Answer 2

Oval

Question 3

Appearance of a diverging lens

Answer 3

Bends inwards

Question 4

What is the principal focus of a convex lens?

Answer 4

The point where rays of light moving parallel to the principal axis converge towards each other and meet at on the opposite side of the lens.

Question 5

What is the focal length?

Answer 5

The distance between the principal focus and the optical centre of the lens.

Question 6

Describe how to measure the focal length of a convex lens. [5]

Answer 6

1. Tape a ruler to a bench and place a white screen at the zero mark.
2. Place the lens in its holder as close as possible to the screen.
3. Slowly move the lens away from the screen until the image of some distant object is as sharp as possible.
4. Using the metre ruler, measure the distance from the centre of the lens to the screen.
5. This is the focal length of the lens.

Question 7

What does a thicker lens usually mean?

Answer 7

There is a smaller focal length.