Using Lenses

P3aL8 Using Lenses

Key Words

Concave lens - lens that bends inwards to the middle.

Convex lens - lens that bulges outwards.

Focal length - distance between the lens axis and the focal point.

Inverted - upside down.

Long sighted - cannot see close up objects clearly

Magnification - making something bigger.

Magnifying Glass - a convex lens close up to an object to make it bigger.

Real - can be projected onto a screen

Short sighted - cannot see distant objects clearly.

Upright - right way up

Virtual - cannot be projected onto a screen.

Grade E

Short sight is usually caused by the converging lens in the eye being too strong, which means it brings the rays together in front of the retina, so that the image on the retina is not focused properly, and is blurred.

Diverging lenses are used in glasses that correct short sight.

Converging lenses are used to correct long sight, where people cannot make the lens fat enough to see close-up objects clearly.

The converging lens brings the rays in so that the image is focused correctly on the retina.

The magnifying glass produces a virtual and magnified image:

magnification = height of image

height of object

Grade C

For a convex lens, the nature of the image depends on where the object is:

Object at	Image at	Size	Orientation	Nature	Use
Infinity	F	Diminished	Inverted	Real	Image on a film (at infinity)
>2F	Between F and 2F	Diminished	Inverted	Real	Image on a film (close up)
2F	2F	Same size	Inverted	Real	Photocopier
Between 2F and F	>2F	Magnified	Inverted	Real	Projector
F	Infinity	Magnified	Inverted	Real	Spot light
<F	<F (on same side)	Magnified	Upright	Virtual	Magnifying glass

A real image can be projected onto a screen.

We can work out the magnification of the lens by the simple formula:

magnification = height of image

height of object

The ray diagram shows the object at a distance bigger than 2F (twice the focal length)

This one is at 2F:

This one is between F and 2F:

This one is at F:

Grade A

There are no notes at this level.