Two kinds of lens
A lens is a piece of clear glass or plastic with curved surfaces that bends (refracts) light. There are two main types. A convex lens is thicker in the middle and converges parallel rays to a single point. A concave lens is thinner in the middle and diverges parallel rays outward.
Key idea
The point where a convex lens brings parallel rays together is the focal point, and the distance from the centre of the lens to this point is the focal length, f.
Images formed by a convex lens
The image from a convex lens depends on how far the object is from the lens:
- Object beyond 2F: image is real, inverted and smaller (as in a camera).
- Object at 2F: image is real, inverted and the same size.
- Object between F and 2F: image is real, inverted and larger (as in a projector).
- Object within F: image is virtual, upright and magnified (as in a magnifying glass).
Example
When you hold a magnifying glass close to small print, the letters look larger and the right way up. The object is inside the focal length, so the convex lens forms a virtual, upright, magnified image.
Images formed by a concave lens
A concave lens always produces the same kind of image, no matter where the object is: the image is virtual, upright and smaller than the object. Because the image is always upright and reduced, concave lenses are used in the spectacles worn to correct short-sightedness, spreading the light before it enters the eye so that it focuses correctly on the retina.
Remember
A ray travelling parallel to the principal axis passes through the focal point after a convex lens; a real image can be caught on a screen, but a virtual image cannot.