Ray and Wavefront Diagrams

When light enters or leaves glass at an angle to the normal it changes direction. This is called refraction.

If we make a piece of glass in the shape shown below we can control what happens to the light that passes through it. We can bring the light together to a focus. The word lens comes from "lenticular" meaning "shaped like a lentil". On this course we only need to know about convex (converging) lenses, not concave (diverging) lenses.

There are two diagrams we can use to show what happens to light from a distant object passing through a lens.

A diagram showing the light as rays. Lines with arrows on showing the direction of travel of the light.

A diagram showing wavefronts. Imagine these to be the crests of the light waves a wavelength apart. (although the actual wavelength would be much smaller)

The easiest way to find the focal length of a lens is to produce an image of a distant object, e.g. some trees in the distance, and measure how far from the lens it is.

On both of these diagrams we know that the light waves are from a distant object, i.e. one that is very far away compared with the focal length of the lens. We know this because both rays and wavefronts are parallel. If they weren't then the object from which they emanate must be nearby.

Remember
Rays or wavefronts from a distant object are parallel and produce an image at the focal length of the lens.