Wavefronts and Rays

Imagine you throw a rock into a pond. Seen from the side, at the level of the water, the ripples look like this:

The distance between one ripple and the next is called the wavelength, λ. The high points are called crests, and the low points are the troughs.

If instead you look down on the pond, as if from a hovering helicopter, the ripples are round, and spreading outwards (diverging). The technical term for ripples is wavefronts. The arrows are pointing in the direction the waves are moving, and they are called rays. Notice that the rays are always perpendicular to the wavefronts. In other words, the wavefront always moves in a direction at right angles to itself.

As the waves move farther and farther from the center, where the rock hit the water, the wavefronts are larger and larger circles. But if you look at a small piece of the wavefront, it nearly looks flat.

This is why rays are often (especially in basic textbooks) drawn parallel to each other when entering a lens — if the light source is very far away, the wavefronts hitting the lens from one point of the object are essentially flat (plane waves), and the rays are parallel to each other.

Images Notice that in the discussion above, it was assumed that the wavefronts were spreading outwards, diverging. This jives with our common sense notion of what waves do — ripples naturally leave the point where the rock hit the water, making larger and larger circles. There are some very special devices, however, that can change the direction of those waves and make them come back together again (converge). These devices are called mirrors and lenses. When these are used to bring light waves back together, they can create a replica or real image of the original source of the light. We'll be discussing this process in considerable detail in the coming days, and learning how lenses and mirrors make such useful things as eyes, telescopes and microscopes.