Chapter 20: Summary

Energy can be transferred from one place to another by two means-by particles or by waves. In stark contrast to the collision between two particles, two waves can readily pass through each other; two waves can occupy the same space at the same time. This is known as superposition. Superposition is a characteristic of all waves-of waves on a rope and sound waves and water waves. It is also a characteristic of light for light is a wave.

The future location of a wave front can be determined by considering each point on the present wave front as a source for new wavelets. The new, future wave front is the envelope of these wavelets. This is Huygens' Principle. If a barrier blocks parts of the wave some of the wavelets will be stopped. This will cause the envelope of the remaining wavelets to bend past the edges of the barrier; the wave bends past the edges of the hole. This behavior is known as diffraction and is characteristic of waves-of all waves. As the hole becomes smaller this bending or diffraction becomes greater.