In treating light as "rays" or using the principal rays to locate images, we are implying that light is composed of tiny bundles or particles that always move in straight lines. That is a reasonable and excellent first approximation.

But, as we look closer, we find that light is really a wave -- an electromagnetic wave.

Is light a stream of particles--like tiny bee-bees? Sir Isaac Newton thought so. The sharp shadows we see tend to indicate that it is.

Is light a wave--like sound or water waves? Christian Huygens, a contemporary of Newton's, thought so. If we look closely enough, as with Young's double slit, we find wave effects.

Actually, we will need both descriptions. And this dual nature of the Universe will be with us throughout our study of atomic and nuclear and particle Physics.

In contrast to particles (or any material objects), waves readily pass through each other. We call this superposition.


Huygens' Principle

Return to Ch 20, Wave Optics: Interference and Diffraction
(c) Doug Davis, 2002; all rights reserved