Light Waves: Interference and Diffraction

Introduction

Now there is no doubt at all that light also comes from the luminous body to our eyes by some movement impressed on the matter which is between the two; since, as we have already seen, it cannot be by the transport of a body which passes from one to the other. If, in addition, light takes time for its passage—which we are now going to examine—it will follow that this movement, impressed on the intervening matter, is successive; and consequently it spreads, as Sound does, by spherical surfaces and waves: for I call them waves from their resemblance to those which are seen to be formed in water when a stone is thrown into it, and which present a successive spreading as circles, though these arise from another cause, and are only in a flat surface.

— Christiaan Huygens, Treatise on Light

Light and Matter

Until now, we have discussed light as though it were made of rays following strict rules of geometry, or of particles which moved and bounced off hard surfaces like perfect billiard-balls.

Early in the seventeenth century, Christian Huygens, a contemporary of Isaac Newton who thought light acted like particles, proposed instead that physicists should be analyzing light as a series of wave fronts striking a surface, and bending toward or away from the normal of the surface depending on whether the new medium was denser or less dense. His suppositions have proven to be a useful way of imagining how light and matter interact in transparent media.

Because light striking a denser surface slows down, the slower-moving wave front "bends" toward the normal line perpendicular to the interface.

During the early 1800s, though, Thomas Young and others performed a series of experiments which demonstrated that, under certain circumstances, light exhibited phenomena we associate with waves. Huygen's theory proved useful in explaining this phenomenon as well.

For further exploration of this material (at least for now), you may like to look at the University of Winnipeg's physics site on the Wave Nature of Light.

Practice with the Concepts

Discussion Points

Does Huygens’ principle apply to sound waves? To water waves where each point vibrates up and down?
Two rays of light from the same source destructively interfere if their path lengths differ by how much?

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