As you all know guys and of course pretty gals, there are many properties of light that can only be understood in terms of a wave-like description. In this article we will examine these in some detail.

Christian Huygens, a contemporary of Newton's, thought light was a wave and proposed a theory or principle that is still useful in understanding the propagation of waves.

Huygens' Principle is that every point on a wavefront is the source of new wavelets and the new wavefront is the envelope of these wavelets.

As a wave encounters an obstruction &emdash; like a slit edge &emdash; the new wave bends around this obstruction.

The large central image above was sent to me by Dan Frederiksen and is by chance similar to the diagram I made above .

The big distinction, with a diffraction grating, is how rapidly the intensity falls away from these maxima. Because there are so many slits to act as sources, any angle other than those for maxima will be dark or nearly dark.

We typically use the word "interference" to describe the superposition and interaction of a few waves as in Young's Double Slit Experiment.

We typically use the word "diffraction" to describe the superposition and interaction of many waves as in the Diffraction Grating or the diffraction effects of a single slit or the diffraction effects of a circular opening or the diffraction effects of light as it goes around any object.

We will look at the diffraction of a single slit.

In the straight forward direction, light from all parts of the slit travels the same distance and arrives "in phase" so there is a bright central maximum.

A single slit diffraction pattern has a bright central maximum surrounded by much smaller maxima.

If we look closely enough, every image is surrounded by a diffraction pattern produced by the wave nature of light as it passes through an opening like the edge of a lens.

The diffraction pattern produced by a circular opening is very similar to that produced by a single slit.

For a slit, the angular distance from the center of the central maximum to the first minimum is

For a circular opening, the angular distance from the center of the central maximum to the first minimum is

If we look at two stars that have a large angular separation, their diffraction patterns hardly overlap, and we clearly distinguish that there are two stars.

The same would be true of two items of cell structure when viewed through a microscope.

As the two objects come closer, their diffraction patterns overlap and it may become difficult to distinguish the two images.

"Rayleigh's criterion" for the resolution of two point sources is that the first minimum of one diffraction pattern occurs at the central maximum of the other one.

Untrained eyes will loose distinction before this and well trained eyes may distinguish two sources even closer than Rayleigh's criterion.

Thin films -- due to reflection and interference -- provide the beautiful colors we seen in soap bubbles and the non-reflective coatings for camera lenses and for glass in picture frames.

Light reflected from both surfaces of a thin film interfer to provide constructive or destructive interference.

There may also be an unexpected phase change at one of the surfaces due to the nature of the media on both sides of the surface.

Let us begin by looking at a thin layer of oil floating on top of water.

Light is reflected from the first surface of a thin film (at point A in this diagram). And some of the light is refracted and passes through the film and continues on to the second surface where it is again reflected (at point B in this diagram). This light passes through the film again and some of it is refracted (at point C in this diagram) and continues on with and interferes with the light originally reflected at the first surface.

The angle if incidence does make a difference.

However, we will restrict our present attention to the situation shown here, where the light is nearly perpendicular to the thin film.

With air on both sides of this thin soap film, the characters of the two reflections are different. One is from a material of low index of refraction to a material of high index of refraction (n₁ < n₂) while the other is from a material of high index of refraction to a material of low index of refraction (n₂ > n₃). These are like the reflections of a pulse in a rope with a fixed end or with a free end. This means there is an extra "phase shift" of one-half a wavelength. This changes our conditions for maxima and minima.