Christian Huygens was the first person to put the wave theory of light on sound footing. Huygens proposed that spherical wave fronts of light spreading out from a point source can be considered as overlapping crests of small secondary waves. Light wave fronts are made up of tinier, overlapping wave fronts. This is Huygen's principle.

Consider flat water waves moving towards a barrier. If a narrow opening is made in the barrier, it acts as a point source for the water waves. By Huygen's principle, spherical wave fronts are produced by the point source. So new spherical waves fan out on the other side of the barrier. Another way to look at this is as the waves being composed of many wave fronts. When the wave runs against a barrier with a small opening, only one of the wave fronts can get through the opening. Since all wavelets are spherical, spherical waves fan out on the other side of the barrier. This is called diffraction. Diffraction occurs in light also. If a light is shined through a small slit or point, light waves will fan out on the other side of the slit.

Just as sound waves can interfere with each other, so can light waves. When the troughs and crests of two light waves meet, they reinforce each other and constructive interference occurs. When trough meets crest, the light waves cancel each other and destructive interference occurs. Consider a beam of light passing through two small slits. When shining a light through just one slit, a simple diffraction pattern occurs. When light passes through two slits, therefore, it might be assumed that the resulting pattern would be the sum of two diffraction patterns. However, this is not so. The light from one slit periodically interferes with the light from the other slit, so that periodic light and dark areas occur on the screen. This convincing experiment to show the wave nature of light was first preformed by Thomas Young in 1801.

A light and dark interference pattern can also be produced by the reflection of light from the top and bottom of a thin film. Consider two pieces of glass. If one of the pieces is wedged up, maybe by placing a very thin piece of paper under one edge, a thin wedge of air will be between the two pieces of glass. If a light is shined on the glass, some of the light will be reflected to our eyes from both the top and bottom pieces of glass. Depending on the thickness of the air wedge at each point, sometimes the top and bottom reflections will be "in phase" and reinforce each other, and sometimes they will be " out of phase" and cancel each other out. At in phase points a light strip occurs, and at out of phase points a dark strip occurs. An interference pattern is produced.

The spectrum of colors seen in a bubble or on an oil slick are produced by light interference, in a phenomenon called iridescence. The colors are produced in the same way an interference pattern occurs on the two pieces of glass. Consider a thin film of gasoline covering another substance like water. Light that hits the gasoline may reflect from either the water or the gas. Depending on the thickness of the gas at each point, destructive or constructive interference may occur. The colors are produced because at places where destructive interference occurs, not all of the visible wave lengths cancel out. Only some colors are subtracted from white light. If red is subtracted out, we see cyan, red's complimentary color. This is iridescence.

Light waves are transverse waves. We know this because light can be polarized. Normal light sources emit non-polarized light. This light travels out in all directions from its source, and can be represented by crossed vectors. In other words, non-polarized light has both a horizontal and vertical component. Polarized light, on the other hand, has only one component, either vertical or horizontal. Polarizers are filters that block out one of light's components. They are composed of many tiny slits in the horizontal or vertical direction. Vertical slits block out horizontal components, and horizontal slits block out vertical components. If two polarizers with oppositely oriented slits are placed next to each other, all the component's of light passing through them will be blocked out, and no light will get through. The reflected glare from most non-metallic surfaces is polarized horizontally, and so polarized glasses have vertical slits that block out this glare.

A hologram is a two dimensional photographic plate illuminated with laser light that allows you to see a reproduction of an image in three dimensions. In conventional photography, light reflected from each point of the object being photographed is directed to one specific point on the film. In a hologram, light reflected from each point of the object hits every point on the photographic plate. The light used to make a hologram must be coherent: composed of only one frequency and composed of light totally in phase. This type of light can only be produced by a laser. A hologram is a recording of the interference pattern resulting from combining two wave fronts. One set of wave fronts is the light reflected from the object, and the other from a reference beam, which is sent directly to the photographic plate without reflecting off of the object. The developed hologram has no discernable image on it- it is simply a photographic patter of microscopic interference fringes. When a hologram is placed in a beam of coherent light, the light is diffracted by the fringes to produce wave fronts identical to those reflected by the original object. Looking through the hologram, one sees a realistic three dimensional image, like you were viewing the object through a window. You can even look around the corners of the object. A potential application of holography is holographic magnification. If a hologram is made with short wavelength light and viewed with long wave length light, the hologram is magnified in proportion to the wavelengths. This could provide huge powers of magnification, without using image distorting lenses.

Past Mechanics II Section Test