How Holography Works :: Holography Holograms Physics

To develop and understanding of how holograms work, one first needs a basic understanding of light and its wave-like properties. First, we will consider investigate the concepts of diffraction and an interference pattern. Consider, for a moment, a person threw a rock into a pond. At the point where the rock hit the water, waves would form and would move away from the source of the wave (the rock) in a spherical shape. This is what a wave looks like when the wave is being emitted from a single point. Now, consider a water wave that is traveling through a small slit in a wall. The waves would emerge form that slit like it would from a point source (the rock). The waves travel through the slit and emerge in a spherical pattern. Now, lets take the next step. Lets say instead of just one slit, two slits were put in the wall. Waves would emerge from each point in a spherical pattern. But this time, the system is more complicated. Because the waves are emerging in spherical patterns, the waves begin to intersect with each other and form an interference pattern. Every wave has crests and troughs. A crest can be thought of as the high point of a wave and a trough can be thought of as the low point of a wave. Every wave can be expressed a sin function, that is, it is periodic. These crests and troughs occur at regular intervals in the wave. So, when these waves from the two slits in the wall start intersecting, the crests and troughs start intersecting with each other; this is known as an interference pattern. If the periods of the waves are lined up, or rather the waves are in phase with each other, the crests and troughs of the two waves overlap and reinforce each other. That is, it looks like there is a wave that is twice as large as one of the original waves at a particular point of intersection; this is known as constructive interference. Now, lets consider a point of intersection where the crest of one wave is present where the trough of the other wave is present. These two waves at this point are out of phase and are experiencing destructive interference. The water would actually look calm at this point, because the waves, in a sense, cancel each other out.