An Introduction to Light Waves
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.
This interference pattern that occurs with water waves also occurs with light waves. And this was proven through Young's Double Slit Experiment. It was done the same way as described above, except with light waves rather than water waves. This interference pattern is essential in the making of holograms. But, before we start looking at holograms, it is first important that we understand lasers and their significance and contribution to the creation of holograms.
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.
This interference pattern that occurs with water waves also occurs with light waves. And this was proven through Young's Double Slit Experiment. It was done the same way as described above, except with light waves rather than water waves. This interference pattern is essential in the making of holograms. But, before we start looking at holograms, it is first important that we understand lasers and their significance and contribution to the creation of holograms.