 # How Holography Works

explanatory Essay
1688 words
1688 words 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.

#### In this essay, the author

• Explains how holograms work by examining the concepts of diffraction and interference patterns.
• Illustrates how a wave looks when it's being emitted from one point. the waves travel through the slit in the wall and emerge in symmetrical patterns.
• Explains that the two slits in a wall form an interference pattern, which intersects with each other and forms crests and troughs.
• Explains that there is a wave that is twice as large as one of the original waves at an intersection; this is known as constructive interference.
• Explains that this interference pattern occurs with water waves and light waves, and proves it through young's double slit experiment. before looking at holograms, it is important to understand lasers.
• Explains how lasers are created and their importance in regard to holography. a rod is shaped into a flash tube, which emits short, intense bursts of broadband light.
• Explains that niels bohr suggested that the radiation of spectral lines created by atoms could be explained if the electrons moved about the nucleus in a fixed orbit.
• Explains that laser light source material provides a particular form of energy state in which the excited atoms can and do pause before returning to the ground state.
• Explains that holography is the means of storing wave information and reproducing it in such a way that none of that information is lost.
• Explains that the interference pattern is important because all of the three-dimensional information about the waves is contained in the two dimensional sample.
• Explains that if the two-dimensional surface is a finite surface, some of the original wave data, the interference pattern, is not recorded and therefore reconstructed.
• Explains that to record the interference pattern, there must be a point of reference by which all of the waves can be measured. a regular light bulb puts out light at many frequencies and wavelengths.
• Explains that the laser emits a single wavelength, that is, coherent light. coherent light waves travel in phase with each other.
• Explains that if we simply illuminate our object with laser light and take a photograph, we would only be recording the different light intensities of the object; we wouldn't have captured any information about the phase of light waves after bouncing off.
• Explains that to capture vital information, we need a standard or reference source to record the phase difference of the light waves and thus capture the information which supplies the vital dimensions and depth, to the holographic presentation.
• Explains that the reference light is emitted in what we will call a plane wave. the object beam strikes the object, and the light deviates in intensity and phase.
• Explains that the hologram, the medium that contains all the information, is nothing more than a high contrast, very fine grain, black and white photographic film.