English subtitles for clip: File:IntroductionToHolography1972.ogv

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This is a hologram.

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Holograms have little in common with traditional photographs, except that both use film.

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Holograms can create completely three-dimensional pictures, and every piece of the film can reconstruct the entire image.

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In order to understand holography, you have to understand something about waves.

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When waves meet, their effects are additive. When two crests meet, we get an increased effect.

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But when crest meets trough, their effects cancel out.

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This interference effect is important in holography.

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Here are two random sources of waves, not of equal amplitude or frequency.

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If we make a time exposure of their shadows, the image is completely washed out because nothing remains in place.

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Here are two pure wave sources of equal amplitude and frequency.

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Notice the areas of motion and calm in the water.

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A time exposure looks like this.

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As long as the frequency and amplitude of the two sources are constant, an interference pattern will be formed wherever they cross.

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These two speakers send out pure sound waves of identical frequency and amplitude.

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The microphone records areas of calm and motion.

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If we could see the three-dimensional interference pattern caused by the two sound sources, it would look like this.

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Light also travels in waves.

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What happens when two light beams cross?

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Interference of light waves should produce bands of light and dark, but we see none.

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In general, when two white light beams cross, no visible interference pattern is produced.

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A red filter will help to make the light monochromatic.

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Even this highly filtered light, however, is not monochromatic enough to produce a noticeable interference pattern.

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Let's examine the light source.

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While our water and sound waves are in step with one another, the light bulb produces light of many wavelengths, which are not in step with one another.

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This is the inside of a laser, a source of intense and spectrally pure light.

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The laser emits light waves more monochromatic than any filtered light source.

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We will use a laser to check for interference by passing its light through this optical device, which splits the beam in two.

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The interference should appear as bands of light and dark.

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Here is another demonstration using laser light.

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A partially silvered mirror allows half of the light to pass through while the rest is reflected.

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Where the two beams of light cross, we will place a film.

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Notice the camera has no lens. It is simply a film holder.

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This will be the path of the light.

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The film looks blank. But watch what happens when we remove the beam splitter, leaving only a single beam of laser light.

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The film reconstructs the original two sources from only one beam.

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How did this happen?

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Within the film emulsion, areas are exposed as light waves move through.

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Where the wave fronts cross each other, the silver emulsion is exposed.

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The developed emulsion is essentially a thick diffraction grating.

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Here is a microscope view of the processed film.

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To understand holography, we can think of the developed layers within the emulsion as a set of microscopic partially reflecting mirrors.

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This film was exposed with two beams interfering with a third.

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Now, by passing one beam through the film, the two other beams are reconstructed.

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If this car were illuminated with a laser, every point on it would reflect some laser light.

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This is how a hologram is made.

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Part of the beam is used to illuminate the car.

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We'll call this the object beam.

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A second beam, called the reference beam, shines directly on the emulsion and interferes with the light reflected from every point of the car.

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The exposure must be made on an absolutely motionless surface.

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Even the slightest vibration will ruin the hologram by smearing out the interference pattern.

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We will expose the film for about ten seconds.

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The film looks blank, but through a microscope we can see interference patterns which bear no resemblance to the real car.

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Now, when the reference beam alone shines on the processed film, an identical image is reconstructed.

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Here is an explanation of what happened inside the emulsion.

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The reference beam from the left interferes with each reflected light beam from the car, creating an interference pattern.

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The emulsion simultaneously records all the patterns caused by all the points.

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Now, when the reference beam alone shines through the film, each pattern reconstructs its own object beam, and thus the whole car appears to be reconstructed.

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What would happen if the emulsion had been here, between the source and the object?

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In this case, the interference produces what is known as a standing wave pattern.

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These are standing waves.

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Therefore, if the light shines through the film and reflects back from the coins, there will be standing waves of light set up in the emulsion.

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Laser light will shine through the film and reflect back from the coins.

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An ordinary white light is used to view this hologram.

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Why does white light reconstruct this kind of hologram?

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A cross section of the emulsion looks like this.

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White light approaches and passes through the film.

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The mirror planes are spaced to reinforce only the wavelength used to make the hologram.

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Because the film shrinks in processing, the green color of shorter wavelength is reinforced rather than the red.

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Here is another white light hologram.

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When a laser shines through a hologram and onto a screen, a two-dimensional image is projected.

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Even a small fragment can project the entire image.

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Holography is an efficient data storage medium, which may soon outdate traditional microfilm.

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Here is a two-channel hologram. Two scenes have been recorded at different angles on the film.

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The channel is selected by tilting the film.

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The lens in the holographic reconstruction actually works.

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This is the principle used in microscope holography.

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A slide is placed in front of the final element of a microscope.

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Later, movable lenses will be able to act with the lens in the hologram,

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allowing us to selectively focus on any point within the slide.

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Through holography, structural flaws can be seen in solid objects.

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The dark lines show a stress pattern.

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By aligning the real object with a hologram image of it,

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a tap of the finger can be shown microscopically bending the steel.

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This circular hologram was made by surrounding the object with a strip of film.

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There are an increasing number of practical laboratory applications of holography.

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Here, the effects of heat are made visible.

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Even very subtle heat effects are clearly visible.

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Holography is in an early stage of development, similar to that of photography a hundred years ago.

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Yet already artists have found in holography a new medium of expression.