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[JayFritoes]

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[lobaz]

Behaviour of a rainbow hologram can be easily understood. First, make a thin transmission H1 hologram of an object, hologram size say 1 mm high and 100 mm wide. Then make a thin transmission H2 hologram out of it say 100x100 mm, H2 in the position of the original object. Suppose you use green light for making the holograms.
When you illuminate the H2 with green monochromatic reconstruction wave, the object is reconstructed. However, H1 shape is reconstructed as well, i.e., light has to pass through 1 mm high slit of original H1:

rainbow0.png (5.64 KiB) Viewed 1029 times

If you want to see the object, you must place your eye to the H1 position (where rays cross), because it is the only position where you can see light from the whole hologram.

If you illuminate H2 with white light (just R, G, B shown for clarity), green component behaves exactly as in the previous case. As H2 is a thin hologram, it has no Bragg selectivity. This means it will also diffract red and blue light. As red is rotated (diffracted) more than green, and blue less than green, you get several ray crossings, one for each colour component:

rainbow1.png (11.78 KiB) Viewed 1029 times

Thus, if you place your eye to point 1, you will see the object in red; in point 2 in green; in point 3 in blue. If you were somewhere between 1 and 2, you would see it in yellow, etc. When you move your eye up and down, you don't see the object in a different view, but in a different colour.

Now, if you put your eye farther from the H2, here is what happens:

rainbow2.png (18.56 KiB) Viewed 1029 times

Light from the lower part of the hologram (dashed line) reaches your eye if and only if it is red, because it must pass the crossing of solid red lines. Thus, you will see the object in red in the lower part, green in the middle, blue in the top part, or in other words, as a rainbow coloured image. That's why it is called 'a rainbow hologram'.
Note that you cannot use laser white (monochromatic red, green, blue), as you would see just three thin horizintal strips instead of a rainbow gradient.

Try to figure out what happens when H1 and H2 are made so that the reconstruction light goes from the top rather than from the bottom.
Got it?