How Holograms Work
You can't make a hologram with ordinary light. Holograms are not created from just the light that scatters off the object. It actually requires two beams of laser light (both of exactly the same frequency) to make the hologram. The first laser (called the object beam) goes through the recording medium (essentially the film where you will capture the hologram) hits the object, and is scattered and reflected back towards the recording medium.
The second beam goes straight through the recording medium and doesn't touch the object - that is called the reference beam. When the scattered light reflecting back from the object meets the reference beam, they create what is called an interference pattern. You may be familar with the term from double slit experiments (which if you aren't familiar with, go check that out and come back - it's really cool and weird physics stuff). This is no different. The secret sauce is the recording medium that converts this interference pattern into an optical element which modifies either the amplitude or the phase of light that passes through it (more about that in a little bit).
The thing is, though, that depending on what direction that light is coming from, it's amplitude (how tall the waves are) or phase (phase is basically like shifting a sine function to the right or left on the x axis - it's still the same wave, but if you overlaid it on the original wave, the peaks and valleys wouldn't line up). is modified differently.
The reason? Light doesn't travel in a straight line (I mean, it's not like a bullet that just travels straight the whole way). If it did, we wouldn't have holograms. Nope. None. Light travels in waves (hard to believe, I know, but they aren't like ocean waves - they are really tiny waves that you can't really see - duh, because to see anything you need light). Like when you look at the graph for a sine function. Up down up down.
So when two beams (waves) of light colide (and typical holographic setups require high frequency lasers and an air table that eliminates any vibrations to get really good resolution holograms), they create an interference pattern, which in terms of the waves is to say that you kind of add them together. If you imagine the two waves as sine functions on an x/y graph, in the places where the tops of the curves are above the line for both, they add together, and places where they are both under the line, they add but in the opposite direction (positive plus positive equals bigger positive, negative plus negative equals bigger negative), and where one is above the line and the other is below the line, you get essentially the difference. Bottom line is that you get a new wave.
That new wave enters the recording medium and modifies it slightly. For simplicity sake, let's just say that some atoms get shifted left and others get shifted right, others up and others down (if you really want to know, look up hologram on wikipedia and you'll get a more detailed explanation). It's more complex than that, but you get the idea. Ok, so now you have captured an image from the light that created the interference pattern by essentially creating a mold, the same way that if you wanted to make a bunch of belt buckles you create a mold first.
To see the hologram, you have to fire light back through the medium. But not just any light. It has to be of the same frequency as the laser that you originally created the image from. I mean, you could use other colored light, but then what you will see will not be the original image (I'm curious what that would look like). The light enters the medium, and because of the atoms that have been shifted around, it will change the properties of the wave, like amplitude or phase as mentioned above, and what you ultimately get the original image that you saw.
Now depending on what direction the light enters the medium, it's going to get changed differently. Why? Because that light might hit an atom when it is in the up position or the down position or anywhere in between (again, I'm kind of paraphrasing what happens in a way that is a little easier to understand). Each combination of interaction will result in a different output. So in a sense, it is as if the atom was a function, and the light's properties were inputs to that function, and if you change the inputs, you'll get different outputs. Pretty straightforward.
Thus as you shift your head around, what you are seeing is light that has entered from different directions and thus modified differently. You might have noticed that when you hold a hologram in your hand and move it around but keep your head steady, you still see stuff move around. Why? Now the light entering the medium is changing the angle of entry which ultimately makes it the same as if it were steady and you were moving around. Either way, it works.
That took a little bit to explain, but it's actually really cool. Any physics folks that want to correct me on any of that, feel free. I'm not a physicist and I'll own up to never having actually done the math behind all this. Write me an email and I'll correct it.
To actually capture a hologram, you need a few things. A setup kind of looks like this:
On the bottom left, marked L, is the laser. It fires up to the right towards a beam splitter, marked DL in this diagram. Then the two beams hit a mirror, marked M1, and then another mirror marked M2. Finally they hit the recording medium and then one of the beams hits the target and the other does not. This is done on an air table that reduces all vibrations so that the beams are extremely stable.
The really fun part of this? You have to do the actual capture in complete darkness. Why? Because any other light that came into contact with the lasers would also cause an interference pattern. It wouldn't be a high % of interactions, but it could be enough that your image would have noise, and you'd be pissed. So you'll have to take the recording medium out of a opaque box, install it in front of the object, fire the laser, and then put the medium back in the box. You then develop the medium, just like you would develop film, so that new light can't alter it any further.
That's essentially how holograms work. Hope you found this helpful. Definitely give me some feedback if you enjoyed this, have more questions, or you want to correct any technical points.