How do you make a hologram?

You've probably got a hologram in your pocket or purse right now. Just look at a credit card. One of my cards has a dove hologram on it, another has a map of the world. Why do our credit cards have holograms on them?

Mainly it's for the bank's protection, in case someone tries to duplicate the card. Since it's very hard to replicate a hologram, there's less of a chance for forgery. But why is it so hard to duplicate a hologram? How does one make a hologram, anyway?

A hologram is essentially a three-dimesional photograph. However, it takes much more work to make a hologram than it does for an ordinary two-dimensional photograph. To learn how holograms are made, we first need to learn a little about lasers.

It's the interference pattern that the photographic film records, unlike a regular photograph, which records the amount of light from a specific point (see the module on pinhole cameras for more information). Also, unlike the photograph, each point of the hologram contains all of the information from the object. So we could destroy almost all of our hologram and still reconstruct the image from a small piece. Try to do that with your regular photograph!

To see the hologram, we have to shine light that's just like the reference beam onto the photographic film after it's been developed. To see the hologram made from a laser, we need to use a similar laser. Now do you see why it's so hard to duplicate a hologram?!

What about the holograms that I can see everyday on my credit card without a laser?