If you’ve ever bought a brand-new DVD, video game, or computer software package, you know you have to tear the plastic packaging to open it. Often, the one final barrier to enjoying your new purchase is a special sticker sealing the package.
The silvery image on the sticker may look like a regular photograph at first, but when you move the package it appears to be something more. It looks three-dimensional but is smooth when you run your finger over it. Is this some type of magic? Not exactly. It’s a type of science known as holography.
Holography is a highly-specialized and technical photographic process. It allows one to record the light scattered by an object as it is subjected to a laser beam. The recording can then be presented in a form, known as a hologram, which makes it appear three-dimensional.
Credit for inventing holograms is usually given to Hungarian physicist Dennis Gabor. His work on optical physics led to breakthroughs in the field of holography in the 1950s. Gabor received the Nobel Prize in Physics in 1971 “for his invention and development of the holographic method.”
Today, holograms can be found on a wide variety of products, from posters and stickers to credit cards and driver’s licenses. Because they are hard to duplicate, they’ve become a popular security measure that can defeat counterfeiters.
Holograms are not only used in products, but scientists have created lifelike holograms that can interact with people. One example is a recent ad campaign in Russia used to deter people from parking in designated disabled parking spots. When someone who is not disabled attempts to park in a spot for disabled persons, a hologram of a person who is disabled appears in the spot and explains why that parking spot is reserved.
To make a hologram, you need a subject, such as an object or person, a laser beam, a half-mirror, and a recording medium, such as metallic film, glass, or photographic film that has added light-sensitive grains. Laser light is required for holography, because it is coherent, which means that laser light waves have a constant phase difference and the same frequency.
The half-mirror is used to split the laser beam into two halves. One of these beams, known as the object beam, bounces off the mirror and reflects off the subject onto the recording medium. The other beam, known as the reference beam, bounces off the mirror and reflects directly onto the recording medium.
The two beams meet and interfere with each other. This interference pattern is what gets imprinted on the recording medium as a hologram. Put another way, the laser light waves were traveling identically before being split by the mirror. When they recombine on the recording medium, you can see how the object beam was changed by reflecting off of the subject.
When you look at a hologram, your eyes compare the light from the two beams. This allows your eyes to see how the subject changes the light rays that reflect off of it. The hologram effectively allows you to see what the subject looks like from any angle, giving the impression that it’s three-dimensional!