H is for Holograms

By Siobhan Fairgreaves

After a little summer break, welcome to the eighth post in this series- and today we’re going to be finding out more about Holograms.

First things first- what even is a hologram, and where would we encounter them?

Holograms are popular optical illusions that seem to make a moving image appear from a still photograph. The moving images appear 3D and can be viewed from different angles. However, holograms aren’t just reserved as fairground gimmicks; we can find them all around us. From our money to our DVDs, holograms are everywhere and they have some very important uses.

Hello dove my old friend, I’ve forgotten the 3-digit security code on the back of my card again| Image: Dominic Alves

Why use a hologram though? Sure, the shiny bits on money and credit cards look nice but what is the point? Holograms on money are used to reduce forgery. When a financial system becomes inundated with forgeries the strength of the currency can be weakened. Incorporating a hologram printed onto a piece of metallic film into the design of bank notes and credit cards means the forgers will find it much harder to copy. Other goods, such as DVDs, games or software programmes may also feature holograms so you can be sure they are genuine products.

Now that holograms are starting to sound important it would be nice to know how they’re made. This is where it gets a bit more complicated, try and bear with me though. The exciting news is that we do get to talk about lasers, everybody loves lasers. We’ll be finding out more about how light works in a few posts time but for now, we need to know that holograms rely on the coherent, synchronised waves of light that make up a laser beam.

Tired of only seeing images of 3D objects from just one boring old angle? Now you don’t need to! | Image: Explain that stuff

The synchronised waves of light in a laser beam can be split using a half mirror which allows half of the laser beam through and reflects the other half. By carefully angling the half mirror and a normal mirror you can ensure that the reflected half of the laser beam hits the object you’re trying to turn into a hologram. The reflected half creates what is known as the reference beam. The other half of the laser beam, having passed straight through the half mirror, creates the object beam which can then also be directed onto the target object.

But how do we know this was the real Dennis Gabor? | Image: Keystone

When these two, usually synchronised, halves of the laser beam recombine on the photographic plate having reflected off the target object they now show the object from different angles. This is because each half of the laser (the object beam and the reference beam) took a different route to get to the same place. From whichever angle you view the hologram you can now see how the light would have hit the object if it had been real. This allows the image to change as you move your head around to capture different angles.

A post about this complex process would not be complete without reference to their creator, Dennis Gabor. Incredibly, Gabor knew how to make a hologram before lasers – which are needed to make holograms work – were even invented. His invention and development of holography (the art of making holograms) in the early 1950s eventually earned him the Nobel Prize in Physics in 1971.

That was definitely one of our tougher posts so far but it just goes to show how often we use complex physics and take it for granted. I know I’m definitely guilty of thinking physics was the least interesting of the sciences because it just didn’t feel as relevant. How wrong was I!

A fun fact to finish off now and something that is sure to inspire a few YouTube searches. If you break a hologram into tiny pieces the whole object can still be seen in each fragment.

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