How does a hologram work? There are two types of holograms: the version that has a 3D image on a flat sticker, or plate, and the 3D kind that appears in the air out of nowhere. Surface holograms work by splitting light, and 3D ones work through several methods.
A flat hologram is an attempt to bridge the gap between what our eyes can see and what a camera can see. We have two eyes, most of us, and they both receive light that is ever so slightly different. Obviously, it depends on size, gender, and lots of other factors, but the average distance between human eyes is about 6.3 cm. That means our eyes detect light from slightly different angles. Our brain takes in this information and uses that knowledge, plus a few other things, to judge the size of an object and its distance. 3D movies make use of this by filming two versions of the movie that are slightly separated. If we move our heads, the light reflecting off the object changes, and our brains recalculate, showing us a 3D image. This is very difficult to do with a photograph because the camera only has one len,s and it takes in light that comes directly to it. The camera records the intensity of the light and the wavelengths to give colors, and that is it. We can judge distances and sizes in the picture by using surrounding information and our knowledge of the world, but it is a 2D picture, and it won’t move. You cannot look at a photograph of a chair from different angles, as you could in real life. Holograms attempt to fix this. They make an image that changes when you view it from different angles.
A surface hologram is made by splitting the light from a laser into two beams. The light from a laser has a much more orderly waveform than the natural light we use to see, and is easier to control. To make the hologram, you put down the object that you are going to use and your photographic plate. This is often a glass plate coated with a very sensitive layer, such as silver halide. The laser is turned on, and the beam is split by a half-mirror. This is a regular mirror, but with a very thin layer of silver, so that half of the laser is reflected away and half passes straight through. The half of the beam that is split is then bounced off another mirror and hits the photographic plate without interacting with the object. This is called the reference beam and is used to give a base rate for the light. The second half is reflected off another mirror, then hits the object, and carries on to the photographic plate. This is called the object beam. Because the light waves are so orderly, when they recombine, we can see the difference between the two of them, which makes the image. The photo plate captures the interference pattern of the light. The waves cancel each other out where they are opposites and multiply each other when they are in phase. This makes the bright and dark places.
The object beam holds a lot more information than the reference beam. When it hits the object, it is not reflected 100% and a lot of the light is absorbed, depending on the brightness of the object. The object will also not be completely smooth, and the light will be reflected in a whole range of directions. The photographic plate has recorded the interference pattern of all of the points of light, so when you move your head, you can literally see the light and dark parts of the object, and you can see the object from different angles, and it moves. The plate has captured and reconstructed the full wavelength of the light that you would see if you looked at the object directly. Interestingly, a photograph captures only one point of light, and if you cut it up, you get smaller and smaller pieces of the image, but a hologram captures every point of light at every place, so if you cut up a hologram, it is smaller, but you still have the complete image on every piece.
3D hologram projections are a little different from that. There are three ways that they can be created. The first type of 3D hologram is merely projected onto glass. It looks 3D, but it isn’t really. They project slightly different images, like a 3D movie, or they rotate the mirror used to reflect the image so that the image appears to move. These are not really holograms. A second way is to have lasers that are very carefully controlled to intersect at certain points. The laser itself is not strong enough to be visible, but when two lasers intersect, the resultant energy is stronger, and it ionizes the air at that point, creating a plasma and a point of light. If you have enough of these, you can trace out a shape. A 3rd way is to use an object and reflect light off it. There is one type that uses ultrasound to very quickly move a polystyrene ball around and reflect light off it. The ball has to move very fast to trace out the whole image. Or electromagnets could be used. The objects have to move incredibly quickly to make the whole image. This is similar to how drones make images in the sky, but on a smaller scale and much more quickly. The 3D holograms that we see in the movies are probably a while away. And this is what I learned today.
Sources
https://www.explainthatstuff.com/holograms.html
https://en.wikipedia.org/wiki/Holography
https://science.howstuffworks.com/hologram.htm
https://blog.scienceandmediamuseum.org.uk/holograms-how-we-can-see-3d-images-using-sound
Photo by Pixabay: https://www.pexels.com/photo/master-card-debit-card-210742/