Holograms are confusing.

They're those weird, 3D ghost images that appear to follow you as you move.

But they're are actually completely flat, 2D films.

And yet they have motion parallax.

That is, as you move your gaze, things in the foreground seem to shift faster than things in the background.

And they give you a stereoscopic view.

That is, you see two different images with each eye, which together create the perception of depth.

3D movies aren't even this 3D.

They don't even have motion parallax.

And that hologram of Tupac at Coachella, it wasn't even three-dimensional in any way.

I digress.

So holograms need some really special properties to appear 3D.

One is that they show different images depending on where you stand in front of the hologram.

This is really unusual.

Let's recall that when you look at a regular photograph, it looks the same no matter which way you look at it.

What you're seeing when you look at the photo is a white light beam.

White light is like a crowd of people.

They're walking with different footstep lengths, like the different wavelengths-- or colors in white light-- in every different direction as the light spreads out.

That beam, reflecting off the photo, is freeze-framing the white light crowd.

Blue light here, red light there.

And it's stagnant.

You can't move the crowd around.

Blue parts will always be here, to the left of those red parts.

With a hologram, you can move.

It provides that parallax effect.

Check it out.

This is the setup to make one type of hologram, although the other types are similar.

It starts with a laser beam, which is split into two.

Then both of the beams go through lenses to make them spread out.

One beam-- the object beam-- bounces off the object you're trying to image and shines onto the holographic film, while the other beam-- the reference beam-- goes straight there.

And then-- well, this is why we used lasers-- laser light looks very different from white light.

It's all going in the same direction.

It all has the same wavelength.

And it's coherent, meaning all the light waves are in phase, or in unison.

This is so key to making holograms.

In cases where two coherent laser beams meet, they can create regular, striped interference patterns, like this.

In our case, the reference beam is a coherent beam, though the object beam is not.

Still, when they meet, they also create an interference pattern, but a special one that encodes the specific 3D information about the object onto the holographic film.

Now, we had a reference beam and an object beam, and together they created an interference pattern.

A plus B equals C. What would happen if we took the interference pattern and sent the reference beam in through the back?

That's kind of like C minus A.

Well, that equals B.

And sure enough, with this type of hologram, sending a laser beam in through the back of the film is how you get out the original object beam.

That's the same beam that was created when the laser first shined off the object.

Since this reproduced beam is a perfect copy of the object beam, you can't tell whether you're looking at the light from the hologram or from the original scene, complete with motion parallax and stereoscopic views.

The other cool thing about holograms is that all parts of the hologram carry information about the object.

So you could cut a little square of the hologram, and you'd still be able to see the entire object in it.

You can't do that with a photograph.

Deep down though, the holographic film is just a bunch of interference patterns.

Up close, the film looks like this, whereas a photographic film just looks like a smaller version of the image.

Much less cool.

Oh, and the Tupac hologram?

That was a stage trick called Pepper's Ghost that dates back as far as the 16th century.

You could even create one at home.

I'll link to some tutorials in the description.

Thanks so much for watching.

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