Lesson Video: Holography Physics

In this video, we will learn how to describe how the phase differences of coherent light waves can be used to produce three-dimensional images.


Video Transcript

In this video, we’re talking about holography. This is the study and practice of making holograms. Many of us have seen holograms before. And in this lesson, we’re going to learn how they’re created.

The first thing to know about a hologram is that it’s a three-dimensional virtual image. And really, it’s that three-dimensional aspect that makes a hologram so special, because if we think about it, we’re quite used to seeing two-dimensional images of objects. And more than that, we know a bit about just how these images are produced.

We know that, given a real three-dimensional object we want to create an image of, we can capture rays of light reflected from or given off by that object. And then if we record the intensity of that incoming light as it divides up across a spatial grid in our imaging device. Then we can produce an image of our object by indicating the brightness level of each one of these squares in our image grid.

So once we have our grid, and each one of these squares is typically called a pixel, we count how much light, that is, how many photons land on each pixel. The more photons, the brighter that pixel and, therefore, the brighter that part of our image. We can say then that creating a two-dimensional image basically involves counting photons as they land at different points in our imager. And this process works very well for representing how a real three-dimensional object looks from a certain perspective. That is, it captures a two-dimensional view of that three-dimensional object.

But what if we wanted a three-dimensional view of this object? It is possible to create an image like that. And this image is called a hologram. But it takes a different setup than the one we have here for recording a two-dimensional image.

In our current setup, we’re essentially treating light as a particle. Our imaging device doesn’t take into account any of its wavelike properties. To create our hologram though, our 3D image, we’ll need to take advantage of the wave nature of light. In order to do that, here’s the kind of setup we might use.

Over here in the top left, we have a laser light source. We can recall that lasers produce coherent light. That is, the light that comes out of a laser has a constant phase relationship. So from our laser, we have these wavefronts of light moving left to right. This light then runs into this optical component called a beam splitter. And that’s just what this device does to incoming radiation. Half of the light reaching the beam splitter passes on ahead, while the other half gets reflected downwards in this case.

Now, at this point, we have two separate beams of light. But they’re still coherent with one another. That is, the light in this beam headed downward has a constant phase relationship with the light in this beam here. And we see that it’s this beam of light which ends up reaching our object. For this reason, the name typically given to this beam of light is the illumination beam. And this makes sense because it’s the beam that illuminates our object. When this light reaches our object, just like we might expect, it gets scattered in all directions. And some, but certainly not all, of that light is scattered downward.

Now, once the light has reflected off of our object, we call the resulting beam by a different name. It’s now called the object beam. And this beam of light is not coherent with the other light in the setup. That’s because different parts of this beam reflected off of different parts of our object, changing the phase relationships within the beam.

As our object beam travels along, eventually it reaches this plate here, a photographic recording plate. This is a lot like film that we might put in a camera to record an image. Now, if our object beam was the only beam of light that landed on our photographic plate, we would essentially be creating an image just like the one we saw earlier, a two-dimensional image of our object. But our setup allows for something else to take place.

Recall that we have this other beam of light which is traveling towards this component, which is a mirror. The mirror reflects this incoming radiation, and it sends it toward the photographic plate. As the light from these two different beams, this one here that we’ll give a name to in a second and our object beam, reaches the photographic plate. Thanks to its wavelike nature, these rays of light interfere with one another.

Just like we expect waves to do, some of this light combines constructively, some destructively, and much of it in between those extremes. And our photographic plate records this interference pattern. Now, here’s the idea. We started out with a coherent beam of laser light. We then split that single beam into two separate beams, which were still coherent up to this point.

One of those beams, called the illumination beam, landed on our object and then was reflected down towards our photographic plate. The other beam though simply reflected off of this mirror and then on towards the photographic plate. So this beam of light here, the lower one in our diagram, has never had its phase relationship altered. This light is still behaving the same way that it behaved when it was emitted by the laser.

Because this beam is unchanged from what it first was, it’s called a reference beam. And it’s the differences between this reference beam and our object beam that are recorded on our photographic plate. And specifically, these differences show up as phase differences between these wavefronts of light.

Here’s why this is important for creating a three-dimensional image of our object. Imagine that we had two waves of light, this one here and this one here, that have the same wavelength. But that they’re out of phase with one another by a half wave cycle. That means that where one wave has a maximum, the other has a minimum. And then, likewise, where the first wave has a minimum, that second one has a maximum, and so on.

Now, if these two waves interfere with one other, like the waves do when they reach our photographic plate in our setup, then that phase difference of a half wave cycle will be recorded. And if we know the original wavelengths of these two waves, we can say how much of a difference in space this phase difference of a half wave cycle corresponds to.

Now, getting back to our setup over here, any phase difference we would see between the light reaching our photographic plate we know is due to the three-dimensional structure of our object. So if a wave in our object beam arrives on the photographic plate one-half wave cycle out of phase with a wave in our reference beam. Then that corresponds to a difference in distance that our two waves have traveled. And that difference is caused by the three-dimensional shape of our object.

So as our photographic plate records lots and lots of phase differences from these two incoming beams, the reference beam and the object beam. What it’s essentially recording are differences in distance that can be used to map out the three-dimensional surface of our object. Now, it won’t be the whole object because not all of it is exposed to the illumination beam. And not all of the light scattered off the object from the illumination beam reaches our photographic plate. But for the light that does, when we compare that object beam with the reference beam, the phase differences that show up can be used to map the 3D surface of the part of the object that was illuminated. And all this information is stored in our photographic plate.

So that’s how a hologram is recorded, by shining coherent light on the object we want to image and then interfering that light that’s reflected off of our object with a reference beam. So that the phase differences between the object and reference beams, which as we saw correspond to differences in distance traveled by those beams, are recorded on our light-sensitive plate.

Now, once this process is complete, if we were to take a look at this photographic plate, where all this information is encoded, there’s no way that we could see an image of our object, in this case an apple. We say that that information, the three-dimensional image of our object, is encoded in the photographic plate. So then, how do we decode it so that once a hologram is recorded, we can see it?

It turns out that the best way to do this is to use a source of light identical to the one we used to encode our hologram. That is, a coherent light source, like light from a laser with the same wavelength as before. So say that we do that. Say that we take our original light source and we shine it on this photographic plate with our encoded hologram.

This light when it interacts with the interference pattern recorded on this plate will diffract. And this diffraction pattern when viewed from a certain perspective displays a three-dimensional virtual image of our object. This is our hologram.

Now, interestingly, being able to see this three-dimensional image at all depends on having the proper viewing perspective. Standing here and looking at the plate this way, we’re able to see this virtual image. But if we were instead, say, standing over here, if we looked at the plate from this direction, we wouldn’t see the image. All this to say, sometimes it’s hard to see a recreated holographic image. But if we’re able to change our perspective or even change the angle of the photographic plate, say, we can experiment and find the range of perspectives. And it is a range from which the hologram is visible.

Another interesting thing about holographic images is that the information used to create this image is stored at every single point in our photographic plate. This is due to the fact that when we were recording our hologram, encoding it, light from the object that landed on the photographic plate landed on every point on that plate. This means that if our plate were to break somehow. Say that we drop it and it breaks in a few pieces and that we were only able to use one of these pieces. When we go to see our hologram, we could still see the whole object as an image. It would just be at a lower resolution than otherwise.

So knowing all this about recording and then viewing a hologram, let’s get some practice with these ideas through an example.

The diagram shows some apparatus used in holography, including a cylindrical object. Which of the following is the apparatus used for? (a) Viewing a recorded hologram of an object. (b) Recording a hologram of an object. (c) Both recording and viewing a hologram of an object.

Okay, so taking a look at our diagram, we see that it shows us this holographic apparatus. Our problem statement talks about a cylindrical object. We can see that here. And along with this, we can identify some other parts of this setup. For example, this element here looks to be a light source, in particular, a laser. We can assume it’s a laser because we know that this apparatus overall is used in holography and these applications require a coherent light source, like a laser.

So these red-colored beams that we see traveling around our apparatus must be beams of laser light. And we can see that, at this point in the laser beam’s path, it’s split so that part of the beam moves on ahead and then part is reflected downward. And then at these two locations in our apparatus, it seems that these elements are mirrors that reflect the incoming laser light. After it’s been reflected, the light in this upper beam is then spread out and then encounters this cylindrical object.

According to our diagram, this light is then reflected off of the object and ultimately lands here. And at this location, at this plate, we see that this light from our upper light beam meets the light from our lower beam. What we see happening then is that light coming from an object, and we call this light our object beam, is interfering with a second beam of light called the reference. And this interference takes place and is recorded on this plate.

This plate is typically called a photographic plate. And it records the light incident on it. And because in this case we have two beams of light interfering when they reach the plate, that interference pattern is what will be recorded. So then what our apparatus overall is accomplishing is it’s mixing together two beams of light, one reflected from our object and one a reference beam. In such a way that the differences between these two beams are recorded.

Another way to describe this process is to say that we’re encoding an image of our object, in this case our cylindrical object. This information is recorded in our plate, where it can then be retrieved later by decoding it using a similar light source. When that happens, it’s possible to view a three-dimensional image, a hologram, of our cylindrical object. But this process and this apparatus is focused entirely on recording that holographic information. We’re not viewing a hologram. And therefore, considering our three answer choices, we can eliminate option (a) as well as option (c), both of which talk about viewing a hologram. The apparatus we’re seeing is used only to record a hologram. And so we’ll choose answer option (b). This apparatus is used to record a hologram of an object.

Let’s recall now some key points about this topic of holography. We saw in this lesson that a hologram is a three-dimensional virtual image. In order to record a hologram of some object, say this spherical one here, a coherent light source, such as a laser, is used to create two different beams. One, called the illumination beam, lands on the object and then when reflected towards a photographic plate is called the object beam. And the other, which doesn’t interact with the object at all, is called the reference beam. The reference beam and the object beam meet and interfere with one another at a photographic plate where that information is recorded.

The depth dimension in a hologram is indicated by the phase differences between the object beam and the reference beam. Once a hologram is recorded, that is, encoded on the photographic plate, it can be decoded and that image seen by using a similar ideally identical light source which when it illuminates this plate diffracts in the same pattern that was recorded originally. This creates a virtual three-dimensional image of the original object, which is visible to the eye when seen from a certain perspective. This is a summary of holography.

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