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
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
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
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
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