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Holograms
How
Holograms Work
BY TRACY V. WILSON
If
you want to see a hologram, you don't have to look much farther than your
wallet. There are holograms on most driver's licenses, ID cards and credit
cards.
If
you're not old enough to drive or use credit, you can still find holograms
around your home. They're part of CD, DVD and software packaging, as well
as just about everything sold as "official merchandise."
Unfortunately,
these holograms -- which exist to make forgery more difficult -- aren't very
impressive. You can see changes in colors and shapes when you move them back
and forth, but they usually just look like sparkly pictures or smears of color.
Even
the mass-produced holograms that feature movie and comic book heroes can look
more like green photographs than amazing 3-D images.
On
the other hand, large-scale holograms, illuminated with lasers or
displayed in a darkened room with carefully directed lighting, are incredible.
They're
two-dimensional surfaces that show absolutely precise, three-dimensional images
of real objects. You don't even have to wear special glasses or look through a
View-Master to see the images in 3-D.
If
you look at these holograms from different angles, you see objects from
different perspectives, just like you would if you were looking at a real
object.
Some
holograms even appear to move as you walk past them and look at them from
different angles. Others change colors or include views of completely different
objects, depending on how you look at them.
Holograms
have other surprising traits as well. If you cut one in half, each half
contains whole views of the entire holographic image. The same is true if you
cut out a small piece -- even a tiny fragment will still contain the whole
picture.
On
top of that, if you make a hologram of a magnifying glass, the holographic
version will magnify the other objects in the hologram, just like a real one.
Once
you know the principles behind holograms, understanding how they can do all
this is easy. This article will explain how a hologram, light and your brain
work together make clear, 3-D images.
All
of a hologram's properties come directly from the process used to create it, so
we'll start with an overview of what it takes to make one.
Making a Hologram
It doesn't take
very many tools to make a hologram. You can make one with:
·
A laser: Red lasers, usually helium-neon
(HeNe) lasers, are common in holography. Some home holography
experiments rely on the diodes from red laser pointers, but the light from a
laser pointer tends to be less coherent and less stable, which can make it hard
to get a good image. Some types of holograms use lasers that produce different
colors of light as well. Depending on the type of laser you're using, you may
also need a shutter to control the exposure.
·
Lenses: Holography is often
referred to as "lensless photography," but holography does require
lenses. However, a camera's lens focuses light, while the lenses used in
holography cause the beam to spread out.
·
A beam splitter: This is a device that uses mirrors
and prisms to split one beam of light into two beams.
·
Mirrors: These direct the beams of light
to the correct locations. Along with the lenses and beam splitter, the mirrors
have to be absolutely clean. Dirt and smudges can degrade the final image.
·
Holographic film: Holographic film can record
light at a very high resolution, which is necessary for creating a hologram.
It's a layer of light-sensitive compounds on a transparent surface, like
photographic film. The difference between holographic and photographic film is
that holographic film has to be able to record very small changes in light that
take place over microscopic distances. In other words, it needs to have a very
fine grain. In some cases, holograms that use a red laser rely on
emulsions that respond most strongly to red light.
There are lots
of different ways to arrange these tools -- we'll stick to a basic transmission
hologram setup for now.
1. The laser
points at the beam splitter, which divides the beam of light into two parts.
2. Mirrors direct
the paths of these two beams so that they hit their intended targets.
3. Each of the two
beams passes through a diverging lens and becomes a wide swath of light rather
than a narrow beam.
4. One beam,
the object beam, reflects off of the object and onto the
photographic emulsion.
5. The other beam,
the reference beam, hits the emulsion without reflecting off
of anything other than a mirror.
6. TRANSMISSION AND REFLECTION
7. There are two
basic categories of holograms -- transmission and reflection. Transmission
holograms create a 3-D image when monochromatic light, or light that is all one
wavelength, travels through them. Reflection holograms create a 3-D image when
laser light or white light reflects off of their surface. For the sake of
simplicity, this article discusses transmission holograms viewed with the help
of a laser except where noted.
Workspace Requirements
Getting
a good image requires a suitable work space. In some ways, the requirements for
this space are more stringent than the requirements for your equipment. The
darker the room is, the better.
A
good option for adding a little light to the room without affecting the
finished hologram is a safelight, like the ones used in darkrooms.
Since
darkroom safelights are often red and holography often uses red light, there
are green and blue-green safelights made specifically for holography.
Holography
also requires a working surface that can keep the equipment absolutely still --
it can't vibrate when you walk across the room or when cars drive by outside.
Holography
labs and professional studios often use specially designed tables that have
honeycomb-shaped support layers resting on pneumatic legs.
These
are under the table's top surface, and they dampen vibration. You can make your
own holography table by placing inflated inner tubes on a low table, then
placing a box full of a thick layer of sand on top of it.
The
sand and the inner tubes will play the role of the professional table's
honeycombs and pneumatic supports.
If
you don't have enough space for such a large table, you can improvise using
cups of sand or sugar to hold each piece of equipment, but these won't be as
steady as a larger setup.
To
make clear holograms, you need to reduce vibration in the air as well. Heating
and air conditioning systems can blow the air around, and so can the movement
of your body, your breath and even the dissipation of your body heat.
For
these reasons, you'll need to turn the heating and cooling system off and wait
for a few minutes after setting up your equipment to make the hologram.
These
precautions sound a little like photography advice taken to the extreme -- when
you take pictures with a camera, you have to keep your lens clean, control
light levels and hold the camera absolutely still.
This
is because making a hologram is a lot like taking a picture with a microscopic
level of detail. We'll look at how holograms are like photographs in the next
section.
Holograms and Photographs
When
you take a picture with a film camera, four basic steps happen in an instant:
1. A shutter opens.
2. Light passes
through a lens and hits the photographic emulsion on a piece of film.
3. A light-sensitive
compound called silver halide reacts
with the light, recording its amplitude, or
intensity, as it reflects off of the scene in front of you.
4. The shutter
closes.
You
can make lots of changes to this process, like how far the shutter opens, how
much the lens magnifies the scene and how much extra light you add to the mix.
But
no matter what changes you make, the four basic steps are still the same.
In
addition, regardless of changes to the setup, the resulting picture is still
simply a recording of the intensity of reflected light.
When
you develop the film and make a print of the picture, your eyes and brain
interpret the light that reflects from the picture as a representation of the
original image.
Like
photographs, holograms are recordings of reflected light. Making them requires
steps that are similar to what it takes to make a photograph:
1. A shutter opens
or moves out of the path of a laser. (In some setups, a pulsed laser
fires a single pulse of light, eliminating the need for a shutter.)
2. The light from
the object beam reflects off of an object. The light from the reference beam
bypasses the object entirely.
3. The light from
both beams comes into contact with the photographic emulsion, where
light-sensitive compounds react to it.
4. The shutter
closes, blocking the light.
5.
Just like with a photograph, the
result of this process is a piece of film that has recorded the incoming light.
However, when you develop the holographic plate and look at it, what you see is
a little unusual. Developed film from a camera shows you a negative view of the original scene --
areas that were light are dark, and vice versa. When you look at the negative,
you can still get a sense of what the original scene looked like.
6. But when you look at a developed piece of film used to
make a hologram, you don't see anything that looks like the original scene.
Instead, you might see a dark frame of film or a random pattern of lines and
swirls. Turning this frame of film into an image requires the right illumination. In a transmissionhologram,
monochromatic light shines through the hologram to make an image. In a reflection hologram, monochromatic or white
light reflects off of the surface of the hologram to make an image. Your eyes
and brain interpret the light shining through or reflecting off of the hologram
as a representation of a three-dimensional object. The holograms you see on
credit cards and stickers are reflection holograms.
7. You need the right light source to see a hologram
because it records the light's phase and amplitude like a code. Rather than
recording a simple pattern of reflected light from a scene, it records the interference between the reference beam
and the object beam. It does this as a pattern of tiny interference fringes. Each fringe can be
smaller than one wavelength of the light used to create them. Decoding these
interference fringes requires a key -- that key is the right kind of light.
8. Next, we'll explore exactly how light makes
interference fringes.
Holograms and Light
To
understand how interference fringes form on film, you need to know a little bit
about light. Light is part of the electromagnetic
spectrum -- it's made of high-frequency electrical and magnetic waves.
These
waves are fairly complex, but you can imagine them as similar to waves on
water. They have peaks and troughs, and they travel in a straight line until
they encounter an obstacle.
Obstacles
can absorb or reflect light, and most objects do some
of both. Reflections from completely smooth surfaces are specular, or mirror-like, while reflections
from rough surfaces are diffuse, or
scattered.
The wavelength of light is the distance from one peak
of the wave to the next. This relates to the wave's frequency, or the number of
waves that pass a point in a given period of time.
The
frequency of light determines its color and is measured in cycles per second,
or Hertz (Hz). Colors at the red end of the spectrum have lower frequencies,
while colors at the violet end of the spectrum have higher frequencies. Light's
amplitude, or the height of the waves, corresponds to its intensity.
White light, like sunlight, contains all of the
different frequencies of light traveling in all directions, including ones that
are beyond the visible spectrum.
Although
this light allows you to see everything around you, it's relatively chaotic. It
contains lots of different wavelengths traveling in lots of different
directions.
Even
waves of the same wavelength can be in a different phase, or
alignment between the peaks and troughs.
Laser light,
on the other hand, is orderly. Lasers produce monochromatic light
-- it has one wavelength and one color.
The
light that emerges from a laser is also coherent. All
of the peaks and troughs of the waves are lined up, or in phase.
The
waves line up spatially, or
across the wave of the beam, as well as temporally, or
along the length of the beam. Light Reflection
You
can make and view a photograph using unorganized white light, but to make a
hologram, you need the organized light of a laser.
This
is because photographs record only the amplitude of the light that hits the
film, while holograms record differences in both amplitude and the phase.
In
order for the film to record these differences, the light has to start out with
one wavelength and one phase across the entire beam. All the waves have to be
identical when they leave the laser.
Here's
what happens when you turn on a laser to expose a holographic plate:
1.A column of
light leaves the laser and passes through the beam splitter.
2.The two columns
reflect off of their respective mirrors and pass through their respective
diverging lenses.
3.The object
reflects off of the object and combines with the reference beam at the
holographic film.
There are a
couple of things to keep in mind about the object beam. One is that the object
is not 100 percent reflective -- it absorbs some of the laser light that
reaches it, changing the intensity of the object wave.
The darker
portions of the object absorb more light, and the lighter portions absorb less
light.
On top of that,
the surface of the object is rough on a microscopic level, even if it looks
smooth to the human eye, so it causes a diffuse reflection.
It scatters
light in every direction following the law of reflection. In other
words, the angle of incidence, or the angle at which the light
hits the surface, is the same as its angle of reflection, or the
light at which it leaves the surface.
This diffuse
reflection causes light reflected from every part of the object to reach every
part of the holographic plate. This is why a hologram is redundant -- each
portion of the plate holds information about each portion of the object.
The holographic
plate captures the interaction between the object and reference beams. We'll
look at how this happens next.
REDUNDANCY
If you tore a
hologram of a mask in half, you could still see the whole mask in each half.
But by removing half of the hologram, you also remove half of the information
required to recreate the scene.
For this
reason, the resolution of the image you see in half a hologram isn’t as good.
In addition, the holographic plate doesn’t get information about areas that are
out of its line of sight, or physically blocked by the surface of
the object.
Capturing the Fringes
The
light-sensitive emulsion used to create holograms makes a record of the
interference between the light waves in the reference and object beams.
When
two wave peaks meet, they amplify each
other. This is constructive
interference.When a peak meets a trough, they cancel one another out.
This
is destructive interference. You
can think of the peak of a wave as a positive number and the trough as a
negative number.
At
every point at which the two beams intersect, these two numbers add up, either
flattening or amplifying that portion of the wave.
This
a lot like what happens when you transmit information using radio waves.
In
amplitude modulation (AM) radio transmissions, you combine a sine wave with a
wave of varying amplitudes.
In
frequency modulation (FM) radio transmissions, you combine a sine wave with a
wave of varying frequencies. Either way, the sine wave is the carrier wave that is overlaid with a
second wave that carries the information.
In
a hologram, the two intersecting light wave fronts form a pattern of hyperboloids -- three-dimensional shapes
that look like hyperbolasrotated
around one or more focal points.
The
holographic plate, resting where the two wave fronts collide, captures a cross-section, or a thin slice, of
these three-dimensional shapes.
If
this sounds confusing, just imagine looking through the side of a clear
aquarium full of water. If you drop two stones into the water at opposite ends
of the aquarium, waves will spread toward the center in concentric rings.
When
the waves collide, they will constructively and destructively interfere with
each other.
If
you took a picture of this aquarium and covered up all but a thin slice in the
middle, what you'd see is a cross-section of the interference between two sets
of waves in one specific location.
The
light that reaches the holographic emulsion is just like the waves in the
aquarium. It has peaks and troughs, and some of the waves are taller while
others are shorter.
The
silver halide in the emulsion responds to these light waves just like it
responds to light waves in an ordinary photograph.
When
you develop the emulsion, parts of the emulsion that receive more intense light
get darker, while those that receive less intense light stay a little lighter.
These darker and lighter areas become the interference fringes.
Bleaching the Emulsion
The
amplitude of the waves corresponds to the contrast between
the fringes. The wavelength of the waves translates to the shape of each fringe.
Both
the spatial coherence and the contrast are a direct result of the laser beam's
reflection off of the object.
Turning
these fringes back into images requires light. The trouble is that all the
tiny, overlapping interference fringes can make the hologram so dark that it
absorbs most of the light, letting very little pass through for image
reconstruction.
For
this reason, processing holographic emulsion often requires bleaching using a bleach bath. Another
alternative is to use a light-sensitive substance other than silver halide,
such as dichromated gelatin, to
record the interference fringes.
Once a hologram
is bleached, it is clear instead of dark. Its interference fringes still exist,
but they have a different index of refraction rather than a
darker color.
The index of
refraction is the difference between how fast light travels through a medium
and how fast it travels through a vacuum.
For example,
the speed of a wave of light can change as it travels through air, water,
glass, different gasses and different types of film. Sometimes, this produces
visible distortions, like the apparent bending of a spoon placed in a half-full
glass of water.
Differences in
the index of refraction also cause rainbows on soap bubbles and on oil stains
in parking lots.
In a bleached
hologram, variations in the index of refraction change how the light waves
travel through and reflect off of the interference fringes.
These fringes
are like a code. It takes your eyes, your brain and the right kind of light to
decode them into an image. We'll look at how this happens in the next section.
HOLOGRAPHIC MAGNIFYING GLASS
If you make a
hologram of a scene that includes a magnifying glass, the light from the object
beam passes through the glass on its way to the emulsion.
The magnifying
glass spreads out the laser light, just like it would with ordinary light. This
spread-out light is what forms part of the interference pattern on the
emulsion.
You can also
use the holographic process to magnify images by positioning the object farther
from the holographic plate.
The light waves
reflected off of the object can spread out farther before they reach the plate.
You can magnify a displayed hologram by using a laser with a longer wavelength
to illuminate it.
Decoding the Fringes
The
microscopic interference fringes on a hologram don't mean much to the human
eye.
In
fact, since the overlapping fringes are both dark and microscopic, all you're
likely to see if you look at the developed film of a transmission hologram is a
dark square.
But
that changes when monochrome light passes through it. Suddenly, you see a 3-D
image in the same spot where the object was when the hologram was made.
A
lot of events take place at the same time to allow this to happen. First, the
light passes through a diverging lens, which causes monochromatic light -- or
light that consists of one wavelength color -- to hit every part of the
hologram simultaneously.
Since
the hologram is transparent, it transmits a
lot of this light, which passes through unchanged.
Regardless of whether they are dark or clear, the
interference fringes reflect some of the light. This is where things get
interesting. Each interference fringe is like a curved, microscopic mirror.
Light that hits it follows the law of reflection, just like
it did when it bounced off the object to create the hologram in the first
place. Its angle of incidence equals its angle of reflection, and the light
begins to travel in lots of different directions.
But that's only part
of the process. When light passes around an obstacle or through a slit, it
undergoes diffraction, or spreads out.
The more a beam of
light spreads out from its original path, the dimmer it becomes along the
edges. You can see what this looks like using an aquarium with a slotted panel
placed across its width.
If you drop a pebble
into one end of the aquarium, waves will spread toward the panel in concentric
rings. Only a little piece of each ring will make it through each gap in the
panel. Each of those little pieces will go on spreading on the other side.
This process is a
direct result of the light traveling as a wave -- when a wave moves past an
obstacle or through a slit, its wave front expands on the
other side.
There are so many
slits among the interference fringes of a hologram that it acts like a diffraction
grating, causing lots of intersecting wave fronts to appear in a very small
space.
Recreating the Object Beam
The
diffraction grating and reflective surfaces inside the hologram recreate the original object beam. This
beam is absolutely identical to the original object beam before it was combined
with the reference wave. This is what happens when you listen to the radio.
Your
radio receiver removes the sine wave that carried the amplitude- or
frequency-modulated information. The wave of information returns to its
original state, before it was combined with the sine wave for transmission.
The
beam also travels in the same direction as the original object beam, spreading
out as it goes. Since the object was on the other side of the holographic
plate, the beam travels toward you.
Your
eyes focus this light, and your brain interprets it as a three-dimensional
image located behind the transparent hologram. This may sound far-fetched, but
you encounter this phenomenon every day.
Every
time you look in a mirror, you see yourself and the surroundings behind you as
though they were on the other side of the mirror's surface.
But
the light rays that make this image aren't on the other side of the mirror --
they're the ones that bounce off of the mirror's surface and reach your eyes.
Most
holograms also act like color filters,
so you see the object as the same color as the laser used in its creation
rather than its natural color.
This virtual
image comes from the light that hits the interference fringes and spreads out
on the way to your eyes.
However, light
that hits the reverse side of each fringe does the opposite.
Instead of moving upward and diverging, it moves downward and converges.
It turns into a
focused reproduction of the object -- a real image that you
can see if you put a screen in its path.
The real image
is pseudoscopic, or flipped back to front -- it's the opposite of
the virtual image that you can see without the aid of a screen. With the right
illumination, holograms can display both images at the same time.
However, in
some cases, whether you see the real or the virtual image depends on what side
of the hologram is facing you.
Your brain
plays a big role in your perception of both of these images. When your eyes
detect the light from the virtual image, your brain interprets it as a beam of
light reflected from a real object.
Your brain uses
multiple cues, including, shadows, the relative positions of
different objects, distances and parallax, or differences in
angles, to interpret this scene correctly. It uses these same cues to interpret
the pseudoscopic real image.
This
description applies to transmission holograms made with silver halide emulsion.
Next, we'll look at some other types of holograms.
HOLOGRAPHY AND MATHEMATICS
You can
describe all of the interactions between the object and reference beams, as
well as the shapes of the interference fringes, using mathematical equations.
This makes it
possible to program a computer to print a pattern onto a holographic plate,
creating a hologram of an object that doesn’t actually exist.
Other Hologram Types
The
holograms you can buy as novelties or see on your driver's license are reflectionholograms. These are usually
mass-produced using a stamping method.
When
you develop a holographic emulsion, the surface of the emulsion collapses as
the silver halide grains are reduced to
pure silver. This changes the texture of the emulsion's surface.
One
method of mass-producing holograms is coating this surface in metal to
strengthen it, then using it to stamp the interference pattern into metallic
foil. A lot of the time, you can view these holograms in normal white light.
You
can also mass-produce holograms by printing them from a master hologram,
similar to the way you can create lots of photographic prints from the same
negative.
But
reflection holograms can also be as elaborate as the transmission holograms we
already discussed. There are lots of object and laser setups that can produce
these types of holograms.
A
common one is an inline setup,
with the laser, the emulsion and the object all in one line. The beam from the
laser starts out as the reference beam.
It
passes through the emulsion, bounces off the object on the other side, and
returns to the emulsion as the object beam, creating an interference pattern.
You view this hologram when white or monochrome light reflects off of its
surface.
You're
still seeing a virtual image -- your brain's interpretation of light waves that
seem to be coming from a real object on the other side of the hologram.
Reflection holograms are often thicker than transmission holograms. There is
more physical space for recording interference fringes.
This
also means that there are more layers of reflective surfaces for the light to
hit. You can think of holograms that are made this way as having multiple layers that are only about half a
wavelength deep.
When
light enters the first layer, some of it reflects back toward the light source,
and some continues to the next layer, where the process repeats. The light from
each layer interferes with the light in the layers above it.
This
is known as the Bragg effect, and
it's a necessary part of the reconstruction of the object beam in reflection
holograms. In addition, holograms with a strong Bragg effect are known as thick holograms, while those with
little Bragg effect are thin.
The
Bragg effect can also change the way the hologram reflects light, especially in
holograms that you can view in white light. At different viewing angles, the
Bragg effect can be different for different wavelengths of light.
This
means that you might see the hologram as one color from one angle and another
color from another angle. The Bragg effect is also one of the reasons why most
novelty holograms appear green even though they were created with a red laser.
Multiple Images
In
movies, holograms can appear to move and recreate entire animated scenes in
midair, but today's holograms can only mimic movement.
You
can get the illusion of movement by exposing one holographic emulsion multiple
times at different angles using objects in different positions. The hologram
only creates each image when light strikes it from the right angle.
When
you view this hologram from different angles, your brain interprets the
differences in the images as movement. It's like you're viewing a holographic
flip book. You can also use a pulsed laser
that fires for a minute fraction of a second to make still holograms of objects
in motion.
Multiple
exposures of the same plate can lead to other effects as well. You can expose
the plate from two angles using two completely different images, creating one
hologram that displays different images depending on viewing angle.
Exposing
the same plate using the exact same scene and red, green and blue lasers can
create a full-color hologram. This process is tricky, though, and it's not
usually used for mass-produced holograms.
You
can also expose the same scene before and after the subject has experienced
some kind of stimulus, like a gust of wind or a vibration. This lets
researchers see exactly how the stimulus changed the object.
Using lasers to make three-dimensional images of objects may
sound like a novelty or a form of art. But holograms have an increasing number
of practical uses.
Scientists can use holograms to study objects in three
dimensions, and they can use acoustical holography to
create three-dimensional reconstructions of sound waves.
Holographic memory has also become an increasingly common
method of storing large amounts of data in a very small space. Some researchers
even believe that the human brain stores information in a manner that is much
like a hologram.
Although holograms don't currently move like they do in the
movies, researchers are studying ways to project fully 3-D holograms into
visible air. In the future, you may be able to use holograms to do everything
from watching TV to deciding which hair style will look best on you.
THE FIRST HOLOGRAM
Dennis Gabor
invented holograms in 1947. He was attempting to find a method for improving
the resolution of electron microscopes.
However,
lasers, which are necessary for creating and displaying good holograms, were
not invented until 1960.
Gabor used a
mercury vapor lamp, which produced monochrome blue light, and filters make his
light more coherent.
Gabor won the
Nobel Prize in Physics for his invention in 1971.
SPECIAL THANKS
Special thanks to Dr. Chuck Bennett, Professor of
Physics at the University of North Carolina at Asheville, for his assistance
with this article.
About Tracy V. Wilson
Tracy V. Wilson
has loved stories and science for as long as she can remember. She joined
HowStuffWorks as a staff writer in 2005, and she spent her first few years at
the site destroying gadgets and mining patents, papers and interviews for the
sake of figuring out what makes things tick. In 2007, she took on the role of
hiring and training HowStuffWorks’ new writers and editors, and she became site
director in 2010. She cohosted the PopStuff pop culture podcast with Holly
Frey; the pair now cohosts Stuff You Missed in History Class. Tracy spends her
downtime much like she spends her time at work: reading, writing, tinkering and
brooking only the most delightful nonsense.
If you tear a
hologram in half, you can still see the whole image in each piece. The same is
true with smaller and smaller pieces.
|
You can create your
own holography table using inner tubes and sand to dampen vibration.
|
In photography, light
passes through a lens and a shutter before hitting a piece of film or a
light-sensitive sensor.
|
In holography, light
passes through a shutter and lenses before striking a light-sensitive piece of
holographic film.
|
You can visualize the interaction of
light waves [b]by imagining waves on water.
|
Light reflection can be specular, mirror-like (left), diffuse or
scattered.
|
When light waves
reflect, they follow the law of reflection. The
angle at which they strike the surface is the same as the angle at
which they leave it.
|
In a transmission
hologram, the light illuminating the hologramcomes from the side opposite the
observer.
|
The interference
fringes in a hologram cause light to scatter in all directions, creating an image in the
process. The fringes diffract and reflect some of the
light (inset), and some of the light passes through unchanged.
|
The holograms found
on credit cards and other everyday objects are mass-produced by stamping the
pattern of the hologram onto the foil.
|
The famous hologram
"The Kiss" shows a sequence of similar, stationary images. Your eye
sees many frames simultaneously, and your brain interprets them as moving
images.
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