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Introduction to

Holography
Part 1 of 3
How does holography work?

December 2015
J
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Yet, holography is an extraordinary
science which has plenty of applications
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After reading this article, you know the answers to the following questions:
 What is holography and how does it differ from photography?
 What causes the optic illusion of the impression of depth?
 What are the properties of light?
 What is light made out of?
 How can different materials be transparent, coloured, or even opaque?
 How does a laser work?
 How do you calculate the amount of light that comes out of a laser?
 What is interference and how can a hologram capture it?
CONTENT
1
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Light
2
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2
The electromagnetic spectrum
2
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4
The laser
2
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Light-sensitivity
4
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INTRODUCTION
Holography is a science which is commonly used as a photographic method of creating a
seemingly three-dimensional image where only a flat surface is used - such as a film or
holographic plate
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This is only an optic illusion, because the image is not
made out of matter, but solely made out of light
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Holography uses the same principles as photography
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Photography captures only the light’s intensity (intensity = amplitude,
the amount of light of a certain point)
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The phase difference causes interference patterns and creates
a seemingly three-dimensional image
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Let us first look at what light exactly is and how it behaves
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2
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Without light, we could not see a single thing
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1 - Properties of waves
If we zoom in on a light beam, we see that light is made out of waves in the electromagnetic
spectrum
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The parts of the wave above the line must be in balance with the parts underneath the line
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The amplitude is the difference of the wave’s height
relative to the equilibrium line
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 λ (lamda) is the Greek letter which represents the wavelength
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A wave starts when it first crosses the equilibrium line, is halfway when it
crosses the line a second time, and ends when it intersects a third time
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Frequency is the amounts of waves per second
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In Hertz is equal to 1 wave in 1 second
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The wavelength can be calculated by the formula λ = v / f
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The speed of light is equal to 3,0 x108 m/s
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2 – The electromagnetic spectrum
Green light has a wavelength of 532 nanometres
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We can calculate the frequency of green light like this: 3,0 x 108 / 5,32 x10-7 = 5,6 x1014 Hz
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This is because we interpret different
wavelengths of light as different colours
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The shortest
wavelengths of the electromagnetic spectrum are gamma rays
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Humans can only observe wavelengths from about 400 to 700 nanometres, and can thus
spectate just tiny sliver of the entire spectrum!
All wavelengths in the electromagnetic spectrum travel at the same speed: 3,0 x 108 m/s
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2
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A green object will reflect wavelengths from
about 500 to 550 nm, while it absorbs other wavelengths
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This is a separate study in quantum mechanics
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Based on the received information, the brain
creates an image
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What we see is in fact not
reality, but just our brain’s interpretation of the world
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What happens to the wave that makes up a light beam when an object reflects it? And what
happens to the object?
In this situation, we think of a light beam as a continuous string of particles – photons
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They have no mass, since they are not made out of matter
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We look at the building blocks of the universe – atoms
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The
nucleus consists out of protons and neutrons
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Objects of regular size are
made out of uncountable many atoms
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If we take just one photon and one atom, we can explain the phenomenon
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Think of it as the planets that circle around the sun
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If a photon meets the atom of an opaque (non-transparent) object, the photon is
absorbed by the electron, which jumps to a higher energy level
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If a photon meets the atom of a transparent object, the photon gives its energy to the electron
again
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Because electrons can only exist on
the levels and not in between, the photon keeps its energy and keeps on flying through the atom
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5

Introduction to HOLOGRAPHY

Part 1 – How does holography work?

Light of different wavelengths have different amounts of energy - the bigger the wavelength, the
lower the energy
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The atoms of a semiconductor, such as yellow-stained glass, have energy levels that differ in
such a way that photons with a low energy cannot cause an electron-jump, while photons with a
higher energy can
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If sunlight shines upon the atom of a semiconductor, some photons get
blocked while others can pass through
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We now understand that light behaves in different ways when interacting with different kinds of
atoms
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This still does not explain why red objects only reflect red light waves and absorb all other
wavelengths
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When a photon hits an electron of an atom of an opaque object, it gets absorbed and the electron
jumps to a higher energy level
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This phenomenon is also known as population-inversion
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The energy that was temporarily stored
gets released in the form of light again – it transforms into a photon
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The difference between the energy levels – which differs
per element - determines the wavelength of the light that is emitted
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The electrons then
fall back to their original energy level, while emitting light with a specific wavelength
determined by the specific way the atom is constructed (the element)
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Reflection is thus a phenomenon of absorption and
emission
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2
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By applying energy in the form of warmth and
electricity to the gasses, the electrons of the atoms jump to a higher energy level
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While falling back, the electrons emit photons
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The
interaction with a photon is not necessary – any form of energy will do the job
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This is called stimulated emission – think of it as the snowball-effect
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It is in fact an abbreviation – Light Amplification by Stimulated
Emission of Radiation
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The mirror in the front reflects 99% of the photons - the other 1% passes
through and forms the beam through a series of converging lenses
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The emission continues until
the battery dies
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Coherent light is pure light, in contrast to sunlight, which contains
light of different directions and wavelengths
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The laser light in toys and laser pens is usually around 2 mW,
and has too little energy to be destructive in any way
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This
means that the total energy is equal to Planck’s constant times the frequency times the amount
of photons
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E=hxfxn
Where n = amount of photons
h = Planck’s constant (6,63x10-34)
f = frequency
Calculation f:
λ = 532x10-9 m (532 nanometre of green light)
v = 3,0x108 ms-1
t = 532x10-9 / 3,0x108 = 1,77x10-15 s
f = 1 / t = 1 / 1,77x10-15 = 5,6391x1014 Hz
E=Pxt
Where E = energy
P = power (100 mW = 0,1 W)
t = time (1 s)
Pxt=hxfxn
Thus: 0,1 x 1 = 6,63x10-34 x 5,6391x1014 x n
n = (0,1 / 6,63x10-34) / 5,6391x1014 = 2,67x1017 photons per second
2
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Interference is a phenomenon that occurs in many ways in nature
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To understand the principle of interference, we use a simple example
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When the waves intersect, the heights
of the waves add up
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A peak
and a through cancel each other out (point 2) – the phase difference is π rad
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The amplifying, weakening and cancelling
out of waves result in a pattern that is called the interference pattern
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7

Introduction to HOLOGRAPHY

Part 1 – How does holography work?

3
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We
shall now explain how a permanent change occurs, since we use it for making a hologram
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The
electrons almost immediately fall back to their original energy level, emitting the energy again in
the form of photons
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The molecules flex because of
the energy, creating isomers with different properties
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This chemical dissolves well
in water
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If we apply the jelly to a small pane of glass, it can record the interference
patterns caused by phase differences of reflected light off an object – the object of which we
want to make a holographic image
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4
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A laser beam is created in a container with gasses by stimulated emission – it shoots out 2
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The beam travels until it reaches a glass pane which is coated by the
light sensitive emulsion
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The interference patterns of uncountable many
light waves that were reflected by the object are recorded by the light sensitive emulsion
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If we now hold the pane
in the light, the reversed phenomenon occurs – the interference patterns are decoded again
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The human brain interprets this phase difference as depth
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