The strange properties of light Light is everywhere. But do you know what it actually is? Really? #Beekhaybee

light

Presented by
Colin Barras
Light is what allows us to understand the world
we live in. Our language reflects this: after
groping in the dark, we see the light and
understanding dawns.
Yet light is one of those things that we don’t tend
to understand. If you were to zoom in on a ray of
light, what would you see? Sure, light travels
incredibly fast, but what is it that’s doing the
travelling? Many of us would struggle to explain.
It doesn’t have to be that way. Light certainly has
puzzled the greatest minds for centuries, but
landmark discoveries made over the last 150
years have robbed light of its mystery. We
actually know, more or less, what it is.
Not only do today’s physicists understand the
nature of light, they are learning to control it with
ever-greater precision – which means light could
soon be put to work in surprising new ways. That
is part of the reason why the United Nations
designated 2015 as the International Year of
Light .
There are all sorts of ways to describe light. But
it might help to begin with this: light is a form of
radiation .
It wasn’t until the late nineteenth century
that scientists discovered the exact
identity of light radiation
This hopefully makes some sense. We all know
that too much sunlight can trigger skin cancer.
We also know that radiation exposure can raise
the risk of developing some forms of cancer, so
it’s not hard to put the two together.
But not all forms of radiation are the same. It
wasn’t until the late nineteenth century that
scientists discovered the exact identity of light
radiation.
The strange thing is, this discovery didn’t come
from the study of light. Instead it emerged from
decades of work into the nature of electricity and
magnetism.
Electricity and magnetism seem like quite
different things. But scientists like Hans Christian
Oersted and Michael Faraday established that
they are deeply entwined.
Oersted found that an electric current passing
through a wire deflects the needle of a magnetic
compass. Meanwhile, Faraday discovered that
moving a magnet near a wire can generate an
electric current in the wire.
Maxwell showed that electric and
magnetic fields travel in the manner of
waves
Mathematicians of the day set about using these
observations to create a theory describing this
strange new phenomenon, which they called
“electromagnetism”. But it wasn’t until James
Clerk Maxwell looked at the problem that a
complete picture emerged.
Maxwell’s contribution to science is huge. Albert
Einstein, who was inspired by Maxwell, said that
he changed the world forever . Among many
other things, his calculations helped explain what
light is.
Maxwell showed that electric and magnetic fields
travel in the manner of waves, and that those
waves move essentially at the speed of light. This
allowed Maxwell to predict that light itself was
carried by electromagnetic waves – which means
light is a form of electromagnetic radiation .
In the late 1880s, a few years after Maxwell’s
death, German physicist Heinrich Hertz became
the first to formally demonstrate that Maxwell’s
theoretical concept of the electromagnetic wave
was correct.
In 1861 he unveiled the first durable
colour photograph
“I am convinced that if Maxwell and Hertz had
lived into the Nobel prize era, they would have
surely shared one,” says Graham Hall of the
University of Aberdeen in the UK – where
Maxwell worked in the late 1850s .
Maxwell holds a place in the annals of light
science for another, more practical reason. In
1861 he unveiled the first durable colour
photograph, produced using a three-colour filter
system that still forms the basis of many forms
of colour photography today.
Still, the idea that light is a form of
electromagnetic radiation may not mean too
much. But this idea helps to explain something
that we all understand: light is a spectrum of
colours.
This is an observation that goes back to the
work of Isaac Newton . We see this colour
spectrum in all its glory whenever a rainbow
hangs in the sky – and those colours relate
directly to Maxwell’s concept of electromagnetic
waves.
Many animals can actually see ultraviolet,
and so can some people
The red light along one edge of the rainbow is
electromagnetic radiation with a wavelength of
about 620 to 750 nanometres; the violet light
along the opposite edge is radiation with a
wavelength of 380 to 450nm.
But there is far more to electromagnetic radiation
than these visible colours. Light with wavelengths
slightly longer than the red light we see is called
infrared. Light with wavelengths slightly shorter
than violet is called ultraviolet.
Many animals can actually see ultraviolet, and so
can some people, says Eleftherios Goulielmakis
of the Max Planck Institute of Quantum Optics in
Garching, Germany. In some circumstances even
infrared is visible to humans . Perhaps this is why
it’s not uncommon to see both ultraviolet and
infrared described as forms of light.
Curiously, though, go to even longer – or shorter
– electromagnetic wavelengths and we stop
using the word “light”.
Beyond ultraviolet, electromagnetic wavelengths
can go shorter than 100nm. This is the realm of
X-rays and gamma rays. You won’t often hear X-
rays described as a form of light.
There is no real physical difference
between radio waves and visible light
“A scientist wouldn’t say ‘I’m shining X-ray light
on the target’. They would say ‘I’m using X-
rays’,” says Goulielmakis.
Meanwhile, go beyond infrared and
electromagnetic wavelength stretches to 1cm and
even up to thousands of kilometres. These
electromagnetic waves are given familiar names
like microwaves and radio waves. It may seem
strange to think of the radio waves used in
broadcasting as light.
“There is no real physical difference between
radio waves and visible light from the point of
view of physics,” says Goulielmakis. “You would
describe them with exactly the same sort of
equations and mathematics.” It’s only our
everyday language that treats them as different.
So we have another definition of light. It is the
very narrow range of electromagnetic radiation
that our eyes can actually see. In other words,
light is a subjective label that we only use
because our senses are limited .
For more evidence of just how subjective our
concept of light is, think back to the rainbow.
Most people learn that the spectrum of light
contains seven main colours: red, orange, yellow,
green, blue, indigo and violet. We are even given
handy mnemonics and songs to remember them.
Look at a strong rainbow and you can probably
convince yourself that all seven colours are on
show. However, Newton himself struggled to see
them all.
In fact, researchers now suspect that he only
divided the rainbow into seven colours because
the number seven was so significant in the
ancient world: for instance there are seven notes
in a musical scale, and seven days in a week.
Maxwell’s work on electromagnetism took us
past all this, and showed that visible light was
part of a larger spectrum of radiation. It also
seemed to finally explain the nature of light.
For centuries, scientists had been trying to pin
down the actual form that light takes at a
fundamental scale as it travels from a light
source to our eyes.
Newton realised that rays of light obeyed
very strict geometric rules
Some thought that light travelled in the form of
waves or ripples, either through air or a more
nebulous “ether”. Others thought this wave model
was wrong and imagined light as a stream of tiny
particles.
Newton preferred this second option, particularly
after a series of experiments he performed using
light and mirrors.
He realised that rays of light obeyed very strict
geometric rules. Shine a ray against a mirror and
it bounced off in exactly the same way a ball
would if it were thrown against the mirror . Waves
don’t necessarily move in such predictable
straight lines, he reasoned, so light must be
carried by some form of tiny, weightless
particles.
The trouble is, there was equally compelling
evidence that light is a wave.
One of the most famous demonstrations of this
came in 1801. Thomas Young’s “double slit
experiment” is the sort of experiment anyone can
replicate at home.
Take a sheet of thick card and carefully make two
thin vertical slits through it. Then get a
“coherent” light source, which only produces
light of a particular wavelength: a laser will do
nicely. Now shine the light through the two slits
onto another surface.
On that second surface, you might expect to see
two bright vertical lines where some of the light
has passed through the two slits. But when
Young performed the experiment, he saw a
sequence of light and dark lines rather like a bar
code.
When the light passes through the thin slits, it
behaves in the same way that water waves do
when they pass through a narrow opening: they
diffract and spread out in the form of
hemispherical ripples.
Where the “light ripples” from the two slits hit
each other out of phase they cancel out, forming
dark bars. Where the ripples hit each other in
phase, they add together to made bright vertical
lines.
Young’s experiment was compelling evidence of
the wave model, and Maxwell’s work put the idea
on a solid mathematical footing. Light is a wave .
But then came the quantum revolution.
In the second half of the nineteenth century,
physicists were trying to understand how and
why some materials absorbed and emitted
electromagnetic radiation better than others.
In 1900, Max Planck solved the problem
That may sound a bit niche, but the electric light
industry was emerging at the time, so materials
that could emit light were a big thing.
By the end of the nineteenth century, scientists
had discovered that the amount of
electromagnetic radiation released by an object
changed depending on its temperature , and they
had measured these changes. But no one knew
why it happened.
In 1900, Max Planck solved the problem. He
discovered that the calculations could explain
those changes, but only if he assumed that the
electromagnetic radiation was held in tiny
discrete packets. Planck called these “quanta”,
the plural of “quantum”.
A few years later, Einstein used this idea to
explain another puzzling experiment.
Physicists had discovered that a chunk of metal
becomes positively charged when it is bathed in
visible or ultraviolet light. They called this the
” photoelectric effect “.
This doesn’t make much sense if light is
simply a wave
The explanation was that atoms in the metal were
losing negatively-charged electrons. Apparently,
the light delivered enough energy to the metal to
shake some of them loose.
But the detail of what the electrons were doing
was odd. They could be made to carry more
energy simply by changing the colour of light. In
particular, the electrons released from a metal
bathed in violet light carried more energy that
electrons released by a metal bathed in red light.
This doesn’t make much sense if light is simply
a wave.
You usually change the amount of energy in a
wave by making it taller – think of the destructive
power of a tall tsunami – rather than by making
the wave itself longer or shorter.
Each quantum packs a discrete energy
punch
By extension, the best way to increase the energy
that light transfers to the electrons should be by
making the light waves taller: that is, making the
light brighter. Changing the wavelength, and thus
the colour, shouldn’t make as much of a
difference.
Einstein realised that the photoelectric effect was
easier to understand by thinking of light in terms
of Planck’s quanta.
He suggested that light is carried in tiny quantum
packets. Each quantum packs a discrete energy
punch that relates to the wavelength: the shorter
the wavelength, the denser the energy punch.
This would explain why violet light packets, with
a relatively short wavelength, carried more
energy than red light packets, with a relatively
longer one.
It also explained why simply increasing the
brightness of the light made less of an impact.
A brighter light source delivers more light
packets to the metal, but it doesn’t change the
amount of energy each light packet contains.
Crudely speaking, a single violet light packet
could transfer more energy to a single electron
than any number of red light packets.
The scientists decided that light behaved
as both a wave and a particle at the same
time
Einstein called these energy packets photons, and
these are now recognised as a fundamental
particle. Visible light is carried by photons, and
so are all the other kinds of electromagnetic
radiation like X-rays, microwaves and radio
waves. In other words, light is a particle .
At this point physicists decided to end the debate
over whether light behaved as a wave or a
particle. Both models were so convincing that
neither could be rejected.
To the confusion of many non-physicists, the
scientists decided that light behaved as both a
wave and a particle at the same time. In other
words, light is a paradox.
Physicists, though, have no problem with light’s
split identity. If anything, it makes light doubly
useful. Today, building on the work of luminaries
– literally “light-givers” – like Maxwell and
Einstein, we are squeezing even more out of
light.
It turns out that the equations used to describe
light-as-a-wave and light-as-a-particle work
equally well, but in some circumstances one is
easier to use than the other. So physicists switch
between them, just as we use metres to describe
our own height but switch to kilometres to
describe a bicycle ride.
Entangled particles can be used to
communicate information
Some physicists are trying to use light to create
encrypted channels of communication: for money
transfers, for instance . For them, it makes sense
to think of light as particles.
This is because of another strange quirk of
quantum physics. Two fundamental particles, like
a pair of photons, can be “entangled”. This
means they share properties no matter how far
apart they are from one another, so they can be
used to communicate information between two
points on Earth.
Another feature of this entanglement is that the
quantum state of the photons changes when they
are read. That means if anyone tried to
eavesdrop on a channel encrypted using the
quantum properties of light, they would, in
theory, immediately betray their presence.
Others, like Goulielmakis, are using light in
electronics. For them it is far more useful to
think of light as a series of waves that can be
tamed and controlled.
Modern devices called “light field synthesisers”
can corral light waves into perfect synchrony with
each other. As a result, they create light pulses
that are far more intense, short-lived and
directed than the light from an ordinary bulb.
They literally took photos of light waves
moving
Over the last 15 years, these devices have been
used to tame light to an extraordinary degree.
In 2004 Goulielmakis and his colleagues
managed to produce incredibly short pulses of
X-ray radiation. Each pulse lasted just 250
attoseconds, or 250 quintillionths of a second.
Using these tiny pulses like a camera flash, they
managed to capture images of individual waves
of visible light , which oscillate rather slower.
They literally took photos of light waves moving.
“We’ve known since Maxwell that light is an
oscillating electromagnetic field, but nobody
dreamed we would be able to capture the light as
it oscillates,” says Goulielmakis.
Seeing those individual light waves is a first step
towards controlling and sculpting them, he says,
much as we already sculpt much longer
electromagnetic waves, like the radio waves that
carry radio and television signals.
A century ago, the photoelectric effect showed
that visible light affects the electrons in a metal.
Goulielmakis says it should be possible to
precisely manipulate those electrons, using
visible light waves that have been shaped to
interact with the metals in a carefully defined
way. “We can control the light, and through it we
can control matter,” he says.
Human eyes are photon detectors that use
visible light to learn about the world
around us
That could revolutionise electronics, leading to
new generations of optical computers that are
smaller and faster than those we have today. “It’s
about setting electrons in motion in ways we
want, creating electric currents inside solids
using light, instead of conventional electronics.”
So there is one more way light can be described:
light is a tool .
That is nothing new. Life has been harnessing
light ever since the first primitive organisms
evolved light-sensitive tissues. Human eyes are
photon detectors that use visible light to learn
about the world around us.
Modern technology is simply taking this idea
even further. In 2014, the Nobel Prize in
Chemistry was awarded to researchers who built
a light microscope so powerful, it was thought
to be physically impossible. It turned out that,
with a bit of persuasion, light would show us
things we thought we would never see.
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