There’s a lot of amazing and fantastic things occurring everyday, everywhere, just yearning
to be explored. It can seem in todays times its harder and harder to find such beauty
in the world but that doesn’t change the fact that it’s there. The mundane yet fascinating
functions of everyday life are often overlooked, continuously plugging away in the background
as everything moves about around them. For this reason it seems appropriate that the
first stop on our journey of discovery is about the very thing that lets us observe
such intricacies in the first place. I am of course talking about Light and why
it’s so interesting
*intro scene*
You probably don’t think about why you can see things too often or even why light exists
but it plays an integral role in just about everyone's life. It’s how we observe our
surrounding world and absorb most of our information, and it plays an essential role in our society.
But if someone asked you what light was where would you begin?
To a scientist the term light usually has a broader meaning than just the thing that
facilitates our ability to see. Light is almost always synonymous with the entire spectrum
of electromagnetic radiation spanning from Radiowaves all the way to the sinister gamma
rays.
Despite their differences in behaviour, the different forms of electromagnetic radiation
are all of the same energy or the same thing. Microwaves, radiowaves, X-Rays and UV are
all sides of the same energetic die; with one of those sides being the visible light
that’s letting you view this video right now.
But still what exactly is it?
At some point in your life you’ve heard the term light waves or waves of light. And
light is always presented as such. But in reality, electromagnetic radiation is a quantum
object and behaves as both a wave and a particle but it is also neither.
Light waves don’t behave like the waves we are used to.
Unlike waves found in our world, light doesn’t have length. Light is actually a singular
oscillation of both magnetic and electric fields travelling through space. Now if we
were to plot this movement we would very clearly see a wave function and shape. Therefore its
understandable to visualize and classify light by the length of their wave period or how
far this point travels through space after completing one full oscillation. But this
fails to explain the interaction of light with matter.
Normal waves apply their energy with time continually increasing the amount supplied.
Light however, despite behaving like a wave, doesn’t do this.
All of the energy within light is passed off seemingly instantaneously in known and distinct
amounts. These units or quanta of light energy are called photons and represent the particle
nature of the way light behaves. All light, all electromagnetic radiation travels
in the form of photons, these singular packets of energy. So unlike normal waves where an
impulse, or the force applied by a wave, is continuous for the entire length of the wave.
Our perception of a steady light source is just a constant stream of photons with the
same rate of oscillation, one after another.
So light or photons exist as an oscillation
in space that we can describe as a wave but behaves as a particle. But why does it exist?
Where does it come from?
Energy is the lifeblood of the universe and is constantly being exchanged and moved around.
You may already know many different ways we can transfer one energy type into another.
Matter is itself pure energy in a highly condensed form and since matter is energy it too needs
a way to transfer itself. So anytime an atom, molecule, or bit of matter
needs to release some kind of excess energy, it can either transfer it thermally to a neighboring
atom or molecule, or more relevantly it can release it in the form of light or a photon.
In order to fully understand this we're gonna have to crack open an atom and see what
makes it tick. How it behaves.
So let’s start here. A simple Bohr Model of hydrogen with a single electron orbiting
the proton in the nucleus. These are the fundamental building blocks of the universe and if we
understand what happens here, we can understand what happens everywhere.
Now normally an atom exists in its ground state. Right here with the electron at this
defined distance from the Proton. If nothing interacts with this atom it will exist like
this, possibly until the end of the universe. Now as one might imagine if hydrogen can exist
in a ground state, it can probably exist in another state. And sure enough an atom can
become excited if it receives the correct amount of energy needed to excite the electron
into a further orbital or distance from the proton in the nucleus.
When an atom is no longer in its ground state it is in what is known as an excited state
and A principal of the universe dictates that excited states or a system with more energy,
is never as stable as the ground state. So to return to stability the excited electron
can either react with another atom… or it can simply return to the ground state and
release the excess energy it received. and THIS my children… this process of an atom
returning to it’s ground state is where essentially all light comes from.
So light comes from the excited or energetic atoms and material returning back to their
ground states, But why does this process produce a photon?
How do we go from an electron to light? It doesn’t really make sense if you think about
it. And thats because we are thinking about particles
when we should be thinking about waves.
As I mentioned previously, light, like all quantum objects, behaves as both particles
and waves. Even here both the electron AND the proton are behaving as both particles
and waves. Due to the protons large mass, it doesn’t behave too much like a wave so
we can continue to think about it as a particle. The electron however has 1/1837th the mass
of a proton and so behaves very much like a wave. So What we imagine as a particle orbiting
a nucleus actually looks more like this, a wave function of the electron possibly existing
in all possible points within a contained volume. Despite its strange appearance this
can actually be represented mathematically as a proper function. Now I won’t overwhelm
you with the mathematics of it all, but this means we can think about electrons as a simple
sine wave to help our visualization. When talking about waves there’s a concept
called overtones that needs to be understood. The rate at which a wave oscillates every
second is called its frequency. If you take that frequency and multiply it by any integer,
so 1, 2 ,3 and so one, the new resulting frequency is what we call an overtone. You can think
of this as the different octaves to the same note played on a piano. Because overtones
are oscillating faster than the fundamental frequency they are more energetic, or they
can deliver more energy. Now when we think of electrons as wave functions,
the ground state is their fundamental frequency and their oscillations reflect that. When
they receive excess energy then they transition into an overtone, or an excited state with
a new wave function that possesses more energy. Eventually the electron will return to its
ground state and as it does this, its wave function will ramp down to its fundamental
frequency and emitt that extra wave energy in the form of well.. A wave.
And finally we understand the birth of photons. Everytime an electrons wave function collapses
back to its ground or lower energy state it produces another wave, light, to compensate
for this loss of energy.
And with this knowledge we can finally look at the different mechanisms that produce the
different types of light. Afterall, light is a spectrum with theoretically infinite
possible wavelengths. So how do we produce one or the other?
What distinguishes one type of light from the other is how much energy their respective
photon is carrying. The energy of a photon is dictated by the rate or frequency of its
oscillations with Faster oscillations providing more energy. When we visualize light as a
wave, this can be represented by its wavelength. Therefore light with shorter wavelengths contain
more energy. So in order to produce a photon with higher
energy, the electron itself that produces it needs to contain more energy. And it does
this by being excited into even higher overtones or even further orbitals. That way when the
electron returns to its ground state it must emit even more energy in the form of light.
The different ways atoms become excited are generally the criteria we use when we describe
light sources and where light comes from.
There are two principal sources of light, incandescence and luminescence. Despite each
using different mechanisms, at the end of the day they both mean the same thing, an
electron gets excited and then returns to its ground or lower energy state.
Incandescent light is the most common light in the universe and it’s biggest player
is Black body radiation. Now Black Body Radiation is another very interesting concept on its
own and possibly deserves a complete seperate video. But to be brief, electron excitation
for black body radiation comes from the temperature of matter itself. All molecules and atoms
are vibrating moving and spinning about in matter. As they bump into each other they
can transfer some of their internal energy to one another. This new excess energy can
then be used to excite an electron into an excited state and as we learned, when the
electron returns to its ground state it produces light. THerefore, Hotter and hotter temperatures
means more and more energy can be transferred amongst molecules. This creates greater electron
excitation which produces not only higher intensity light, but also brighter light since
more energy in an object also means more molecules and atoms can become excited. This is how
our older lightbulbs worked by superheating a small wire filament until it was hot enough
to glow.
Luminescent light is the second light source and is comprised of light not produced by
heat. There are three main subcategories and again I remind you, at the end of the day
they all simply describe excited electrons moving to a ground or lower energy state.
The first subcategory comprises changes to chemical bonds and chemistry. We can see this
happen in triboluminescence and chemiluminescence, where an alteration in the chemical make-up
of matter changes or lowers its internal energy thus emitting the excess energy in the form
of light. The second is electroluminescence and this
is what we use for most of our lights now. This effect occurs by passing other electrons,
or a current, through a material. Depending on the mechanism used in the lightbulb, this
current will excite electrons who then subsequently fall back to a ground state. Light bulbs like
LEDs that use this method are FAR more efficient than incandescent bulbs as very little of
the energy supplied to it is lost to the generation of heat.
The third luminescent light source is the re-emittance of light and this is composed
of two more subcateories. Fluorescence and Phosphorescence.
Both of these principles involve matter absorbing light or a photon themselves, and then emitting
it away.
Fluorescence is interesting because most of the time the light it re-emits is a lower
energy than the light it absorbed. This makes certain minerals glow under UV light and is
what was happening at your local roller rink or bolling alley in the 90s. It’s also whats
still happening in flourescent lights today. Electrons shoot through ionized air with mercury
atoms which are excited by electroluminescence to produce UV light, which is absorbed by
the phosphor powder(not to be confused with phosphorous) on the inside of the bulb and
then re-emitted as visible light. Phosphorescence is the final light type to
discuss and its the same principal as fluorescence, but these atoms for some reason are able to
take their sweet time when transitioning back to their ground state. As a result, a phosphorescent
objects appear to glow as every single atom within it decides to return to its ground
state at it’s own pace.
These different light sources account for essentially all the light in our universe
and certainly in our society. The remaining energetic light types, X-rays and Gamma rays,
are generally formed by different process and I could probably spend another 15 minutes
talking about them. But the overarching theme to both those process’ and the ones mentioned
previously, is that when matter receives or is in possession of too much energy, it will
eventually release it in the form of light.
To cap things off So now you may be wondering, if light comes
in a spectra of different energy levels, why do we see the light that we see? You’ve
probably heard of the mythical Mantis Shrimp with its 12 color receptors for all the fantastical
colors it could possibly see or that birds and bees can see UV light and maybe you felt
a little left out. After all why do we see just the visible light spectrum and not some
other kind of light? Well the easy answer is that we simply evolved to see that part
of the spectrum and nature decided there’s no evolutionary benefit to being able to see
other lightwaves. But the truth is there is no law dictating which part of the spectrum
an organism can see, it’s all circumstantial.The reason we see what we consider the visible
spectrum is because of our own atmosphere. Matter absorbs light, that means the matter
in our atmosphere, as non-dense as it is, absorbs some light as well. And it’s better
at absorbing some wavelengths than others. If we plot out all the different wave lengths
of light coming from the sun and their absorption by the atmosphere, we’d find a large dip
of absorption in the visible range. Since this is the most abundant light with the highest
energy or smallest wave-length, organisms on this planet have evolved to be sensitive
to it. This generates interesting thought experiments,
for if we ever did meet an intelligent life, their view of the universe would be heavily
dictated by the composition of their atmosphere and who knows how that could change someones
perception and attitude towards the universe.
And thats just one of the many different ways you, me, and life on earth is unique.