Hi, I'm Steve, you can call me Steve.
Today's episode is another installment
in the Science of Geekery series. It's
all about frickin' laser beams. Lasers. I
am not nearly bald enough to pull that
off. Although, give it time. We're all
familiar with sci-fi shows with laser
guns that shoot a beam of coloured light.
It's helpful because we can actually
tell who's shooting at who, and a lot of
the times the beams will be colour-coded so we can tell the good guys from
the bad guys. Thing is, I've worked
with lasers and they don't actually do
that. They're usually invisible except
for the dot on the thing they're
shooting at, and the colour depends on
the energy and not who's holding it.
"LASER" is an acronym. It stands for Light
Amplification by Stimulated Emission of
Radiation, which sounds complicated but
isn't. So light, obviously, is light.
Amplification just means "to make more of",
the same way the amp on your speakers
makes more sound. Emission of radiation
sounds like that should involve giant
glowing monsters, but it doesn't.
Radiation doesn't automatically mean
radioactive. It's just any form of energy
that's moving in a straight line. Heat is
radiation, radio waves are radiation and
 light is radiation. Basically the
acronym is saying it's increasing light
by giving off light. It's redundant, but
those extra letters on the end make
it easier to say. The important bit here
is the stimulated. A stimulus is
something that causes a reaction. It
turns up in biology a lot when they're
poking organisms to see what they do.
Basically we get no light emitted until
we give it a prod. This is how a laser
works. Inside it there's material that
absorbs energy. The technical term for
this material is gain medium,
which I actually did not know until I
started researching this video. Instead
of gain medium, I will probably just call
it "the stuff". So in the fancier lasers,
like the ones I used in my Masters, "the
stuff" is a mixture of noble gases and in
the cheap ones like this it's a crystal.
To make light, "the stuff" absorbs energy,
usually electricity and in this case
it's from the batteries, and in the large
ones it's from the wall. So the electrons
in the atoms in "the stuff" have absorbed
the energy from the electricity and they
jump to a higher energy state. This is
called the excited state. The thing about
being excited is it's hard to maintain
that indefinitely, so eventually the
electrons get tired of being excited and
one of them will relax down to the
ground state, and it releases that
absorbed energy as a photon, which is a
particle of light. If you watch a lot of
my videos you'll probably notice I talk
about photons a lot. My Master's was in
photochemistry, which is the chemistry of
the light, so it's just an occupational
hazard. The chamber that stores "the stuff"
has mirrors at each end, so the photon can
bounce off the mirror, fly back into "the
stuff" and then interact with another
excited electron, force it to release its
energy as another photon, and now you
have two. That's the stimulus, the S in
LASER, that interaction of the photon
with the excited atom, and if you do that
a whole bunch of times you get a whole
bunch of photons. That's the
amplification, the A in LASER. One photon
turns into entire crapload of photons,
and one of those mirrors at the end is
slightly transparent, which means some of
those photons, instead of bouncing off,
can escape in a nice narrow beam
like that, which is not a nice narrow
beam but I blame the webcam and the fact
it's bouncing off my hand. Now you probably
notice you can't see anything except
where it hits my hand. Remember, all of
these photons are moving in the same
direction and in order for you to see it,
a photon has to enter your eye and
strike your retina, and that's not
happening here, unless one of these
photons happens to make an abrupt 90 degree turn, and that doesn't
happen unless it hits something. So you
can see the dot on my hand here because
life is being scattered and reflected
after smacking into my palm. Photons are
going in all directions now, including to
my eyes and to the camera pick up. In
order to see that the beam itself, you
need to look directly into the laser, and
seriously do not look directly into the
laser. I actually tried to shine it into the
webcam earlier and I almost blind myself
with the reflection, so this is not a toy.
It's a cat exerciser, but it's not a toy.
One thing I want to point out here is on
camera this laser beam looks like a big
blob, whereas you're probably used to a
small cold dot. So this is an artifact of
the camera itself. What's happening is
when the laser hits my palm, it's actually
penetrating my skin a little bit. Human
skin is somewhat transparent to red
light. So you can see it is glowing red
through. This laser also has a white
light, which looks reddish white coming
through my thumb. So what's happening is
the laser penetrates my hand and that
light is reflected inside my tissue and
re-emitted in sort of this broad smear,
and the webcam pics all of that up
and it looks all the same. So I see a dot
with a halo but you see a circle,
so that's why it looks the way it does,
because technology. You'll notice that
this laser is red like most lasers on TV.
You can get green lasers and blue lasers
as well, and during grad school I used
ultraviolet lasers. The colour depends on
the energy of the photons coming out and
that depends on the gap between the
excited and the ground states in "the
stuff" and that can change depending on
what type of "stuff" you use. So I
discussed how the energy of a photon
affects its colour in my why is the sky
blue video and I will link that down
there. Enough science, now for some geekery.
So very few shows actually do lasers
correctly. They'll depict slow-moving,
visible bolts or beams, and this is kind
of unavoidable sometimes. It's a rule of
drama. it increases the tension and
conveys what's actually happening if you
can see the laser bolts being shot at the
heroes. What does annoy me it is when
people dodge laser beams. Lasers are made
of light, and therefore they move at the
speed of light. Also, the thing you see
them by is the thing they are, which is
light, so by the time you see the laser
beam, it's already hit you. Many shows will
get around this by calling their
directed energy weapons blasters or
phasers or PPG's and describing them as
shooting plasma or particles or some
sort of technobabble that isn't the
light. I always approve of going around
rules of science if you can't avoid
breaking them. One person who got this
right was Isaac Asimov. Asimov was a
chemist, so I may be kind of biased in
liking him, and his stories generally
conformed to the laws of science as they
were known at the time of writing, except
for the starships and the talking robots.
Anyway, in the novel Foundation and Earth,
the protagonists land on a planet
and are attacked by a pack of wild dogs.
If they'd had a sci-fi laser that shot
colourful bolts or made pew-pew-pew noises, they
could have scared the dogs off. Instead
they had a silent, invisible, real world
laser that was quite effective at
killing the dogs... except the dogs didn't
realize it was a threat because they
couldn't see or hear it. It's also an
example of real world science adding to
the tension of the story. So that's
lasers: electrical energy converted into
lights in a way that does not resemble
what you see on TV. Thank you for
watching, I've been Steve.
