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Lasers are a unique
and visually fascinating phenomenon.
They were invented in the 1950's,
and ever since, have been used
to shed light on so many things.
From pointers, to printers, to data transmission,
to brain scans--
and not to mention, holograms.
Before we focus on lasers,
let's explore how light waves behave,
what they have in common with other waves,
and what sets them apart.
Light waves have a lot in common with other waves.
In another segment, we told you how sound waves
bend around doorways and other objects.
Well, light waves bend too.
That's diffraction.
The bending of a wave around a barrier.
Now, light waves are about a million times shorter
than sound waves.
And light waves will bend around objects
whose dimensions are about the same size
as a light's wavelength.
That's about the size of a thin piece of wire,
or the edge of a razorblade.
So, light waves bend on a scale
much smaller than sound waves.
That's why you can hear someone around a corner,
but you can't see them.
Light behaves like a particle as well as a wave,
which is called wave particle duality.
This idea becomes important as we explore
how light interacts with itself
and its surroundings.
Now, let's check out some waves you can see
to explain how waves behave.
Waves in the ocean can be quite a challenge
for anyone steering a ship.
Especially if those waves are hitting the ship
from several directions.
When waves collide with each other,
they can make bigger waves or smaller ones.
Light waves and sound waves do the same thing.
So, how do they get bigger or smaller?
It's what we call interference.
When waves overlap or come together.
It's the overlapping of waves
forming a wave of increased
or decreased amplitude.
Waves consist of alternating peaks and troughs.
High points and low points.
If the peak of one wave
overlaps with the trough of another wave,
the two are completely out of phase with each other.
That's called destructive interference,
which diminishes the amplitude of the combined waves.
When that happens with sound waves,
what do you get?
No sound at all.
The sound of silence.
What happens with a light wave?
We'll get to that a little later.
So what happens to waves if the peaks of one wave
overlap with the peaks of another wave,
and the troughs of these waves match up too?
Then they're completely in phase.
The waves will constructively interfere.
The two smaller waves combine and produce a bigger wave,
which increases the amplitude of the combined waves.
That's called constructive interference.
When that happens with sound waves,
what do you get?
A louder sound.
As peaks are stacked one on top of the other.
What about with light?
Well, you'll see in just a moment.
What do you think happens when you shine a laser
through a very narrow slit onto a screen?
What shape of light appears on that surface?
Is it the same size and shape as the slit?
Did you think that it would look something like this?
It's because of two wave properties
called diffraction and interference.
Thanks to diffraction, the bending of light,
the light covers a much larger area of the screen
than you'd expect.
And thanks to interference, the overlapping of waves,
we have a series of vertical lines.
The concept of interference
was explained by Christiaan Huygens.
Interference occurs because each point of a wave
behaves as a separate light source.
The yellow dots in the image
represent those individual points of light,
and the principle that Huygens discovered.
Huygens' principle states that every point on a wave
behaves as a separate wave or wavelet.
This means that these independently behaving waves
will interact on the screen.
Some in phase with one another
and some out of phase.
That bright spot at the center of the screen
is where the waves crashed into each other
in phase, creating constructive interference.
Their peaks and troughs matching up perfectly.
Creating a bigger, brighter light wave
than any one of them alone.
Those areas of constructive interference,
seen as bright spots on the screen,
are called maxima.
Alternating with the maxima are dark regions,
where the waves are out of phase when they collide.
The troughs and peaks match up,
canceling out one another.
The areas of destructive interference
are seen as dark spots on the screen
and are called minima.
Because of how the wavelets combine
when they move through the slit,
the maxima and minima will always alternate
between constructive and destructive
interference patterns.
Check out the Closer Look for information
on the mathematical relationships
and what creates variations in this pattern.
Okay, so what do you think happens if we add another slit?
Would you see a second column of bright and dark spots
next to the first?
Maybe a cross pattern?
Take a guess.
What appears on the screen has become known as
Young's Double-Slit Experiment.
Thomas Young discovered the wave properties of light,
and this was one of his experiments.
It took another hundred-plus years
to discover the particle properties of light.
With two slits, the interference pattern
goes to a whole new level.
Within each peak from a single slit pattern,
there's a series of smaller peaks and troughs.
Maxima and minima.
What do you think will happen if I increase the distance
between the slits?
As you can see,
the distance between the maxima decreases.
What if I decrease the wavelength of light
traveling through the slits?
The distance between the maxima decreases.
For now, let's focus on a pretty amazing application
of light diffraction and interference.
Namely, holograms.
Holograms use laser light to make 3D images.
To learn more about how that's done,
let's go to Georgia Tech,
to talk to Dr. Ben Klein,
who's an associate professor
in the Department of Electrical and Computer Engineering.
So, Dr. Klein, how are holograms made?
So, a hologram is a reproduction of the light
that was bouncing off of some object.
Say, if you're taking a hologram of an apple,
it would be a reproduction of the light waves
that were bouncing off of the apple.
So, if I can make a copy of that light wave
reaching your eyes,
I can fool you into thinking there's an apple
in front of you.
So, making a hologram is a tricky thing.
It's much trickier than making a photograph.
We take a laser beam,
and we split the laser beam
to travel on two paths.
So, one path avoids the apple,
and the other path bounces off of the apple.
And then we bring those two paths back together.
And the light waves interfere
and make this kinda cross-hatch pattern,
which we record in the hologram,
and it records as sort of a grading,
that will later be played back.
You would be able to turn your head
and see a slightly different perspective
on the apple, because the entire light wave
that came off the apple was recorded
and is being played back,
rather than two individual perspectives,
or one individual perspective.
Thank you, Dr. Klein.
And with that, that's it for this segment
of "Physics in Motion".
And we'll see you next time.
(announcer) For more practice problems,
lab activities and note-taking guides,
check out the "Physics in Motion" toolkit.