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Light rays are amazing phenomena,
traveling at incomprehensible speeds
across the vastness of the universe.
They have properties of both waves and particles.
Scientists have debated light's essential nature
for centuries.
The end result is that we have a dual theory of light.
You may think you need a superpower to slow
or bend a ray of light,
but there are things all around you
that change the way light behaves,
and also change what you see.
We're going to talk about refraction,
the bending of light,
in this segment of "Physics in Motion."
Experts have used what we know about refraction
to make lenses and telescopes
that can enable us to see things
from the infinitesimal, to the infinite.
But let's bring it back down to earth.
We've all see this, right?
That's refraction, the bending of light
as it moves from one transparent medium to another,
from air to water, in this case.
Other kinds of waves, like sound waves,
can also refract,
but we're going to focus on light today.
Now, what causes refraction?
Basically, it's because the speed of light changes
depending upon the medium that it's traveling through.
The angle of light rays can also change
as they move from one medium to another.
How much it changes depends on the refraction index,
a ratio that describes how much light bends
as it moves from one medium to another,
which depends upon the density of the medium.
To get an idea of refraction indices,
here are a few examples.
A vacuum is one.
Air is 1.0003.
Water is 1.330.
Salt is 1.520.
Diamond is 2.420, and lead is 2.600.
The higher the index of refraction,
the more the material tends to bend light.
To calculate the index of refraction,
divide the speed of light in a vacuum
by the speed of light in a medium.
The formula looks like this.
N, the index of refraction,
equals the speed of light in a vacuum, c,
divided by the speed of light in a medium, v.
Let's take our water and ruler example.
The light is traveling at its usual speed
through the air,
a little less than 300 million meters per second.
But when the light hits the water
and makes a ruler look like it's in a different place,
what exactly is happening?
Let's analyze it.
The angle of incidence, the angle at which
the light ray hits the water, stays the same.
That's measured from the Normal, which is a line
perpendicular to the material,
but when the medium changes
to one with a different refraction index,
the angle at which the light moves
changes also relative to the Normal.
If it hits the medium that makes it slow down,
like water, it bends towards the Normal.
If it goes from a slower to a faster medium,
it bends away from it.
Since water is more dense than air,
the angle moves which way?
Towards Normal, right,
and that's what you're seeing here.
Who exactly figured this out?
Willebord Snell, a Dutch astronomer and mathematician
who lived almost 500 years ago.
He's credited with determining the angle of change
resulting from the light traveling from one medium
to another.
He came up with a formula we use
to calculate the change,
naturally called Snell's law.
Let's take a look at it.
N sub one, the index of refraction
in the first medium,
multiplied by sine theta sub one,
theta sub one being the angle of incidence,
equals n sub two,
the index of refraction in the second medium,
times sine theta sub two,
theta sub two being equal to
the angle of refraction in the second medium.
When you do this calculation,
you can look up the n values,
the indices of refractions,
from charts,
and when you measure the one angle,
you can figure out the other angle.
But there's more.
The way light strikes an interface
affects how we perceive the image.
Let's go poolside to check this out.
Before we get into the pool,
let's look at some clouds,
so we can see what angles do
to our perception.
See?
Just regular looking clouds.
Now, let's go underwater and look straight up.
You see the clouds in the same place as before
because the light is traveling perpendicular
to the interface between the air and the water.
It's not bent from the normal.
Here's a diagram of what's happening
to the light at the pool.
Now, let's shift our gaze
and look at an angle
towards the edge of the pool.
The images have shifted because of refraction.
The light is bent according to Snell's law.
If you look at an even sharper angle
towards the surface,
you see only a strange silvery line
that doesn't make sense.
You're looking at what we call the critical angle.
This is the angle of incidence which causes
an angle of refraction of 90 degrees.
In this case, as a ray of light passes
from the less dense medium to a more dense medium.
The critical angle can be found mathematically by
taking the sine inverse of the index of refraction
n sub two, divided by the index of refraction,
n sub one,
when n sub one is greater than n sub two.
In this case it will be the sine inverse
of the index of refraction of air, 1.00,
divided by the index of refraction of water, 1.33,
giving us a critical angle of 48.8 degrees.
So let's look up at an angle greater than 48.8.
You see the bottom of the pool reflected
in the surface of the water.
A little crazy, but true.
At this angle, and at incident angles
greater than this angle,
the rays are no longer refracted,
but totally reflected.
The image we see is a reflection
of the bottom of the pool.
If the light strikes the interface
at an angle greater than the critical angle,
the light is reflected internally.
That's called internal reflection.
Here's a diagram of what's happening to light
that's being internally reflected.
Internal reflection can come in very handy
in our everyday lives.
For instance, fiber optic cables
use internal reflection
just like we saw in the water
to keep light signals within the fiber,
no matter how you bend it.
That way, the fibers can carry far more information
than electrical wires of the same size.
Let's look at how this internal
reflection phenomenon works.
I have here a laser and a bottle filled with water,
which has a hole in it.
Now, I've covered up the hole,
but if I uncover it,
the water flows out, of course.
Let me cover it back up for a second.
What do you think will happen if I uncover it,
and then shine a laser through the hole?
Let's try that.
It looks like the stream of water
holds the laser light in.
This is total internal reflection in action.
The laser beam is hitting the water-air boundary
at an angle greater than the critical angle,
causing the light to reflect back into the stream.
Refraction has other tricks up its sleeve.
Okay, not tricks.
It's all science, but it's still pretty amazing.
You know how a prism bends the light
and breaks it into different colors.
When that happens, the beam of light
is being refracted,
but when the light hits the glass,
some wavelengths bend more than others.
The shortest wavelengths, the violet ones,
bend the most.
The longest ones, the red ones, bend the least.
Though the wavelengths are bending,
the frequency of the waves
stays the same during
all of this.
In other words, as the speed of light is reduced
when it hits a slower medium,
the wavelength is shortened proportional to the change,
but when light is refracted, the frequency is unchanged.
It is a characteristic of the source of the light.
The medium doesn't affect it.
If you've ever wondered why a diamond
is so specially sparkly, it's like a raindrop.
A combination of refraction, dispersion,
which is where white light is broken down into colors.
And internal reflection, light rays
pulling out all the stops
to create the twinkle and shine of a gem.
As scientists began to understand how light behaves
and how refraction works, it opened a lot of doors.
One big development followed the discovery
that a light ray traveling along the normal,
perpendicular to the medium,
doesn't change direction at the boundary.
Only speed.
That led to two momentous inventions,
lenses and refracting telescopes
that allow us to see things too tiny for the naked eye,
or light years away.
That's it for this segment of "Physics In Motion,"
and we'll see you next time.