Normally you would not expect to see a
fish
tank in a laser lab, but it turns out that
a fish tank filled with water is a nice
way
to demonstrate how light propagates from
one medium into another.
in this demonstration what you're seeing
is the laser beam inside the
water encountering the air-water interface
and
being reflected back down into the water.
If you look closely you will see that no
light is emerging Out of
the water, into the air, you can see
nothing on this card, as I
move it across here.
That's manifestation of a phenomenon known
as total internal reflection
and is diagrammed on the picture on the
board over there.
Total internal reflection can happen when
you're, the light
is traveling from a high-index medium like
water to
a lower-index medium like air, and when
the light
wave approaches the surface at a
sufficiently steep angle.
In that case 100% of the light will be
reflected back down into the water.
And as I change the angle,
as you see here, you can see that no light
is coming out into the air.
All of the light is remaining inside the
water on that first reflection there.
Total internal reflection is the basis
upon which fiber optics work.
Light is confine, confined inside the
fiber optic by
total internal reflection, and that keeps
100% of the
light in so that light can be transmitted
for
a very long distance at very, with very
low loss.