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Normally you would not expect to see a
fish

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tank in a laser lab, but it turns out that

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a fish tank filled with water is a nice
way

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to demonstrate how light propagates from
one medium into another.

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in this demonstration what you're seeing
is the laser beam inside the

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water encountering the air-water interface
and

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being reflected back down into the water.

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If you look closely you will see that no
light is emerging Out of

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the water, into the air, you can see
nothing on this card, as I

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move it across here.

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That's manifestation of a phenomenon known
as total internal reflection

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and is diagrammed on the picture on the
board over there.

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Total internal reflection can happen when
you're, the light

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is traveling from a high-index medium like
water to

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a lower-index medium like air, and when
the light

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wave approaches the surface at a
sufficiently steep angle.

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In that case 100% of the light will be
reflected back down into the water.

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And as I change the angle,

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as you see here, you can see that no light
is coming out into the air.

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All of the light is remaining inside the
water on that first reflection there.

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Total internal reflection is the basis
upon which fiber optics work.

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Light is confine, confined inside the
fiber optic by

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total internal reflection, and that keeps
100% of the

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light in so that light can be transmitted
for

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a very long distance at very, with very
low loss.