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Unit 9: Light Page 11

Textbook Reading

Imagine tossing a rock into a still It doesn’t give off any light on its
pond and watching the circles of own but you can see it because

ripples form and spread out into light waves bounce off the apple
rings. Now look at the ripples in into your eye. If you shut off the
the water… notice how they spread light, then you can’t see the apple.
out. What makes the ripples move In this same way, the sun is a light
outward is energy, and there are source, and the moon is a light
different reflector.
kinds of
energy, such The Speed of Light
as electrical
Light can change speed the same
(like the stuff
way sound vibrations change
from your
speed. (Think of how your voice
wall socket), mechanical (a
bicycle), chemical (a campfire) and changes when you inhale helium
others. and then try to talk.)

The “speed limit” of light is
The ripples are like light. Notice
the waves are not really moving 186,000 miles per second – that’s
the water from one side of the fast enough to circle the Earth
seven times every second, but
pond to the other, but rather move
that’s also inside a vacuum.
energy across the surface of the
water. To put it another way,
You can get light going slower by
energy travels across the pond in a
aiming it through different
wave. Light works the same way –
substances. In fact, you can get
light travels as energy waves.
high-energy particles to travel
Only light doesn’t need water to
faster than light through water.
travel through the way the water
When this happens you get
waves do - it can travel through a
something equivalent to a sonic
vacuum (like outer space).
boom for light (called Cherenkov
Radiation) which emits a cone of
Light Reflection or Source?
blue light.
A candle is a light source. So is a
campfire, a light bulb, and the sun.
An apple, however, reflects light.

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