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shown. So it was easy to see how another light particle of a different
light acts like a wave. wavelength, which is why glow-in-
the-dark toys don’t reflect back the
James Maxwell predicted light same color light they were charged
would be an electromagnetic wave with. If light were a wave, the
(more on this in Unit 10) in the atom would emit back the same
1860s after doing several color light, but it doesn’t. So
experiments with electricity and clearly light acts like a particle in
magnetism. He further predicted this case.
that this wave would travel at
speed c (where c = 186,000 miles So we have two seemingly
per second). opposing points of view – light
sometimes acts like a wave, and
In the 1800s, scientists had sometimes acts like a particle. So
observed the photoelectric effect, which is it?
in which a light particle hits a free
electron and knocks it out of a It turns out to be both. Light is
metal plate. A particle (like a both a particle and wave, and
marble) can do this, but waves furthermore, these two ideas
don’t act this way at all. actually complement each other.
You need both in order to describe
all the different ways that light
behaves. We’ll cover this in a lot
more detail in Unit 9.
Ideal Gas Law
In 1905, Einstein explained the We live in a sea of air called the
photoelectric effect by suggesting atmosphere. Everything around us
that light comes in bundles and has atoms pushing on it equally in
behaves like it was a particle. all directions, a lot like a room full
of continuously-bouncing ping pong
Glow-in-the-dark toys work on a balls.
similar principle: the light particles
hit the electrons and transfer some Think of each ping pong ball as a
of their energy to the electron. The molecule. If we raise the
result is that the electron emits temperature of the molecules, they
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