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Unit 9: Light Page 15
Light comes in packets called Usually, the amount of energy a
photons. This idea seems a bit photon has determines whether it’s
strange, because most of your a particle or a wave. Lower energy
everyday experience with light is photons (like radio and microwave)
when it acts like a wave. travel like waves, and higher
energy gamma rays interact like
If you try to detect light, it behaves particles.
like particles (by the photoelectric
experiment, the Compton effect, or In most cases, light behaves like a
even on your video camera pixels). wave – a disturbance moving
energy from one spot to another.
If you don’t try to detect light, it You can measure its wavelength
acts like a wave and has wave (the distance between two peaks)
interference patterns. You’re on and frequency (the number of
the larger scale when you don’t try peaks passing a point each
to detect the individual packets of second). I’ll show you how to do
light, so you don’t notice that light this with the experiments in this
is quantized. unit.
It would be like trying to go the When we think of light as tiny
store and buying 2 tablespoons of packets (‘quanta’ of energy) called
eggs. They just don’t come photons, then we’re talking about
packaged that way. There’s a light acting like a particle. Photons
minimum set package for the carry no mass, but a fixed amount
amount egg you need in a recipe. of energy. The energy depends on
That’s like trying to detect a the wavelength. For example, blue
photon. light (shorter-wavelength photons)
has more energy than red light
However, if you’re making 2 million
(longer-wavelength photons).
cookies, you’d order eggs by the
truckload and no longer worry Einstein received a Nobel Prize for
about if you had exactly the right figuring out what happens when
amount down to the single egg – you shine blue light on a sheet of
you’d say “give me 25 pounds of metal. When he aimed a blue light
eggs” or something similar. That’s on a metal plate, electrons shot off
when light acts like a wave – it’s the surface. (Metals have electrons
more of a continuous effect. which are free to move around,
which is why metals are electrically
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