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Unformatted text preview: miliar interference pattern seen in Figure 1.1. The dark patches
of the interference pattern occur where the wave from the ﬁrst slit arrives
perfectly out of sync with wave from the second slit, while the bright points
are where the two arrive in sync. For example, the bright spot right in the
middle is bright because each wave travels the exact same distance from their
respective slit to the screen, so they arrive in sync. The ﬁrst dark spots are
where the wave from one slit traveled exactly half of a wavelength longer than
the other wave, thus they arrive at opposite points in their cycle and cancel.
Here, it is not the intensities coming from each slit that add, but height of
the wave. This diﬀers from the case of bullets: I12 (y ) = I1 (y ) + I2 (y ), but
h12 (y ) = h1 (y ) + h2 (y ), and I12 (y ) = h(y )2 , where h(y ) is the height of the
wave and I (y ) is the intensity, or energy, of the wave.
Before we can say what light does, we need one more crucial piece of
information. What happens when we turn down the intensity in both of these
In the case of bullets, turning down the intensity means turning down the
rate at which the bullets are ﬁred. When we turn down the intensity, each
time a bullet hits the screen it transfers the same amount of energy, but the
frequency at which bullets hit the screen becomes less.
With water waves, turning down the intensity means making the wave
amplitudes smaller. Each time a wave hits the screen it transfers less energy,
but the frequency of the waves hitting the screen is unchanged.
Now, what happens when we do this experiment with light. As Young
observed in 1802, light makes an interference pattern on the screen. From this
observation he concluded that the nature of light is wavelike, and reasonably
so! However, Young was unable at the time to turn down the intensity of light
enough to see the problem with the wave explanation.
Picture now that the observation screen is made of thousands of tiny little
photo-detectors that can detect the energy they absorb. For high intensities
the photo-detectors individually are picking up a lot of energy, and when
we plot the intensity against the position y along the screen we see the same
interference pattern described earlier. Now, turn the intensity of the light very
very very low. At ﬁrst, the intensity scales down lower and lower everywhere,
just like with a wave. But as soon as we get low enough, the energy that
the photo-detectors report reaches a minimum energy, and all of the detectors
are reporting the same energy, call it E0 , just at diﬀerent rates. This energy
corresponds to the energy carried by an individual photon, and at this stage
we see what is called the quantization of light.
Photo-detectors that are in the bright spots of the interference pattern 1.1. THE DOUBLE SLIT EXPERIMENT 5 report the energy E0 very frequently, while darker areas report the energy E0
at lower rates. Totally dark points still report nothing. This behavi...
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This document was uploaded on 09/22/2013.
- Fall '13