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Quantum physics came into being because classical physics
could not explain experimental results on the atomic
scale. Max Planck was the first to propose quantized
energy levels in 1900, and Einstein explained the photoelectric
effect in 1905. (Einstein was awarded the Nobel Prize
in 1921 for his explanation of the photoelectric effect,
NOT relativity as most people think.)
Three conclusions from the photoelectric effect could
only be explained using quantum theories:
- the emission of an electron from a clean metal
surface is independent of intensity of the light
- the emission of an electron does depend on frequency
and has a minimum "threshold" frequency
below which no emission occurs, and
- there is no "time lag" while the electron
"soaks up" energy from a weak light source.
The electron is emitted immediately, even if the light
is very feeble. All you need is a high enough frequency.
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| In your activity,
you used a mercury light source which produces an intense,
nearly white light. To select the various frequencies
or wavelengths, the white light was passed through a diffraction
grating producing a "rainbow" of color, much
like the prism I showed
you earlier. |
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Diffraction gratings are extremely useful
devices for separating component wavelengths and hence
the identification of those components in a source. You
can identify the chemical composition of various compounds
on earth as well as the composition of stars by observing
their "spectra".
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A diffraction grating uses interference, like in a
two slit case, but with a large number of slits, about
1000 per millimeter. As the number of slits increases,
the maxima get narrower and sharper, while the minima
spread out. The result for N slits is fine "spikes"
for the maxima separated by broad dark bands. This is
useful because if you have many wavelengths in a source,
an ordinary two-slit approach which gives wide maxima
will have the various colors overlapping too much. The
grating spreads out the colors making their angular
positions easy to identify.
The lack of a time lag in the photoelectric effect
suggested that light travels in discrete little bundles
or packets called "photons". The Compton Effect,
or the wavelength shift in the scattering of X-rays,
also demonstrated that light must be composed of these
packets. However, the interference of light can only
be explained if light is a wave.
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| If you shine light that is so dim through
a double slit arrangement, the light arrives on the screen
as single photons. You can detect where these photos hit
the screen one-by-one. At first the photons appear randomly
spread across the screen. |
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But after a time, you see the emergence of bright bands
and dark bands. Somehow the single photons "know"
that there are two slits and the photons interfere with
themselves! This suggests that light travels as a wave,
but when it interacts with matter, as in the photoelectric
effect, it does so as a particle.
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How can you detect one single photon?
You use a device called a "photomultiplier"
which works using the photoelectric effect. A single photon
hits a small metal plate with a very low work function,
so a single electron is emitted. |
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| That electron is accelerated through
a potential and strikes another metal, where its energy
frees several electrons. Those new electrons are accelerated
across another potential where each one frees several
electrons. This "cascading" effect will eventually
produce a current that is high enough to be measured.
Hence, the arrival of one photon can be detected. |
