MOTION MOUNTAIN

134 3 what is light?
vocabulary. On the other hand, we have not yet finished the study of the vacuum; vacuum
will keep us busy for the rest of our walk, starting with the following part of adventure, on
Challenge 139 d quantum physics. In fact, quantum physics shows that all experimental values in Table 15
require amendments.
Challenge 140 s
Challenge 141 s
Challenge 142 s
Challenge 143 e
Ref. 95
Challenge 144 s
Challenges and fun curiosities about light, polarization and
the geometric phase
Since light is a wave, something must happen if it is directed to a hole less than its
wavelength in diameter. What exactly happens?
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On a sunny day at moderate latitudes on the Earth, sunlight has a power density of
1 kW/m 2 . What is the corresponding energy density and what are the average electric
and magnetic fields?
Spectrally pure light is often called ‘monochromatic’. Why is this a misnomer?
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Electrodynamics shows that light beams always push; they never pull. Can you confirm
that ‘tractor beams’ are impossible in nature?
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It is well known that the glowing material in light bulbs is tungsten wire in an inert
gas. This was the result of a series of experiments that began with the grandmother of
all lamps, namely the cucumber. The older generation knows that a pickled cucumber,
when attached to the230 V of the mains, glows with a bright green light. (Be careful; the
experiment is dirty and dangerous.)
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Light beams have an effective temperature and entropy. Though not often discussed
nowadays, the thermodynamics of light has been explored in great detail by Max von
Laue (b. 1879 Koblenz, d. 1960 Berlin) in the years between 1900 and 1906. Von Laue
showed that usual light propagation in empty space is a reversible process and that the
entropy of a beam indeed remains constant in this case. When light is diffracted, scattered
or reflected diffusively, the effective temperature decreases and the entropy increases.The
most interesting case is interference, where entropy usually increases, but sometimesdecreases.
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We saw that light has energy, linear momentum, angular momentum, entropy, temperature,
pressure, chemical potential and, as we will see in the next volume, consists of
quantons. It makes thus sense to state:
⊳ Light is a substance.
Enjoy exploring this conclusion.
Motion Mountain – The Adventure of Physics copyright © Christoph Schiller June 1990–November 2015 free pdf file available at www.motionmountain.net