MOTION MOUNTAIN

102 3 what is light?
primary
infrared
rainbow
secondary
infrared
rainbow
secondary
visible
rainbow
primary
visible
rainbow
Ref. 62
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Ref. 63
F I G U R E 59 The same rainbow
in the visible and in the infrared,
showing how infrared comes
before red (© Stefan Zeiger).
wave oscillate in a second!The frequency value, between 375 and750 THz, is exactly as
predicted. The frequency value is so high that its detection was impossible for a long
time. But with these modern experiments the dispersion relation of light,ω=ck, has
finally been confirmed in all its details, and to extremely high precision.
The result of all these experiments is: light waves, like all other waves, can be distinguished
by their wavelength or frequency values. The most important categories are
listed in Table 14. For visible light, the wavelength lies between0.4 µm, corresponding
to violet, and0.8 µm, corresponding to red. The wavelength of a visible harmonic light
wave determines itscolour.
Light is a wave. This also ends a discussion that led to intense debate in the Middle
Ages: How narrow can a light beam be?
Light and other electromagnetic waves
The experiments mentioned so far showed that electromagnetic waves exist, that they
move with the same speed as light, and that light is a wave. To confirm that light waves are
indeed electromagnetic is more difficult. The most convincing proof would be to repeat
Hertz’s experiments for light. In Hertz’s experiment, shown in Figure 53, the receiver is
a simple open metal circle; when the wave – more precisely, its magnetic field – arrives,
a spark is generated and the wave is thus detected.
In an almost incredible feat of miniaturization, in 2009, the research group of KobusKuipersmanagedtomake
metalringsmuchsmallerthanamicrometre, andrepeat
Hertz’s experiment for light. An impression of their experiment is given in Figure 60.
They could clearly discern the maxima and minima of waves, as well as their polariza-
Motion Mountain – The Adventure of Physics copyright © Christoph Schiller June 1990–November 2015 free pdf file available at www.motionmountain.net