We saw in a previous chapter how the spectroscope splits up light-waves
into their colours. But nature is constantly splitting the light into
its different-lengthed waves, its colours. The rainbow, where dense
moisture in the air acts as a spectroscope, is the most familiar
example. A piece of mother-of-pearl, or even a film of oil on the street
or on water, has the same effect, owing to the fine inequalities in its
surface. The atmosphere all day long is sorting out the waves. The blue
"sky" overhead means that the fine particles in the upper atmosphere
catch the shorter waves, the blue waves, and scatter them. We can make a
tubeful of blue sky in the laboratory at any time. The beautiful
pink-flush on the Alps at sunrise, the red glory that lingers in the
west at sunset, mean that, as the sun's rays must struggle through
denser masses of air when it is low on the horizon, the long red waves
are sifted out from the other shafts.
Then there is the varied face of nature which, by absorbing some waves
and reflecting others, weaves its own beautiful robe of colour. Here and
there is a black patch, which _absorbs_ all the light. White surfaces
_reflect_ the whole of it. What is reflected depends on the period of
vibration of the electrons in the particular kind of matter. Generally,
as the electrons receive the flood of trillions of waves, they absorb
either the long or the medium or the short, and they give us the
wonderful colour-scheme of nature. In some cases the electrons continue
to radiate long after the sunlight has ceased to fall upon them. We get
from them "black" or invisible light, and we can take photographs by it.
Other bodies, like glass, vibrate in unison with the period of the
light-waves and let them stream through.