MOTION MOUNTAIN

what is light? 123
F I G U R E 76 A green flash above the setting Sun and one above the Moon, showing also the colour
change of the Moon rim (© Andrew Young and Laurent Laveder/PixHeaven.net).
Ref. 83, Ref. 84
Ref. 85
F I G U R E 77 Milk and water simulate the evening sky (© Antonio
Martos).
Rayon-vert.The green flash is often seen on tropical beaches, for example in Hawaii, and
from the decks of ships in warm waters.
Even pure air splits white light. However, this effect is not due to dispersion, but to
scattering. Wavelength-dependent scattering, mainly Rayleigh scattering, is the reason
that the sky and distant mountains look blue and that the Sun looks red at sunset and
sunrise. (The sky looks black even during the day from the Moon.) You can repeat this
effect by looking through water at a black surface or at a lamp. Adding a few drops of
milk to the water makes the lamp yellow and then red, and makes the black surface blue
(like the sky seen from the Earth as compared to the sky seen from the Moon) as shown
in Figure 77. More milk increases the effect. For the same reason, sunsets are especially
red after volcanic eruptions.
In the evening, however, the sky is blue for another, far less known reason: at the time
around sunset, the sky is blue mainly because of the ozone layer. Ozone is a blue gas.
Without ozone, the sky would be yellowish during sunsets.
Motion Mountain – The Adventure of Physics copyright © Christoph Schiller June 1990–November 2015 free pdf file available at www.motionmountain.net