88 The Sky 
They also contribute to the indistinctness of distant views 
usually referred to as optical haze and the shimmering ob- 
served near the surface of the water or on deck on hot days. 
One of the most spectacular of refractive phenomena is the 
so-called green flash. If you observe the sun as it is setting, 
on an evening when the horizon is exceptionally clear, just 
as the last portion of the sun’s disk disappears below the 
horizon you will see a bright yellow flash, a green one, and 
then possibly a blue and a violet one. This occurs because in 
dispersion the blue and green light are bent more than the 
red and yellow. As a result they are the last to disappear. In 
the morning at sunrise the same thing may be observed but 
in reverse order: blue appearing first, then green, yellow, and 
finally the red disk of the sun itself. The green flash is not a 
very easy thing to observe, but it is worth repeated trials 
because of its beauty and the fact that very few people have 
ever seen it. So much for phenomena due to refraction of the 
air itself. We now turn to optical effects due to refraction by 
particles suspended in the air: raindrops, ice crystals, and 
snowflakes. 
RAINDROPS CAUSE ILLUSIONS TOO: RAINBOWS 
The rainbow is not as simple a thing as it may first seem. 
Sometimes a single rainbow stretches from horizon to hori- 
zon. Often you will see a number of concentric rainbows, 
some with their colors running in an opposite direction to 
that of their neighbors. All of the different rainbows may be 
accounted for by means of rigorous mathematical theory; 
however we will describe here how they are formed in a 
qualitative way. 
Rainbows occur wherever there are a sufficient number of 
