Bad Astronomy: Misconceptions and Misuses Revealed, from ...

BLUE SKIES SMILING AT ME 41
nitrogen and oxygen (N 2 and O 2 ) in the air can intercept that light.
Almost like little billiard balls, photons—the fancy name for particles
of light—bounce off these molecules and head off in a different
direction every time they hit one. In other words, nitrogen and
oxygen molecules scatter the incoming sunlight like bumpers in a
pinball machine.
In the mid-1800s the brilliant British physicist Lord Rayleigh
found out a curious thing: this scattering of light by molecules depends
on the color of the light. In other words, a red photon is a
lot less likely to scatter than a blue photon. If you track a red photon
and a blue photon from the Sun as they pass through the air,
the blue photon will bounce off its original course pretty quickly,
while a red one can go merrily on its way all the way down to the
ground. Since Lord Rayleigh discovered and quantified this effect,
we call it Rayleigh scattering.
So, what does this have to do with the sky being blue? Let’s
pretend you are a nitrogen molecule floating off in the atmosphere
somewhere. Nearby is another molecule just like you. Now let’s say
that a red photon from the Sun comes at you. As Lord Rayleigh
found, you don’t affect the red photon much. It pretty much ignores
you and your friend and keeps heading straight down to the ground.
In the case of this red light, the Sun is like a flashlight, a shining
source of red light in one small part of the sky. All the red photons
the Sun emits come straight from it to some observer on the ground.
Now let’s imagine a blue photon coming in from the Sun. It
smacks into your friend, rebounds off him, and obligingly happens
to head toward you. From your point of view, that photon comes
from the direction of that molecule and not the Sun. Your molecule
friend saw it come from the direction of the Sun, but you
didn’t because it changed course after it hit him. Of course, after it
hits you that photon can rebound off you and go off in another
direction. A third nitrogen molecule would see that photon as
coming from you, not the Sun or the first molecule.
Now you’re a person again, standing on the ground. When a
blue photon from the Sun gets scattered around, at some point it
will hit some final air molecule near you, go through a final scattering,
and head into your eye. To you that photon appears to