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

BLUE SKIES SMILING AT ME 47
of colors. It just produces more green than any other color. When
they are all combined, our eye still perceives the light as white.
Or yellow. Take your pick.
Okay, I lied a minute ago; I still have one more question. If the
sky isn’t blue because it reflects the color of the oceans, why are
the oceans blue? Do they reflect the sky’s color? No. Of course,
they do reflect it a little; they look more steely on overcast days
and bluer on sunny days. But the real reason is a bit subtler. It
turns out that water can absorb red light very efficiently. When
you shine a white light through deep water, all the red light gets
sucked out by the water, letting only the bluer light through. When
sunlight goes into water, some of it goes deep into the water and
some of it reflects back to our eyes. That reflected light has the red
absorbed out of it, making it look blue. So the sky is blue because
it scatters blue light from the Sun, and the oceans look blue
because that’s the only light they let pass through.
PPP
At the start of this section, I promised you’d understand all this
well enough to explain it to a five-year-old. If a little kid ever asks
you just why the sky is blue, you look him or her right in the eye
and say, “It’s because of quantum effects involving Rayleigh scattering
combined with a lack of violet photon receptors in our retinae.”
Okay, that might not work. In reality, explain to them that the
light coming from the Sun is like stuff falling from a tree. Lighter
things like leaves get blown all around and fall everywhere, while
heavier things like nuts fall straight down without getting scattered
around. Blue light is like the leaves and gets spread out all over the
sky. Red light is like the heavier material, falling straight down
from the Sun to our eyes.
Even if they still don’t get it, that’s okay. Tell them that once
upon a time, not too long ago, nobody knew why the sky was
blue. Some folks were brave enough to admit they didn’t understand
and went on to figure it out for themselves.
Never stop asking why! Great discoveries about the simplest
things are often made that way.