is explained in another
way. Suppose I throw balls of all colors to you, having trained you to
keep all the balls except the red ones. I throw you a blue ball; you
keep it. I throw a red ball; you throw it back. I throw a green ball;
you keep it. I throw a yellow ball; you keep it. I throw two balls at
once, yellow and red; you keep the yellow and throw back the red. I
throw a blue and yellow ball at the same time; you keep both balls.

Now suppose I change this a little. Instead of throwing balls, I shall
throw lights to you. You are trained always to throw red light back
to me and always to keep (absorb) all other kinds of light. I throw a
blue light; you keep it, and I get no light back. I throw a red light;
you throw it back to me. I throw a green light; you keep it, and I get
no light back. I throw a yellow light; you keep it, and I get no light
back. I throw two lights at the same time, yellow and red; you keep
the yellow and throw back only the red. But yellow and red together
make orange; so when I throw an orange light, you throw back the red
part of it and keep the yellow.

Now if we suppose that instead of throwing lights to _you_ I throw
them to molecules of dye which are "trained" to throw back the red
lights and keep all the other kinds (absorb them and change them to
heat), we can understand what the dye in a red sweater does. The dye
is not really trained, of course, but for a reason which we do not
entirely understand, some kinds of dye always throw back (reflect) any
red that is in the light that shines on them, but they keep all other
kinds of light, changing them to heat. Other dyes or coloring matter
always throw back any green that is in the light that shines on them,
keeping the other colors. Blue coloring matter throws back only the
blue part of the light, and so on through all the colors.

So if you throw a white light, which contains all the colors, on a
"red" sweater, the dye in the sweater picks out the red part of the
white light and throws that back to your eyes (reflects it to you) but
it keeps the rest of the colors of the white light, changing them to
heat; and since only the red part of the light is reflected to your
eyes, that is the only part of it that you can see; so the sweater
looks red. The "green" substance (chlorophyll) in grass acts in the
same way; only it throws the green part of the sunlight back to your
eyes, keeping the rest; so the part of the light that reaches you f