visible-light
The color of the bulb at the bottom of a lava lamp can be changed. A special bulb can be put in that changes through the colors, blue green and white. This means in some circumstances, the lava, although green, will appear black. What color bulb will make it appear black?
I know the answer is blue, but when the light is shined against the green liquid, I understand it will be absorbed, but why will it turn black?
Thank
This has as much to do with biology as with physics. The long answer on the biology is here. The biology in summary: the human eye has only three different "color-sensitive" elements, and uses a complex combination of the amount of response it sees from each of these to assign a "color" to the image.
Because there are only three sensitivities in the human eye, there are a variety of different techniques using only three basic (in some schemes called primary) colors to represent colors to humans.
The actual frequency of the light emitted from the part of the rainbow we call green may have the same effect on the human eye as something we get by mixing our blue crayon with our yellow crayon on a piece of paper. But it is easy to build a detector which will trivially differentiate between monochromatic green from one slice of a rainbow, and a mixture of the light reflected from blue and yellow crayon pigments.
In principle, humans might have evolved a different eye with four or five different "color" detectors, in which case the schemes needed to make color images would probably need to have four or five basic colors, and the images we see from our current three color representations would seem to be washed out, missing something important. But the eye didn't develop that way.
The sky does not skip over the green range of frequencies. The sky is green. Remove the scattered light from the Sun and the Moon and even the starlight, if you so wish, and you'll be left with something called airglow (check out the link, it's awesome, great pics, and nice explanation).
Because the link does such a good job explaining airglow, I'll skip the nitty gritty.
So you might be thinking, "Jim, you half-insane ceiling fan, everybody knows that the night sky is black!" Well, you're only half right. The night sky isn't black. The link above explains the science of it, but if that's not good enough, try to remember back to a time when you might have been out in the countryside. No bright city lights, just the night sky and trees. Now when you look at the horizon, can you see the trees? Yes, they're black silhouettes against the night sky. But how could you see black against black? The night sky isn't black. It's green thanks to airglow (or, if you're near a city, orange thanks to light pollution).
Stop, it's picture time. Here's an above the atmosphere view of the night sky from Wikipedia:
And one from the link I posted, just in case you didn't check it out:
See, don't be worried about green. The sky gets around to being green all the time.
Best Answer
If you shining only blue light on a substance, the green liquid, which absorbs blue light then no light is coming from the green liquid. Absence of light is blackness.
Update
One of the best examples of these ideas about colour occurs in nature - green leaves.
The light which hits the leaves is used in photosynthesis.
Many people are under the impression that it is green light which the plant needs for photosynthesis.
That is not so as it is the red and blue light which is needed.
So the leaves absorb all the colours contained in white light absorb the red and the blue ends of the spectrum and re radiate the green leaves.
Hence the leaves are seen to be green.
Here is an absorption spectrum to illustrate this point.