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?



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.


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.

enter image description here

  • $\begingroup$ But won't it still have green light reflected? $\endgroup$
    – nzkiwi888
    Nov 6 '16 at 8:16
  • $\begingroup$ Where is the green light coming from? $\endgroup$
    – Farcher
    Nov 6 '16 at 8:37
  • $\begingroup$ True. If white light were shone against the green liquid it would appear green still right? $\endgroup$
    – nzkiwi888
    Nov 6 '16 at 8:58
  • $\begingroup$ Remember that no lamp (except lasers) produces spectrally pure colours. The "blue" light will certainly contain other colours as well, including green. Similarly, the liquid will also reflect some other colours besides green. Hence the liquid will still reflect part of the light and will look greenish. The exact colour depends on the "blue" of the light, and the "green" of the liquid. $\endgroup$
    – hdhondt
    Nov 6 '16 at 9:03
  • 1
    $\begingroup$ @Dmitry It is possible to have a material which absorbs red and green wavelengths which also reflects yellow wavelengths. $\endgroup$
    – Farcher
    Sep 10 '18 at 5:26

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