Light Absorption and Transmission

[mangoplant]

Hello all, I had a question out of curiosity. Say you have a green piece of paper. This paper is green because it absorbs the wavelength of lights corresponding to the other colors of the visible spectrum and reflects (transmits) green light, thus appearing green. If one were to shine a laser that is also green (the same green as the paper), would the paper heat up or would nothing happen? I would imagine it would heat up if it were a laser that was not green as the paper would absorb the light, but what if the laser is green?

 

[UltrafastPED]

If the paper absorbs the light, it will heat up.

Your specification would lead you to believe that the "green paper" is a perfect reflector; it is not. All you really know is that it absorbs the other visible light; it will also absorb a portion of the green light.

You would need to conduct tests of the absorption and reflection intensities in order to know how much of the green light is actually absorbed.

For any "real paper" a great deal of the light is absorbed. The shinier the paper, the greater more likely that it is reflecting more strongly, and absorbing less.

 

[mangoplant]

Ah I see. So say there were some theoretical liquid that had a single absorption peak at some random wavelength, say 300 nm. If I understand you correctly, only light with 300 nm wavelength would cause this theoretical liquid to heat up? All other light that is not 300 nm wavelength would pass through the liquid without interacting with it?

 

[Drakkith]

Yes, if the liquid absorbed ONLY radiation with a wavelength of 300 nm then all other would pass through or be reflected. Only the 300 nm light would heat it up. But there are no real materials that can achieve this. All real materials absorb at least some small part of all wavelengths.

 

[ehild]

Ah I see. So say there were some theoretical liquid that had a single absorption peak at some random wavelength, say 300 nm. If I understand you correctly, only light with 300 nm wavelength would cause this theoretical liquid to heat up? All other light that is not 300 nm wavelength would pass through the liquid without interacting with it?

I would not say that the material does not interact with light with wavelength it does not absorb. The speed of light is slower in the material than in vacuum. The material slows it down by interacting with it. The material also reflects some of the incident light and changes its direction of propagation. Only this interaction does not consume energy, similarly to elastic collision.

ehild