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Two transparent plastic sheets say of red and blue color overlap as shown in figure . An observer looks at a clear sky through the sheets.

figure

He will observe light coming through sections as,

SECTION 1: Red color, less bright than usual

SECTION 2: Blue color, less bright than usual

SECTION 3:Dark,almost no light

Now the questions

(1).Why does say red sheet transmits(in section 1) red component and absorbs all other colors?(same for section 2)

(2).Why does brightness decreases in section 1 and 2?

(3).Why does observer see almost no light from section 3?

(4). According to me If "a colored sheet (as in section 1 or 2) transmits its own color component and absorbs all other colors" is true then why didn't observer observe purple color through section 3?

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1 Answer 1

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The red sheet absorbs all wavelength except "red". The same is true for the blue sheet, except that here the "special colour" is "blue".

  1. Why does say red sheet transmits red component and absorbs all other colours?

This is due to the atomic levels of the material. I suppose that the band structure of the red sheet is such that the occupied states are not coupled to unoccupied states by all wavelength (in the visible range) except red. Thus, all wavelength get absorbed except red.

  1. Why does brightness decreases?

Consider that we use white light with a constant intensity $I_0$. White light consists of all possible wavelength (in the visible range). The red sheet absorbs all wavelength except the "red" once. Therefore, the intensity naturally decreases.

  1. Why does observer see almost no light from section 3?

If all wavelength except "blue" are absorbed first and then all wavelength except "red" are absorbed, there is nothing left.

  1. why didn't observer observe purple color through section 3?

Please have a look at the spectrum of visible light. Purple sits at one end of the spectrum of visible light (high energy side), while red sits at the other end of the spectrum (low energy).

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