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We all know that light has constant speed, and variable wavelength, and frequency. Suppose I have a big red glass square sheet. When white sun rays pass through it, their colour changes from white to red. I am unable to understand this effect

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    $\begingroup$ Please do not edit your question to change it entirely. If you are currently blocked from asking questions, then you must wait until that restriction is removed. Do not try to find ways around it. $\endgroup$ – tpg2114 Aug 11 at 16:45
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"White light" contains all the colors and when the light passes through the red glass all other colors are absorbed but the red goes through.

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  • $\begingroup$ This is an incomplete answer. The color red can also be perceived by a combination of frequencies, if only this combination of frequencies passes the glass and all the rest are absorbed, the glass will look red. $\endgroup$ – LostCause Jul 14 at 20:03
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    $\begingroup$ @LostCause I did not know that frequencies "perceived" anything... $\endgroup$ – user207455 Jul 14 at 20:07
  • $\begingroup$ @LostCause, if you have a more complete answer, please post it. $\endgroup$ – David White Aug 11 at 19:23
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    $\begingroup$ "The color red can also be perceived when the retina receives any of a range of frequencies. If only photons in this frequency range arrive at the retina, while excluding photons in the ranges that perceive green or blue, then red is perceived by the brain." $\endgroup$ – Ross Presser Aug 11 at 22:25
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First think, why is that glass sheet Red? Answer: it permits only certain wavelengths to pass through, absorbing all others. Those that passed are predominantly red, as perceived to our eyes.

White light being a combination of all colours contains the wavelength range of red, which is the only part of the spectrum that the glass lets pass through.

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Essentially, white light is a combination of all colours in the visible spectrum. Now, when the light passes through the red glass, The glass absorbs all the colours of shorter wavelengths/higher frequencies and allows only red light(longer wavelength) to pass through it. I highly recommend you to study Color subtraction from this link:

https://www.physicsclassroom.com/class/light/Lesson-2/Color-Subtraction

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