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On the wall , i Walter had put a poster & when I removed the poster. I shone blue + red + white light on my white wall in the dark room.

Under blue light only , I am able to see so many details. Also , I see a reflection of a bit green color. It’s because of my camera it is looking like violet color. Not just wall but even on floor , the colour of dust appears to be green. Why is that ?

Under red & white light , I do not see these tiny details. Especially under red , I don’t see any of these details. Why is that ?

I do have studied about photo electric effect & think that it is somewhere used here. But I m not able to think how ?

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As a guess, you might be picking up some fluorescence. Fluorescence absorbs light with a shorter wavelength and reemits is at a longer wavelength. Blue to green fluorescence is pretty common, but you are unlikely to notice it since the fluoresced color is so low compared to the excitation light. The room has to be dark and the excitation light needs to be a short wavelength where your eye and camera are less sensitive. Black lights which emit UV light make it easier to perceive fluorescence. Some white surfaces like high brightness printer papers have fluorescent brighteners. Fluorescence is a very important phenomenon in laboratory diagnostics testing.

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  • $\begingroup$ Ok. Why is it so visible in dark room especially? $\endgroup$
    – S.M.T
    Sep 24 at 14:50
  • $\begingroup$ @S.M.T Fluorescence is a very faint phenomenon. Normally you would never notice it. The room has to be dark and the excitation light needs to be a short wavelength where your eye and camera are less sensitive. Black lights which emit UV light make it easier to perceive fluorescence. $\endgroup$
    – Eric S
    Sep 24 at 15:06
  • $\begingroup$ It's also possible there isn't fluorescence but rather SMT's eyeball & neurons are interpreting the reflected colors as green-ish. We really need a proper grating spectrometer to verify what's happening in this situation. $\endgroup$ Sep 27 at 12:23
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    $\begingroup$ @CarlWitthoft Just trying a black light is probably easier. If there is fluorescing surfaces, a black light will very likely excite it. $\endgroup$
    – Eric S
    Sep 27 at 16:18

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