I tried to look up for answer about why don't white objects form images like mirrors do. Answers suggested the reflection wouldn't be specular. But how could you be convinced, as projectors use them well to reflect light and produce clear images?


1 Answer 1


If a screen reflected as a mirror, a viewer would see the image of the projector with a light beam coming out. Good materials for a screen for conventional projection of images have to scatter light back as point sources on their surface. White surfaces do that, they scatter light as point sources incoherently thus the eye perceives "white" .

The wavefront of the beam of the projector has a space distribution which carries the image , i.e. different frequencies and intensities for the different colors of the image projected. These hit in space different point sources which reflect back the specific color as if it is coming from that point source, and thus the image is formed for the observer.

  • $\begingroup$ Thanks, but i am still concerned about what the difference between the light coming from the projector and light coming from a person standing in front of the curtain would be. Wouldn't the light coming out of the person (or his clothes) too be the combinations of different frequencies and intensities for different colors. Why doesn't the screen show same effects in this case? $\endgroup$
    – Nabin
    Nov 19, 2016 at 6:55
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    $\begingroup$ a person is not a light source to start with,the mirror reflection is secondary and is not strong enough as a light source. In addition the point sources of a screen, in contrast to a mirror,disperse incoherently the light falling on them . the important thing from the light of the projector is that it is coherent, the phases between the frequencies are fixed and the beam trajectory is linear. This is not the case from the light reflected from a person on a white screen. $\endgroup$
    – anna v
    Nov 19, 2016 at 7:09
  • $\begingroup$ "a person is not a light source" tats new $\endgroup$ Nov 19, 2016 at 7:25

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