:yuck: I wanna ask if there is coloured shadow ??If so, how to make it??
You might say that anything that selectively transmits certain optical wavelengths has a colored shadown. Tinted glass or colored saran wrap, for example.
Color would mean that light is getting through which makes it inherinitly, not a shadow.
i don't think that is the definition of a shadow.
imagine a room (more likely a stage) where there are a bunch of identical red filter lamps flooding the place. then imagine one single cyan (blue-green) lamp that is shining on you or some other object that has enough brightness so that most of the places where this cyan light mixes with the red light, it looks pretty much white. but you or that object casts a shadow because there is just one cyan light source, however there is red light everywhere (because there are many different red light sources in different locations). where there is the absence of cyan light because you or that object is blocking it, there is a shadow. but that shadow will be red.
rbl is correct. A shadow is going to be the ambient colour of the space minus whatever light is being blocked.
That's why shadows on white snow are blue; it's not an illusion. The shadowed areas of snow are still well-lit by strongly-blue sky light, whereas the more yellow light from the sun is blocked.
Note by the way, that in rbj's example and my example we've used complimentary colours (cyan-red, yellow-blue), but this is merely coincidence. On a stage lit with red and green spotlights the shadows from objects blocking the red spotlight will be green.
Nice example, rbj. I thought of an even better example of a colored shadow with partial transmission: a lunar eclipse! When the sun, earth, and moon are lined up in space, then the moon sits in the earth's shadow. It's not black, however. Why? Because there is a small amount of red light refracted from the atmosphere along the edges of the earth, the same light we see during a sunset.
We should keep in mind when considering these things that a "shadow" is a concept with relatively little scientific significance. They generally arise when a direct source of light is shining on a surface that's being obscured by another object. In human perception, this effect is exaggerated by two biological facts:
1) The human eye tends to exaggerate color and shade contrast. This ability has obvious evolutionary benefits for picking out certain objects (say, a predator) on a continuous background.
2) We only see a small part of the electromagnetic spectrum.
That second one means that we can't tell whether a particular shadow extends beyond visible wavelengths (~400-800 nanometers). A human, for example, is optically thin to some radio and gamma-rays, so if we could see that part of the spectrum, there would no noticable shadow.
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