Q.62. Which property of light is involved in the formation of shadow? Explain in which case you will not get penumbra at all.

I think rectilinear propagation of light.

A "penumbra" is that region around the umbra where the shadow is only partial, or imperfect. You get these when the light source is larger than a single point. These form because while some of the light from the source gets blocked by the shadowing object, not all of it does. If you are in the penumbra looking towards the light source, you will see part of it visible, and part of it blocked. A point light source will either be all visible or all blocked, but an extended source can be partially viewable beyond the edge of the shadowing object. The same is true of multiple sources. 


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