When a light source illuminates an opaque object, a shadow forms behind the object — the region where light does not reach. The sharpness of its edge, however, depends on the size of the source. If the source is point-like, the shadow’s edge is sharp: from every point behind the object, either the source is visible, or it is not. If instead the source has a finite extent, an intermediate zone of penumbra appears, where only part of the source is visible: there is light, but weakened, and the shadow fades gradually.
An extended source (the lamp) casts a sharp central shadow surrounded by two bands of penumbra, where only part of the light arrives.
You are an ant on the screen
Imagine being an ant on the screen. If you see all of the lamp, you are in full light. If you see only part of it, you are in penumbra. If you do not see it at all, you are in shadow.
Collegamenti
Argomenti: Ottica
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