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Light & Colour · Question 04 of 20

How shadows work

A shadow is not a thing — it is an absence with edges, and the edges tell you about the light.

Plate II — Umbra & penumbra straight-line rays · extended source
Grow the lamp and watch the shadow’s sharp edge dissolve into fuzz.
lampumbrapenumbrasolid lines: blocked from the whole lamp · dashed: blocked from part of it
FIG. II — UMBRA & PENUMBRA
Lamp size soft-edged
Small lamp → sharp shadow. Big lamp → fuzzy halo.
Object position midway
Full shadow (umbra)
124units
Fuzzy fringe (penumbra)
131units
Grow the lamp and the shadow's edges go fuzzy — places on the wall that can see part of the lamp are only part-dark. Shrink the lamp to a dot and the shadow snaps sharp. Push the lamp big enough and the dark core disappears entirely.
The short answer

Light travels in straight lines, so when something blocks it the light simply can't reach the floor behind — that dark patch is the shadow.

What's actually happening

Light is the straightest thing in the universe. It does not flow around obstacles the way water or sound does — if a straight line from the light source to the floor passes through your body, that bit of floor goes dark. A shadow is just the full map of all the blocked straight lines.

The interesting part is the edge. A truly point-sized light source draws razor-sharp shadows: every spot on the floor either sees the source or it doesn't. But real sources — the sun, a lamp, a window — have width. Spots near the shadow's edge can see part of the source, so they are only partly dark. That part-shadow fringe is the penumbra; the fully-dark core is the umbra. The bigger the light source looks from where the shadow falls, the softer and fuzzier the edge.

Solar eclipses are this exact picture at planetary scale. The Moon casts an umbra a few hundred kilometres wide that races across Earth — stand inside it and the sun is fully blocked; stand in the vast penumbra around it and you get a partial eclipse. Same physics as your hand over a lamp, scaled up a billion times.

Try it at home Sharpen and soften a shadow
  1. 1In a dark room, hold your hand a few centimetres above a table and light it with a phone torch from high up — the shadow has crisp edges.
  2. 2Now tape a sheet of baking paper just in front of the torch to spread the light. The same hand now casts a soft, fuzzy-edged shadow.
  3. 3Move your hand toward and away from the table and watch the fuzzy penumbra grow and shrink. You are resizing the light source as seen from the shadow.