Why does a pencil's shadow have a fuzzy edge sometimes and a sharp edge other times?
Did this make sense?
Your thumbs help us make it clearer.
Thanks! That helps us make it clearer.
After you watchWhy does a pencil's shadow have a fuzzy edge sometimes and a sharp edge other times?
The short answer
A broad light source produces a partly lit shadow edge. Its width depends on both source size and geometry: moving the object close to the receiving wall narrows the band.
The whole story
How it works
The model samples 25 points across one fixed panel and tests which straight rays are blocked by the pencil. The hidden slider moves the pencil from 55% to 85% of the source-to-wall distance. The computed penumbra width decreases as the remaining gap shrinks.
What people get wrong
The light size is not the only thing that matters. A broad panel can make a narrow shadow edge when the object is close to the receiving surface.
The catch
A broad source often softens shadows, but the same source can produce a narrow edge when the object is close to the receiving surface. These rays omit diffraction and ambient reflections.
Questions kids ask
Why does a pencil's shadow have a fuzzy edge sometimes and a sharp edge other times?
A broad light source produces a partly lit shadow edge. Its width depends on both source size and geometry: moving the object close to the receiving wall narrows the band.
What is different in the hidden experiment?
After moving an object under a point light, predict how its gray shadow band changes when moved toward a wall under a fixed broad panel. Trace the rays and compare penumbra widths.
What limit should I keep in mind?
The light size is not the only thing that matters. A broad panel can make a narrow shadow edge when the object is close to the receiving surface.
Talk about it
- Before running the hidden experiment, explain your prediction.
- Compare the final picture with your prediction. Which visible change supports your explanation?
- What did the experiment keep fixed?
For grown-ups
The model samples 25 points across one fixed panel and tests which straight rays are blocked by the pencil. The hidden slider moves the pencil from 55% to 85% of the source-to-wall distance. The computed penumbra width decreases as the remaining gap shrinks. The light size is not the only thing that matters. A broad panel can make a narrow shadow edge when the object is close to the receiving surface.
Keep going
What else makes you wonder?
- If a big light makes soft shadows, what would your shadow look like under three lamps spread across the ceiling at once?
- The Sun is a real disk, not a point — so what makes a star, which is far bigger, still cast such a razor-sharp shadow?
- If the dark middle of a shadow can shrink to nothing on a cloudy day, could you ever make a shadow that is all fuzz and no dark part indoors?
