What are the factors that affect the sharpness of a shadow?

I would think that the distance between the light source and the object, the distance between the object and the shadow, and the size of the light source would all have an effect.

How do they affect the shadow exactly? What is the full explanation?

  • $\begingroup$ The full explanation is in both answers. $\endgroup$ – Muze Jan 13 '16 at 0:09

The finer light source smaller than the object casting the shadow will make the most defined shadow.

Figure adapted/modified from "Shadows" via School physics by Keith Gibbs

enter image description here

  • $\begingroup$ exe. A florescent tube vs led light. The led light will make a more defined shadow. $\endgroup$ – Muze Dec 27 '15 at 6:09
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    $\begingroup$ You need to cite the source of this image (schoolphysics.co.uk/age11-14/Light/text/Shadows/index.html) or wherever it originally came from. Otherwise you are plagiarizing and that is taken very seriously here. $\endgroup$ – pentane Mar 29 '16 at 13:12
  • $\begingroup$ @pentane I modified it to make it mine. $\endgroup$ – Muze Mar 29 '16 at 23:29
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    $\begingroup$ Oh in that case put something like "Image source: Schoolphysics by Keith Gibbs (modified)" or "Figure adapted from <source>" $\endgroup$ – pentane Mar 30 '16 at 12:17

If you are talking about the partially shaded outer region of a shadow then yes it would be called the penumbra and would be described as Jen says. Or like the area of a partial eclipse.

If your talking about factors that affect the sharpness of a shadows edge then it would have to do with distance, wavelength, light source and sharpness of the illuminated objects edge.

Light is being diffracted by the edge of the object being illuminated. This is called knife edge or straight edge diffraction and it produces a diminishing fringe pattern along the shadows edge that looks like this. enter image description here

If the edge is sharp enough (like a razor edge) and the light source is monochromatic you can see these fringe patterns. The term I use is single edge and I write about it in my paper on the web site at the top of my page.

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    $\begingroup$ Only minor correction: If Zero on the x-axis is the line of the geometrical shadow, than the intensity distribution for photons starts inside the shadow (some negative distance). This is because the electric field component of the photons are equally distributed and this is, what we observe as fringes on the screen. For electrons it is different. The first fringe starts away from the geometrical shadow. This is because the moving electrons are repelled by the surface electrons of the razor blade or some other sharp edge. $\endgroup$ – HolgerFiedler Dec 27 '15 at 14:02

Although the Sun is much wider than the Earth it produces sharp shadows on a clear day since its angular width from Earth is very small. The slightest mist reduces the sharpness of the shadow.

At night a bare light-bulb produces a much sharper shadow than the same bulb inside a diffuser, such as a Chinese lantern.


protected by Qmechanic Sep 25 '16 at 13:26

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