Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

What factors affects the size of a shadow and how would you derive the diameter of a shadow of a circular object on a flat screen?

share|cite|improve this question
It's based on the distance and orientation of the light source, whether it becomes an ellipse or a larger circle ;-) – Waffle's Crazy Peanut May 30 '13 at 13:19
If the light source isn't pointlike, the shadow doesn't have sharp edges. – Ben Crowell May 30 '13 at 15:36
up vote 2 down vote accepted

Instead of the Circular object I use a cardboard with a circular cavity that will behave same as the object, instead the lighted portion on screen would be shadow in that case :)

enter image description here

$$\tan\theta=\dfrac r d =\dfrac R{d+D} $$

share|cite|improve this answer

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.