Or for example on the bottom of a pool as light is focused by waves on the surface of the water. It seems awkward to just call it a shadow since it implies being darker than the surroundings.

2 Answers 2


They are called caustics, the randomly fluctuating patterns of bright lines at the bottom of the pool. They are caused by reflection and refraction of sunlight at the perturbed pool surface illuminated by the sun.

The rippling caustics seen below are so called because they are caused by concentration of sunlight (with the resultant potential of causing a burn).

More details on this phenomenon can be found here.

(source: toronto.edu)

  • Can you mention some references?
    – Kris
    Sep 17, 2013 at 7:30
  • Unfortunately I do not subscribe to physics journals and would not be able to point you to any authoritative source. Hope this link helps - http.developer.nvidia.com/GPUGems/gpugems_ch02.html
    – user49727
    Sep 18, 2013 at 16:52
  • If you think it helps, you should include it in your answer.
    – Kris
    Sep 19, 2013 at 7:36

In the case you describe, the opposite of a shadow is a focus, plural (usually) foci:

  1. a. A point at which rays of light or other radiation converge or from which they appear to diverge, as after refraction or reflection in an optical system: the focus of a lens. Also called focal point.
    The Free Dictionary
  • I don't think this definition applies for cases where it is an only slightly brighter spot on a shadow cast onto an object that is not actually in the focal plane.
    – Random832
    Sep 15, 2013 at 20:54

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