When sun light falls on my bathe tub i noticed that the shadow of any small particle floating on the water surface is bigger than the particle and also it is quite circular i.e. Deform from It's actual shape. Generally it tends to become a circle.

  • $\begingroup$ Like the Sun and you? $\endgroup$
    – John Donne
    Jun 4 '18 at 17:56
  • $\begingroup$ Like the sun and a grain of sand. $\endgroup$ Jun 4 '18 at 18:00
  • $\begingroup$ For any light source that is not emitting perfectly collimated or converging light (that is, the light rays are parallel or move closer to one another) the shadow will be larger than the object (just consider the shadow cast by any object illuminated with a point light source). That the shape of the shadow of a small object becomes circular in the case of the sun is due to the fact, that the sun is circular (and the small objects acts like the aperture of a camera obscura, with the only difference that it blocks light instead of letting it through). $\endgroup$ Jun 4 '18 at 18:05
  • $\begingroup$ @Sebastian The OP isn't totally satisfied by the camera obscura explanation. See physics.stackexchange.com/questions/410064/… $\endgroup$
    – PM 2Ring
    Jun 4 '18 at 20:52
  • $\begingroup$ Well for objects floating on the surface, the meniscus due to surface tension is certainly relevant as well. But it is never the less true, that any object will cast a shadow that is larger than the object in sunlight. $\endgroup$ Jun 5 '18 at 0:01

In this case, the likely explanation is that the surface of the water near the "floater" is not flat due to surface tension. This causes the light rays entering the water near the particle to be bent, causing a "shadow" under the particle.

You might have also noticed that the swirling water when the drain is opened casts a "shadow" (without the presence of any particles) because the water surface is not flat.

For example, you can see the bending of the water surface in this photo.

enter image description here

And here is an example of the cast "shadows".

Surface Tension Shadows


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