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