Caustics are the effects of light cast onto an object via reflection off or refraction through another object.
Swimming pool rendered without caustics
Reflective caustics added to swimming pool
To calculate caustics, the mental ray renderer uses the photon map technique. (Ray tracing can’t generate accurate caustics, and they aren’t provided by the default scanline renderer.)
You enable caustics on the Render Setup dialog 
Indirect Illumination panel Caustics And Global Illumination rollout. In addition, you must designate:
The settings for generating and receiving caustics are on the Object Properties dialog mental ray Panel.
Refractive caustics rendered with the default of Radius turned off.
Radius size is based on scene extents; specifically, 1/100 the radius of the full scene.
Radius value explicitly set to 1.0
Radius value increased to 2.5.
Filter type changed to Cone.
Photon count increased to 50,000 (in Global Light Properties group) for greater detail in the caustics.
