This is from the Maya Help Files on the topic below

Global illumination and caustics workflow

Generally, the workflow is as follows:

Make sure that at least one light source in the scene emits photons.
See Turn on photon emission for a light source.

Make sure that at least one surface is casting or receiving caustics.
By default, all objects cast and receive caustics, though typically not all have to in order for you to achieve the look you want. To fine-tune the look of global illumination or caustics or to reduce render times, you can specify exactly which objects should cast and, or receive photons. See Flag objects to cast and receive photons.

Tip
To see the distribution of photons for global illumination, you can generate a 3-dimensional visualization map in shaded or wireframe scene views.





Make sure raytracing and the desired global illumination and, or caustic effects are enabled.
See Render with global illumination.

Render with mental ray for Maya.

Tip
Combine global illumination with Final Gather for the most physically accurate lighting effects. See Combine global illumination with final gather.