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From Odwiki

Photon is a term used in PhotonMapping. It represents a small amount of light. The photon is sent out from the light source (backwards ray-tracing) and bounced around the scene. At some point, the photon may be stored in a PhotonMap.

When a photon strikes a surface, there are three possibilities:

• The photon is reflected. (This is called specular reflection) - imagine light from the sun bouncing off a mirror and striking the opposite wall.
• The photon is transmitted. In almost all real materials, the light gets refracted due the difference in material densities.
• The photon is absorbed. The surface then emits radiant energy in all directions, which can be solved by Irradiance calculations. The process of absorbing photons is called storing.

When there is a non-planar surface in the case of refraction (for transmitted photons) or reflection, it would cause the focussing or blurring of light which cause the light ripples on the bottom of a pool, the focus point of a magnifying glass or the bright spot on a surface in the centre of a shiny brass ring.

Please note that photons are not split. Each packet of light (quanta) is treated individually and the photon's future is usually decided by a technique called RussianRoulette in which three probabilities are provided for the three different photon fates.

The PhotonMap keeps track of all the photons from every light source. This provides a way of estimating the light which falls on a surface through all LightPaths.