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bainroot

This slide illustrates how the input energy must be greater than the energy coming out, because some energy can be absorbed by the surface, but it is impossible for negative output energy or output energy greater than input energy. The symbol rho stands for reflectance, and is a ratio. White objects will have reflectance 1, black objects will have reflectance 0.

jeyla

Note that this assumes that the surface is passive--meaning there is no additional energy source. Light reflecting onto a burning piece of wood would likely reflect more energy than is coming in from a light source.

hanrahan

In terms of more energy coming out than coming in ...

The reflected light energy is always less than the incoming light energy. But there can be extra emitted energy such as from flames or any other source of light. In that case the outgoing energy could be greater than the incoming energy.