The dependence of absorption coefficient on wavelength causes different wavelengths to penetrate different distances into a semiconductor before most of the light is absorbed. The absorption depth is given by the inverse of the absorption coefficient, or α^-1. The absorption depth is a useful parameter which gives the distance into the material at which the light drops to about 36% of its original intensity, or alternately has dropped by a factor of 1/e. Since high energy light has a large absorption coefficient, it is absorbed in a short distance (for silicon solar cells within a few microns) of the surface, while red is absorbed less strongly. Even after a few hundred microns, not all red light is absorbed in silicon. The variation in the absorption depth for "blue" and "red" photons is shown below.

The blue photons are absorbed very close to the surface but most of the red photons are absorbed deep in the device.

The absorption depth is simply the inverse of the absorption coefficient.