In a medium with large optical depth, such as snow that is more than a few inches deep, multiple scattering is rampant, that is, photons are scattered repeatedly each time changing direction.

• 在一个象雪这样具有大的光学厚度（几英寸厚）的介质中，非常容易出现多散射，即光子反复地被散射，每次散射的方向不同而已。