Why are snowflakes white?

 

This Christmas decoration got me thinking about snowflakes. It turns out that the actual crystals are colourless but they have many surfaces that reflect. Tere are two types of reflection shown below. The top one is called specular reflection. It is the sort of reflection you get from mirrors that enable you to see your reflection. As you can see, the flat surface reflects all rays incident at the same angle away at the same angle of reflection. The bottom one is called diffuse reflection and shows what happens when the surface is uneven. As ever, angle of incidence = angle of reflection, but the different surfaces mean that the angles of incidence are actually all different. The result is no clear image. If all wavelengths are reflected in this way, the result is white.