If you ask anyone for a description of snow, you would most likely hear them say “white”. In fact, this is so embedded in our perception of the world, that it is a mere fact for most. However, when one stops to think about it, he soon discovers that it isn’t that obvious or straightforward at all. So why is snow white?

When we look at clean water or ice cubes, we see that they are clear and translucent, even individual snowflakes follow this rule (see second picture). 

Snowflake Closeup

So why is it that a larger collection of snowflakes (or a snowball) is white. What process causes this phenomenon?

To answer this question we first have to understand the workings of light. A ray of light can be described as both a particle or a wave and its exact workings are quite complex, so this question we work with light as a wave.
Like any wave, a wave of light (or Photon) moves with a certain frequency (the rate it goes up and down), which in turn determines the color of the light. If you then combine a bunch of photons with all the frequencies of all the colors, you get white light.

This means that all colors can be made from white light by systematically taking other colors away. This is what happens with almost anything around us, as most materials absorb light with certain frequencies and reflect others. This process, which is the cause of most things having a color, is called absorption.
For example, if we look at a banana, all the light that falls on it gets absorbed, except for the yellow light which gets reflected back into our eyes. This is why we see bananas as yellow.

Now going back to the case of the snowflake, we all know that ice cubes and water are clear, but the surface is also reflective. This is because when the light hits the surface, some of it can pass through while the rest gets reflected. (see 3rd picture)

Light going trough glass

Now while a block of ice or one snowflake only has two faces/sides in the path of a light ray, a snowball has billions.

So if we look at a snowball, like in a hall of mirrors, all the loose and randomly arranged snowflakes reflect a bit of the light back and forth between themselves and eventually back out of the snowball. All the frequencies of light get absorbed almost equally, so the remaining light is usually still a collection of all the frequencies and thus white. We call this a diffuse reflection (see 4th picture)

This means that the loose arrangement of the snow is crucial for this effect. 

Light going trough glass

Something that we can clearly see with compressed or melting snow, which becomes slightly transparent.

Now you may wonder if the almost equally absorbed light causes any visible effects. The answer to that is yes.
Red light gets absorbed slightly better than blue light, causing a small imbalance in the total. This is what gives snow a bluish hue when it is not directly shined upon.

Clouds, finely crushed glass, salt, sugar and foam are white for the same reasons.

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