又到冬天了，漫天飞舞的雪花总能给人带来无限的遐思。在欣赏这美妙雪景的同时,不知您有没有想过: 雪为什么是白色的? 如果雪变成了红色或是绿色,还能给您同样的遐想吗?

One of the reasons that so many people love snow is that it coats everything in a clean, "pure" white blanket. Snow wouldn't be snow if it wasn't white. But why is snow white?

To understand where the whiteness comes from, we need to back up and look at why different things have different colors in the first place. Visible light is made up of many different frequencies of light. Our eyes detect different frequencies as different colors. Different objects have different colors because the atoms and molecules¹ that make up the object have different vibration² frequencies. Basically, the electrons of the particle will vibrate a certain amount in response to energy, depending on the frequency of the energy. In the case of light energy, the molecules and atoms absorb a certain amount of light energy depending on the frequency of the light, and then emit this absorbed energy as heat.

A couple of different things can happen to the light frequencies that are not absorbed. In some material, when a particle re-emits the photons, they continue to pass through to the next particle. In this case, light travels all the way through the material, so the material is clear. In most solid material, the particles re-emit most of the unabsorbed photons out of the material, so no light, or very little light, passes through and the object is opaque³. The color of an opaque object is just the combination of the light energy that the object's particles did not absorb.

So, since snow is frozen water, why does it have a distinctive⁴ color? Ice is not transparent⁵; it's actually translucent⁶. Snow is a whole bunch of individual ice crystals arranged together. When a light photon enters a layer of snow, it goes through an ice crystal on the top, which changes its direction slightly and sends it on to a new ice crystal, which does the same thing. Basically, all the crystals bounce the light all around so that it comes right back out of the snow pile. It does the same thing to all the different light frequencies, so all colors of light are bounced back out. The "color" of all the frequencies in the visible spectrum⁷ combined in equal measure is white, so this is the color we see in snow, while it is not the color we see in the individual ice crystals that form snow.

vibration frequencies: 振幅

opaque：不透明的

photon：光子