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Union University Department of Physics

The Science Guys



Science Guys > February 2001

February 2001

Why is water clear but when it freezes it is often white, as in snow?

The transparency of water is amazing: the water of an unspoiled reef or Oregon’s Crater Lake is renown for its clarity, for one can peer about 100 feet through it. Yet everyone knows that even a minor snowfall blankets the ground in white. The inch of snow we received in January was sufficient to blanket the area with the white stuff, turning our area briefly into a winter wonderland. More to the point, why is water or a chunk of ice clear, but snow isn’t?

Pure water is colorless, whether in a vapor, liquid, or solid phase. That is, the molecules of water cannot absorb visible light and so cannot yield colors like dye or pigment molecules. But this doesn’t mean that water does nothing to the light that strikes it. You can model water like a pane of glass, which both transmits and reflects light. A good mirror reflects almost all light, and allows none to be transmitted through it. Conversely, high-quality windows allow almost all light to transmit through them with little reflection or glare. But if you look closely, you will notice a faint reflection of yourself on most windowpanes.

Water and glass not only reflect but also refract light. This means that as a light beam enters water or glass, the light bends. You know this from the spoon-in-a-glass trick: if you put a spoon in a glass of water, you notice that the handle of the spoon makes an abrupt "break" at the water/air interface. The same happens as light enters a piece of ice: it will bend. Now, if you just have one solid sheet of ice, the bending isn’t much and neither is the reflection off the ice’s surface; most of the light penetrates the ice, and the ice appears clear.

Now, what if you aren’t looking through an inch of solid, smooth ice but a multitude of dainty snowflakes? Snowflakes form directly from the vapor phase of water into the lovely crystals snowflakes are so famous for. These ice crystals form with hexagonal (six-sided) symmetry, so each crystal has many sides or crystal faces. When incident light hits a snowflake, some of the light is reflected off the crystal back towards the observer. However, most of the light penetrates the crystal and is bent, or refracted. Now, a snowflake has such a complex, intricate structure that this light hits internal crystal faces and bounces around inside the crystal. The combination of reflection and refraction is so efficient for an ice crystal that, ultimately, most of the light actually bounces back towards the light source and the observer. Since all the colors in sunlight add to give white, what we see when we look at snow is white: the sunlight that has reflected off and refracted through the water (ice) crystals to come back at us.

In fact, because snow reflects light so well, you can get what is called "snow blindness" if you go outside on a bright, sunny day after it has snowed. The light is so bright, that when you go inside it appears very dark, and it takes a minute or two for your pupils to dilate enough to see well.

The myriad of water droplets and ice crystals which form clouds are great at bending light and thus clouds, like snow, reflect most of the light and appear "white." Satellite images show that clouds appear bright white when viewed from above. From below thick clouds appear dark, because cloud tops reflect most of the light into space.