Why Is Ice Clear but Snow White?
― The Same "Ice," Yet Looking So Different
An ice cube from your freezer is a clear lump you can see straight through. Yet snow falling from the sky, or lying on the ground, looks brilliant white. Both are frozen water, so why do they look so different?
Pick up a piece of ice and you can see the shape of things on the other side, even if only faintly. A snowman or a pile of snow, on the other hand, is a dazzling white that lets no light through.
Ice and snow are exactly the same thing: frozen water. Yet one is clear and the other is white. The difference is thought to come not from colour itself, but from how the ice is put together.
So how is the structure of snow different from ice?
Inside a big block of ice, the ice is almost uniform, so light is thought to travel through without being disturbed.
Each time light crosses the boundary between a snow crystal and air, it is turned aside (scattered) and sent off in every direction.
Let's look at this difference in how light is scattered, step by step.
Why a single block of ice is clear
When light crosses the boundary between two different materials (for example, air and ice), some of it is reflected and some is bent (refracted). A big block of ice has almost no such boundaries inside it. Apart from a little reflection and refraction at the surface, light is free to travel straight on, so you can see the scene on the other side.
Snow is made of countless tiny ice grains and air gaps
Snow forms when water vapour in the air freezes, and it is thought to be a pile of countless very fine ice crystals. Between the crystals are lots of air gaps. So inside snow there are countless boundaries between two different materials: ice and air.
It looks white simply because countless colourless, clear bits of ice bounce light around in all directions.
Light changes direction again and again at ice–air boundaries
Light travelling through snow is reflected and refracted a little each time it hits a boundary between an ice crystal and air. Because there are so many boundaries, the light is turned again and again (multiple scattering) and finally leaves the snow, scattered in every direction. When this light, scattered every which way, reaches our eyes, snow looks dazzlingly bright and white, or so it is thought.
Clouds, soap bubbles, milk and crushed glass are things that are really colourless, clear or translucent, yet look white. You can find many such examples around you. For many of them, as with snow, one reason is thought to be that countless tiny particles or bubbles are gathered together and scatter light many times over.
Try it yourself
- Get a piece of clear ice (one from an ice tray or freezer is fine).
- Once you have checked that you can see through it, wrap it in a clean cloth and crush it into small pieces (take care to stay safe).
- Look at the crushed ice (like shaved ice) and check how its clearness has changed.
- Confirm that the more finely it is crushed, the whiter it looks.
Just by crushing one clear piece of ice into small bits, you can see with your own eyes how it turns whitish, like snow.
Summary
Ice looks clear because there are few boundaries to disturb light, so it passes straight through. Snow looks white because it holds countless tiny ice grains and air gaps, and at their boundaries light changes direction again and again and scatters every which way, or so it is thought. Even with the same "frozen water," how it is gathered together makes the look completely different.
The whiteness of snow is not a colour laid on top. It is a clear collection of ice, built up from countless tiny bounces of light.
For those who want to know more ― terms, numbers and links to textbooksFrom middle-school science to topics under active research, each part is labelled by level
- Middle schoolCovered in middle-school science
- High schoolCovered in high-school "Basic Physics"
- High school+High-school "Physics," or advanced or sidebar material in textbooks
- UniversityUniversity-level specialist optics, not taught in high school
- ResearchNot yet settled even at university level; researchers are still studying it
Middle schoolTerms: words for how ice and snow look
- Refraction: light changing direction at the boundary between two different materials.
- Scattering: light hitting something and spreading out in various directions.
- Multiple scattering: light scattering over and over again.
High schoolChecking with a formula: how much straight-going light is lost at each boundary
Suppose part of the light changes direction each time it crosses a boundary. Let's use a simplified model to calculate the share of light that keeps going straight.
Share of straight light = previous share × 0.5 (at each boundary)
| × 0.5 | From a rough model in which about half of the light changes direction at each boundary |
| After the 1st boundary | 100 × 0.5 ＝ 50 |
| After the 2nd boundary | 50 × 0.5 ＝ 25 |
| After the 3rd boundary | 25 × 0.5 ＝ 12.5 |
| After the 4th boundary | 12.5 × 0.5 ＝ 6.25 |
| Result | After passing just 4 boundaries, the straight-going light drops to about 6% |
Real snow is thought to contain far more boundaries than this, and even this simplified calculation lets you picture how light struggles to pass straight through even a very thin layer of snow.
* The figure of "50% at one boundary" is a simplified, rough model for understanding the mechanism. The real share is thought to vary with the size and packing of the ice grains.
High school+Why every colour is scattered alike
Snow does not take on any particular colour and looks evenly white because ice absorbs hardly any wavelength (colour) of visible light and lets it through easily. Light of all colours is scattered in the same way and reaches the eye mixed together, so it is thought to be seen as white.
UniversityOptical models of multiple scattering
In optics, researchers use statistical methods to model the way many tiny particles scatter light again and again, as in snow or clouds. Such models are also thought to be important for estimating the reflectivity of snow (albedo) accurately.
ResearchWhat is still unclear
- How finely the shape and density of snow crystals affect reflectivity (albedo) is a topic still being studied in detail in optics and snow and ice science.
- How the reflectivity of snow and ice changes with global climate change, and how that affects rising temperatures (a phenomenon called the snow–ice albedo feedback) is also regarded as an important research question in climate science.
- Research and development is also under way on remote sensing technology for observing the state of snow and ice accurately from satellites.
Even a pure white snowy landscape holds a rich subject, where optics and climate science meet and research still continues.
Links to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Middle school | Science: properties of light | Basic terms: refraction, scattering, multiple scattering |
| High school | Basic Physics: reflection and refraction of light | Calculation of light lost at each boundary |
| High school+ | Physics: wavelength and colour of light (advanced) | Why colours are not absorbed differently |
| University | Optics | Statistical models of multiple scattering and reflectivity |
| Research | Snow and ice science, climate science (under research) | Snow–ice albedo feedback, remote sensing technology |
- Explanations of the reflection, refraction and scattering of light in physics textbooks.
- Explanations of the optical properties (reflectivity) of snow in snow and ice science materials.
- Research reviews on the snow–ice albedo feedback in climate science.
- Research reviews on modelling multiple scattering in optics.
- Explanations of satellite monitoring of snow and ice in remote sensing.
* The share of light changed at a boundary is a simplified, rough model for understanding the mechanism. The real optical properties of snow are thought to vary with the shape and density of the crystals.
* This article is a general-audience science explainer. When playing with ice or snow, take care not to slip on slippery ground, and watch out for illness caused by the cold.