1. Why a clear block of ice turns white when it is crushed
When you break a clear ice cube, the pieces suddenly look white and cloudy. Before breaking, you could see through the ice. After breaking, you cannot. This happens because ice crystals are packed tightly together. Light passes through them with very little absorption or scattering, so most of the light goes straight through. This makes a pure block of ice look transparent [1].
Snow is a collection of tiny crystal grains. When light hits snow, it scatters off the surfaces of grains facing many different directions. This makes snow look white [1]. Crushed ice is thought to be in a similar state to snow. However, this article does not use specific data on crushed ice itself. The puzzle here is the color white. Where does this white color come from?
2. What white light is made of: a mix of all colors
Sunlight and fluorescent light are white light, which means they contain a mix of many different colors [1]. Materials that scatter all colors of light equally, like snow, appear white. On the other hand, if a material absorbs or scatters specific colors strongly, it takes on that color [1].
So, crushed ice is not white because a white color was added to it. It is white because the ice scatters light without choosing a specific color. The mixed light reaches your eyes directly. But where in the ice is the light being scattered?
3. How light bends at the boundary between ice and air
Ice and air have different refractive indices, which measure how much light bends. For visible light with a wavelength of 546 nanometers, ice has a refractive index of about 1.31, while air is about 1.0003 [2]. Because of this difference, at the boundary between ice and air, some light reflects and some bends as it travels.
When you crush ice, the number of these boundaries increases suddenly. New surfaces, cracks, and bubble walls all become boundaries between ice and air. Light changes direction at each of these points, so it reaches your eyes from many directions. Research on snow explains that when ice crystals are large enough relative to the light, reflection and refraction do not depend much on the color of the light. Studies suggest that with snow about a few centimeters deep, it looks white without any color [3].
Individual ice grains are too small to add color. The white appearance comes from light that has passed through many surfaces and mixed together [3]. Does this same idea apply to things other than ice?
4. Why sugar and salt look white for the same reason as crushed ice
Sugar and salt also look white because they are collections of tiny crystal grains [1]. If you take just one large crystal, it looks transparent [1].
Even though individual grains are transparent, when many are packed together, there are many boundaries. Light changes direction at these boundaries, making the pile look white. This is the same principle as with ice. The spaces between the grains are filled with air. What happens if water fills those spaces instead of air?
5. How water filling the gaps in crushed ice changes the way light reflects
The refractive index of water is about 1.33, which is very close to ice’s value of about 1.31. Because of this similarity, when water replaces the air in the spaces between ice grains, light bends less at the boundaries [2].
Research on snow suggests that when liquid water replaces air between grains, the grains act as if they are larger. In near-infrared light (which is invisible to the eye), wet snow reflects less light than dry snow [2]. It is important to note that this research focuses on near-infrared light in snow. It does not directly state what happens to the visible whiteness or whether wet ice becomes transparent. How crushed ice looks as it melts is left for you to observe with your own eyes.
6. Why thick ice looks blue: ice absorbs red light
Ice has a property that makes it slightly more likely to absorb red light. The distance light can travel in ice before being absorbed (the absorption length) is estimated at about 2 meters for red light with a wavelength of 700 nanometers. For blue-violet light with a wavelength of 400 nanometers, this distance is estimated at about 200 meters [2]. This is a difference of about 100 times.
Snow grains have a radius of about 100 micrometers. Even if light refracts or reflects 1000 times, the total distance it travels inside the ice is estimated at only about 20 cm [2]. This is just one example for 1000 bounces, not a fixed value. Since 20 cm is much shorter than 2 meters, red light is not absorbed much, so snow stays white.
However, in thick ice like glaciers, light travels a long distance. Red light is gradually absorbed. In glacier crevasses and icebergs, sea ice with few bubbles is said to take on a blue tint [2]. Ice with many bubbles looks white.
Note that explanations for the blue color of glaciers vary slightly by source. The Hokkaido University material mentions that scattering and absorption differ by color as light travels through ice [1]. This article focuses on Warren’s explanation, which provides specific values for absorption lengths.
7. How to observe clear and crushed ice yourself at home
You can try this at home. Put clear ice (from an ice tray is fine) in a cup. Use the back of a spoon to crush it slightly. Compare whether you can see through the ice before and after crushing it.
Watch how the whiteness changes as the crushed ice melts. Leave it for a little while to see if the whiteness fades as water fills the spaces between the grains. The results may change depending on the amount of ice and the temperature.
For official information, the Hokkaido University Institute of Low Temperature Science has a resource called "The Door to Snow and Ice Science" that discusses the blue color of glaciers.