Where does the white go when the snow melts?

FontedesneigesNow that winter and snow are here, how about having some fun with a science lesson and some activities about the science of light, color and reflection? This wonderful article comes from Ray Williamson of Williamson Consulting, and was originally published in the “Light Touch” feature of OSA’s Optics and Photonics News in January, 1995.

When the snow melts, where does the white go?
The riddle like nature of this question tickles me. How can something so bright leave no trace? (Grape juice wont do that). We’ll see the answer soon, and we’ll have some fun in the kitchen. Let’s first ask, “is white a color?” In a sense it is, for it is all colors added together. That makes it a unique color, but it still obeys the color laws of addition and subraction. White does give two unique results: A white object will return the color of any light shined on it unchanged, and white dilutes any pure colors mixed with it rather than creating a new color. Two interesting riddles remain…

How is white different from “silvery” , which also reflects any color unchanged? And, how is white different from clear, which passes any color unchanged?
The words shiny and transparent might come to mind. Here it is, in one big gulp. Younger children should probably take smaller bites.

  • A white object scatters all colors in all directions
  • Clear objects let all colors pass through
  • Colored objects absorb some of the colors inside them
  • Silvery objects are always shiny – they reflect all colors, but scatter light less than white objects

 

Surface reflections are one key to whiteness. Any object, even a clear one or a black one, reflects some light at it’s surface. Some things with special optical properties (such as mercurochrome, iridescent sunglasses, or butterfly wings) reflect some colors better than others. But most surfaces reflect all colors about equally.

This is easiest to see with smooth, shiny surfaces. Look carefully at the reflection of a light bulb from shiny surfaces like a blue lake, red jello, coffee, or a black car. Regardless of what color is underneath, the color of the light bulb’s reflection is unchanged. Of course, the light bulb may be any color, but the color of its reflection is unchanged, which was a property of white objects.

So why dont those things look white?
Because we can only see their surface reflection of the light source when we look in exactly the right direction. Beneath their surfaces, light is absorbed. Red Jello absorbs everything but red. Coffee absorbs all colors, eventually. Black paint absorbs all colors in a thin layer. Rear surface reflections are another key to whiteness. In the right conditions for total internal reflection, essentially all the light is reflected. See how white a clear diamond looks in diffuse light.

Scattering is another key to whiteness
When a strong reflection shows us surrounding details, we call it silver. When we can’t see those details, we call it white. A mirror looks silvery, but a diamond looks white because the tiny facets confuse the reflected image. A metal paper clip looks silvery due to its glints and highlights. One big bubble looks silvery, but countless tiny ones look white. What happens if we jumble many tiny surfaces together? Light bounces in all directions. Any light not absorbed is eventually scattered or reflected. Many reflections, some total internal reflections, in many directions means lots of scattered light of all colors. Have you noticed that many powders are white or nearly white? Powders have many surfaces. White paint contains a powder that reflects very well and absorbs very little.

Fun in the kitchen! We can do some real science – testing a theory by checking its predictions and using a theory to learn what we can’t see directly. Let’s see if we can apply the principles of surface reflection and scatter to explain what we see in the kitchen.
The water coming out of an aerated faucet has lots of bubble, the surfaces of which reflect in all directions. How does this look? Where does the white go when we fill a glass with this water?
Salt looks very white at first glance. Acrually, each crystal is clear, but we’re seeing lots of surfaces. If we get up really close with a magnifying glass, we can see how clear and shiny each crystal is.
If we get up really close, with a magnifying glass, we can see how clear and shiny each crystal is. What happens when we put salt in water and stir it up? What happens if we let salty water evaporate?
What about milk? If we put milk in water, the white dosen’t go away. Why?? Because milk contains scattering particles that dont dissolve in water!
If we roughen a clear ice cube by hitting it with the back of a spoon or scraping it with a key, we will make lots of irregular surfaces. How will this look? What would happen if we dripped some water on it?
When many shiny surfaces are put together smoothly, all the reflections come out together. How should a new roll of clear plastic kitchen wrap look on the roll? Silvery? Look, and see!
This one’s magical: put a little liquid dish soap in a large bowl, fill it quickly with water from the faucet to get lots of suds, and leave it alone for about 20 minutes. The top will look ghostly and sky blue. Those bubbles have gotten so thin that light from the front and back surfaces interfere, and you can barely see them. Now take your finger and “draw” a trail through the bubbles. What happened? You broke some thin bubbles and their soap and water slipped down to thicken some others and change thier optical properties so they no longer interfered. Are they silver or white?
Based on our disscussion, when snow melts, where does the white go? Snow is made of countless clear crystals jumbled in every direction. Each front surface reflects a little bit. The crystals are clear and many rear surfaces reflect totally. There are so many surfaces that almost everything is reflected. So snow is very white. When the snow melts, all those surfaces join together until there’s only one surface on the top of the water, which reflects only a little bit in a certain direction.
So now you know – what happens to the white when the snow melts.