Photograph by David Lynch
Written by Martin Richard
Last week we looked at the longest glacier in the Alps. We saw it from the outside, looking down from a nearby mountain.
In this week’s photo, we are looking into a glacier. It’s white on the top, but turns blue deeper inside. The white is from the snow. That’s not a surprise, since glaciers are made from snow. Where does the blue color come from?
A swimming pool is blue at the deep end. A glacier is bluer the deeper it gets.
Maybe they both turn blue for the same reason?
Yes, because they both have something to do with how light behaves when it goes through water.
What we want to know is, why does white light turn blue when it goes through a lot of water? Whether water is liquid (sloshing in a pool), or ice (frozen in a glacier), the light turns blue.
To understand the color of glaciers and swimming pools, you have to understand rainbows!
White light is actually light of all colors mixed together. You can separate white light into its colors by shining it through a prism. Here is a picture of a prism casting the spectrum of white light.
When you shine light on a prism, the light has to cross the edge of the glass. Crossing the edge pulls the light apart into its colors. To go through a prism, the light has to go through two edges so the colors get separated even more, and now you can see them.
When you shine light through a prism you get a rainbow of light. All the colors of a beam of light are called its “spectrum.” A rainbow shows the spectrum of while light.
Raindrops act like prisms...millions of them. Millions of raindrops have millions of edges and cast millions of spectrums. (Ooops! That’s not quite right. We don’t say “spectrums.” Spectrum is an old word and we still use the old plural, which is “spectra.” So let’s correct that sentence to say …)
Millions of raindrops have millions of edges and cast millions of spectrums spectra. We see all those spectra in the shape of a bow.
Dump those raindrops in a swimming pool – or in a glacier! – and the drops are not separate any more. All the edges are gone! No edges means no prisms means no rainbows!
The light in the pool or the glacier has to go through a lot of water. Water absorbs the red and orange colors. So you can’t see the reds and oranges; they are trapped in the water! The more water you have, like in the deep end of the pool, the more red and orange gets taken out.
But water does NOT trap blue light! Blue goes through!
So the glacier does not TURN the light blue; the blue was always there. The deep, frozen water of the glacier TAKES AWAY most of the colors EXCEPT blue.
What is really important in this week’s photo is what you DON’T see: you don’t see the red and orange light that was trapped by the water of the glacier. What you DO see is the blue light that escaped and made it all the way to your eyes.
We got this picture of the glacier here: Blue Glacier Ice. It is not written for kids, but you're welcome to check it out!