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Red Sails in the Sunset
My seven-year-old daughter caught me off guard the other day during a cookout. As we were watching the sunset, she asked, “Daddy, why is the sky red?” Now, as chemists, we are well trained in the ways of spectroscopy and always ready to answer the “Why is the sky blue?” question with platitudes of refraction and scatter. But I was unprepared for the red question.
The answer. During a lunar eclipse, the moon appears red, even though it is in the Earth’s shadow. Sunlight is refracted through the atmosphere around the edges of the Earth, removing the blue end of the spectrum and leaving only the red. This is the same effect that causes red sunrises and sunsets.
The color of the sky depends on the wavelength of the sunlight scattered according to the Rayleigh scattering model. Briefly, the Rayleigh model says that shorter (bluer) wavelengths bend more than longer (redder) wavelengths. When the sun nears the horizon, sunlight travels a longer path through the atmosphere around the edges of the Earth. Thus, the blue light bends more and is removed. Particles, dust, pollution, and even moisture can result in scattered colors due to Mie scattering, which is different from the Rayleigh scattering effect. Therefore, colors seen during dusk and dawn arise from a combination of Rayleigh scattering and particle scattering. Now, how to explain the facts without sterilizing the beauty?
The explanation. My little girl has trouble with terms like wavelengths and Rayleigh scattering. But it wasn’t as hard to explain as it sounds. In fact, it was kind of fun and a good bonding experience for us. First, I got out my handy-dandy prism. Everyone who has young, precocious kids needs a science kit that includes a prism. Then, I found a friendly sunbeam tumbling through the kitchen window. I placed a piece of white paper on the table and played with the prism until I could project a clear spectrum. Then I spread out the spectrum across the paper. I asked my little girl to trace the pretty rainbow on the paper with her colored pens and then color in the segments.
I then gently bent the paper to show her what a rainbow looks like. I gave her the cardboard tube from a roll of paper towels and asked her to look through it. Then I turned the bent paper toward her so that she could see the red, but the blue portion was below her line of vision, cut off by the tube. “I can see it, Daddy! I can see it!”, was the very fulfilling response. The excitement of the moment was worth it, and justified, in my mind, the sloppy science.
By Definition
- Refraction is the bending of light rays at sunset when they pass from a fast medium—the vacuum of space—to a slow medium—the atmosphere.
- Rayleigh scattering is the scattering of light by air molecules. The phase function of Rayleigh scattering preferentially extracts the blue wavelengths from sunlight, making the sky appear blue and the sun appear yellow.
- Mie scattering occurs when dust particles in the air are about the same size as the light wavelengths.
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