Paint the Sky Blue

Thus if one is to be five times as distant, make it five times bluer.

Leonardo

Do you remember asking your parents, “Why is the sky blue?” As a painter, Leonardo da Vinci was fascinated by colors. In his notebooks, he described and tried to scientifically explain the subtle colors of leaves on trees, the blueness of the sky, and the spectrum of the rainbow. One particular observation he found intriguing was the “bluing” of distant mountains—the farther away the mountains were, the bluer they appeared.

Leonardo wrote that the atmosphere is blue because of the darkness above it: The black above, mixed with white light, made a blue sky. There is a lot of blue sky between an observer and a distant mountain, he explained. The blue atmosphere tints mountains bluish exactly like red glass tints reddish any objects seen through that glass. Thus, Leonardo described, the farther away an object is, the bluer it should be painted: One that is five times distant should be five times bluer.

Leonardo's observations were accurate, although his hypothesis was not entirely correct. Today we know that the sky is blue, clouds are white, sunsets are red, and distant mountains appear bluish because of light scattering. The earth's atmosphere is composed primarily of nitrogen and oxygen molecules, which are very small. When light strikes those tiny molecules, they vibrate. Fast frequencies, such as 75 million vibrations per second, generate violet light. Slow frequencies, such as 43 million vibrations per second, generate red light. The tiniest atmospheric molecules can vibrate very rapidly—scattering the violet and blue frequencies of light far more than green or red, and so the sky appears blue. Bigger molecules vibrate more slowly, scattering slower frequencies of light. Water droplets in clouds vary in size, so each water droplet scatters a different frequency of light. Mixing all the frequencies of light together generates “white” light, and thus clouds appear white.

During a sunset, the sky changes color from blue to yellow to orange to red. As the light travels through more and more atmosphere, it encounters more and more particles. Blue frequencies are scattered over and over again; all that is left for your eyes to observe is the slower frequencies. The lower the sun in the sky, the more blue light is scattered away, so your eye sees the leftovers.

Leonardo was right about the blue mountains! We see objects because they reflect light. Very little light from the distant mountains actually reaches your eye. But there is a lot of atmosphere between you and those mountains, and that atmosphere scatters blue frequencies. So, just as red glass tints objects red, atmospheric scattering tints distant mountains blue!

Go outside and look at the sky. Is it blue? Is the color above the same as the color at the horizon? Can you see distant mountains or hills? Are they shades of blue?

Activity

  1. What is light scattering? Write a few sentences for your answer.
    [anno: Light scattering refers to what happens when light waves strike particles in the atmosphere, such as molecules of oxygen and nitrogen. When the light hits these particles, they vibrate. The different particles vibrate at different speeds. Small particles vibrate more quickly than larger particles. Smaller particles generate violet light. Larger particles generate red light.]
  2. Do you think that violet light or red light has a longer wavelength? Why?
    [anno: Red light has a longer wavelength than violet light because red light has a lower frequency than violet light.]
  3. Imagine that you could hear the waves made by light. What would violet light sound like? What would red light sound like? How would these two colors sound differently? If you could choose an instrument to represent each color, what would each instrument be? Write a few sentences for your answer.
    [anno: Answers will vary. Students may suggest that violet light would have a high-pitched sound since it has a much faster frequency than red light. Students may compare red light to a timpani drum and violet light to a piccolo.]