Paint the Sky Blue
by Claudia Vanderborght
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?
- Why are clouds white?
[anno: Clouds are white because they contain droplets of water of different sizes. Since objects of different sizes reflect different frequencies of light, and a single cloud contains enough drops of water to represent most of these frequencies, the frequencies mix together to make the clouds appear white.]
- Imagine you were standing on the Moon and looking out across the Moon's surface. You look down at your feet and see gray dust covering Moon's surface. Now you look out at the side of a crater that is about 20 miles away. Do you think the crater would appear blue or white? Why? Write a few sentences to explain your answer.
[anno: Answers may vary but could include that since the Moon does not have an atmosphere trapping certain elements, there is nothing in the air to reflect the light, so the crater would appear gray and not blue.]