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Tell Me Why...Answering common questions about nature and the sky!

By Katrin Gross

Why is the sky blue?

On a sunny day we all know the sky looks blue, but sunlight itself is not blue. Sunlight is made up of many different colors of light–all the colors of the rainbow! We can think of light as waves, like waves in the ocean, and different colors of light have waves of different sizes and frequencies (how many waves pass by in a certain amount of time). When the sun is shining, these different waves of light have to travel through the atmosphere to reach us. But the atmosphere is full of obstacles in the form of gas molecules, and they scatter the waves of sunlight into their different color components. But not all colors scatter the same because their waves interact with the obstacles differently. When we look up at the sky, we see mainly blue because the blue waves of light scatter more than the other colors. This spreads blue light across the sky so that we can clearly see it. That same scattering also causes red sunsets! When the sun is setting, the light has to travel through twice as much atmosphere and all of that blue light gets scattered away until the only wavelength of light that’s left is red, and that’s what we see!

Why are plants green?

The color of plants are also determined by the waves of light we see–in fact, all colors are the result of different waves of light! Everytime we look at an object, some colored waves of light are absorbed and some are reflected back at us. The colors reflected back at us are the colors we recognize the object as. Therefore, we see green when we look at plants because green waves of light are reflected back at us. That’s because plant cells contain molecules called chlorophyll which help them turn sunlight into sugars that plants use for food. Chlorophyll prefers to use red and blue waves of light instead of green, so the green waves of light are reflected away. When leaves change color in the fall, they are using different light absorbing molecules to make sugars, which use different colors of light. Some molecules don’t use red light, so we see red plant leaves!

Why is the Moon bright?

At night, the Moon looks like it’s shining in the sky and sending light down to Earth. But the Moon doesn’t actually make any of its own light. Just like the Earth, the Moon is made of rock and metal which don’t produce light like the gaseous Sun. Instead, the Moon looks bright because it reflects light from the Sun. When it’s night time here in Philadelphia, the Sun is still shining on the other side of the planet (in Australia for example). That means that when the Sun isn’t shining on us it is still shining on the Moon because the Moon is far enough away (238,855 miles!) that it can still receive light from the Sun. Then the Moon can reflect that light back on us and light up the night sky!

Why do fireflies glow?

If you’ve ever seen a firefly, you know that their bodies can create light just like the Sun or a lightbulb can. However, fireflies make light very differently through what’s called bioluminescence. Bioluminescence is when a living organism produces light. In fireflies, light is produced through a chemical reaction that uses a chemical called luciferin. This light-producing reaction occurs in a specialized light-producing organ in the fireflies’ body. The light-producing reaction that uses luciferin also needs oxygen to start. Since fireflies are able to control when oxygen can enter the light-producing organ, they can control when they glow!


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