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“Why is the sky blue?” may seem to be a simple question, but it actually touches some of the deepest aspects of astronomy and skygazing because it deals with light. “Seeing” something whether it’s a table across the room or a galaxy millions of light years away consists of light entering our eyes and the brain taking the light and creating a picture within our brain. So before asking the question “Why is the sky blue?”, it’s necessary to know the answer to the question, “What is light?”
Light consists of weightless particles (that contain no mass) traveling at about 186,000 miles or 300,000 kilometers per second. These particles of light are called photons. But light also exhibits the characteristics of a wave. Traditionally, wave activity is explained by imagining a pebble being dropped into a lake, but that imaginary lake has been filled with imaginary pebbles years ago. So instead, imagine a long rope tied to the side of a wall and a girl shaking rhythmically one end of the rope up and down. Each up and down shake of the rope is a cycle and the number of cycles per second is the wave’s frequency. If you observe the rope from the side, you will see a wave pattern of alternating crests and troughs. The distance from one crest to another is the wavelength. When the girl shakes the ropes more rapidly, the frequency will naturally increase, but the wavelength will shorten.
Light acts in many ways similarly to the shaking rope. It also has a frequency and a wavelength. As the frequency increases, the wavelength decreases and as the frequency decreases, the wavelength increases. The frequency of light determines its color. Blue light has a higher frequency and a shorter wavelength than red light. Our eyes are sensitive to only a small part of the spectrum of frequencies. There is ultra-violet and infra-red light just beyond our eyes’ range, radio waves with much lower frequencies than light, and gamma rays with much higher frequencies. Sunlight contains many different colors which can be separated through a prism or a rainbow into different colors.
In short, keep in mind the following three facts:
The atmosphere around the Earth is largely made up of two colorless gases: oxygen and nitrogen. Red and blue light reacts very different from each other to oxygen. Because the wavelength of blue light is roughly the size of an atom of oxygen, blue light interacts with the oxygen and is scattered by it, while red light, with its longer wavelength, goes right pass the oxygen atoms. If the Earth had no atmosphere, the sun’s light would travel directly from the Sun in a straight line towards our eyes and we would see the Sun as a very bright star in sea of blackness. But because the Sun’s blue light is scattered by the oxygen in the atmosphere, blue light from the Sun enters our eyes from all sorts of different angles and we see the entire sky as blue. The atmosphere scatters violet light even more effectively, but our eyes are more sensitive to blue. Wherever we look towards the sky, some light is bouncing off an oxygen atom and entering our eyes, making the sky appear to be blue.
Who hasn’t enjoyed watching a sunset as the Sun’s red disk sinks below the horizon. Why does the Sun now appear red? During sunrise and sunset, the Sun’s light must pass a greater distance through the atmosphere in order to reach our eyes because instead of dropping directly through the atmosphere, it reaches the Earth at an angle. The same scattering effect on the blue light, also takes place, but the blue light is unable to pass through the extra distance and reach our eyes. This leaves only the red light which passes, unhindered through the atmosphere and reaches our eyes in a direct line with little or no scattering. We see the Sun’s disk red because its blue light has been blocked by the atmosphere. We don’t see the entire sky red because there is no scattering and the red light reaches us in a direct line.