Why is the Sky Blue?

FAS Astronomers Blog, Volume 33, Number 8.

There are many profound questions in the world of science and astronomy. Some have answers, and some do not. Douglas Adams, in The Hitchhikers Guide to the Galaxy, addressed one of these in his search for the answer to the ultimate question of life, the universe, and everything. He, however, avoided a more fundamental question that is on the mind of many school children today. It is not “how did the Universe begin?” or “where did the planets come from?” It is, “Why is the Sky Blue?”

To understand the answer to this question, we need to start by focusing on light itself. 

Light is a wave. Ok, it is also a particle, but we’ll leave that for another time. Light is a transverse wave. It moves through the vacuum with a finite speed and oscillates perpendicular to the direction of motion. Sound, on the other hand, is a longitudinal wave. Sound waves move through a medium, such as air, and oscillate in the direction of motion. It both cases, light and sound waves are described by their frequency (number of cycles per second), and equivalently by their wavelength (the distance between two waves of light).

Light is also electromagnetic radiation. This radiation comes in all different forms that are distinguished by their frequency and wavelength. The electromagnetic spectrum ranges from low frequency/long wavelengths (radio waves, microwaves, and infrared waves) through visible light to high frequency/short wavelengths (ultraviolet waves, X-rays, and gamma rays). 

We see visible light, which is only a very small portion of the entire electromagnetic spectrum. 

Visible light is broken down into different colors, which are nothing more than different wavelengths and frequencies. The light we see ranges from the low frequency/long wavelength red up to the high frequency/short wavelength violet. Does anyone remember ROY G BIV, or R(ed) O(range) Y(ellow) G(reen) B(lue) I(ndigo) V(iolet)? What we see as white light is just all the colors mixed in together.

Light travels at a constant speed in a vacuum, which is around 186,000 miles per second. We designate this speed with the constant c. Light does, however, slow down as it travels through other mediums such as water and glass. Because of this, as light passes through something like a glass of water, an object placed in the water appears to bend. We say that the light is refracted by the water. The degree to which things “bend” is a function of the material’s refractive index, which is the ratio of the speed of light in a vacuum to that of light in the medium (n = c / v). The refractive index for a vacuum is 1.0, for water is 1.3, and for glass is 1.5 or higher depending on the type of glass. 

The different colors of light have slightly different refractive indices because they travel at slightly different speeds in various mediums. As such, when light is sent through a prism or is refracted by the water droplets after a rainstorm, we notice that the different colors of light bend at different angles, so we see all the colors of the rainbow. The higher the frequency, the slower light travels through a medium such as water or glass and the more it bends. 

The Sun’s light is mostly white light containing all the colors of the visible spectrum. It just so happens that blue light has the largest frequency (and the shortest wavelength). As such, it is scattered more by the particles in the Earth’s atmosphere than red light. So, we see a blue sky. On the horizon, light has much more of the atmosphere to pass through. The blue light is scattered and absorbed much more. This allows the red and yellow light to pass through. Therefore, we often see a red-looking Sun on the horizon.

If we want to be a bit more technical, thanks to the folks at Scientific American, the light from the Sun is absorbed by the molecules in the atmosphere. This causes the electrons to oscillate, and they produce more light of the same frequency. Because blue light has a higher frequency than red light, these oscillations are more pronounced. So, there is more blue light produced and sent out into our line of sight. When we look at the Sun’s light on the horizon, the light must travel a longer distance allowing the blue light to be absorbed and emitted many more times, and, in essence, it gets out of the way. As such, we see the red light, although it isn’t produced as much. 

Evidently, again thank you Scientific American, blue light scatters 10 times more than red light. Although, one might wonder, why don’t we see a violet sky? Violet light has a higher frequency than blue light, so it should refract more. However, we see a blue sky because there is more blue light coming from the Sun than violet light, so the blue light is more noticeable. 

P.S. The answer to the ultimate question of life, the universe, and everything is …… 42. Sorry, you need to read the book for the full explanation.