Question #7: Why is the sky blue?

This question comes from Sandra on Facebook.

Sandra, your question relates back to another question about the electromagnetic spectrum. When we consider the sky we have to think about the wavelengths of light. White light is coming from the sun to Earth. White light contains all the visible colors, we see this as a rainbow of colors through a prism. Of those visible colors, red has the lower energy and the longest wavelength, blues have the higher energy and the shorter wavelength.

Our air is full of particles, oxygen, carbon dioxide, hydrogen, dust, nitrogen, water droplets and so on are present in the atmosphere. We can imagine that the sky is full of atoms and molecules floating around though they are too small for us to see with our eyes. (Until the dust and water droplets condense to make clouds or there is too many particles and we see haze or smog.)

Now, imagine a wavelength of light coming into the atmosphere. If it is a red wave it is coming in in wide waves. This red light has less of a chance of bumping into particles. If the wave is blue though it is zigzagging back and forth a lot more and has more chance of bumping into a particle. When a blue wave bounces into a particle it is dispersed. That means it is deflected from that particle and maybe into another and so on until it is eventually deflected toward the ground. This deflection is called the scattering of light.

So we have red light coming in straight and blue bouncing off in every which direction like this diagram:

From NASA.gov

With all those blue light waves bouncing in every direction no matter where you look there will be blue light entering your eyes.

The next question is why blue instead of violet? Violet has more energy and an even shorter wavelength.

There are four reasons for this. The first is the light itself. The light coming from the sun is not constant nor does it always represent every color in the visible light spectrum. Some of the highest energy light, the violets, are absorbed by the upper atmosphere. The blues and violet are scattered around but we are also getting some light that is reaching us unscattered. So, the violet and blue are lightened. We can see this effect if you are in an open landscape. On a bright, sunny, cloudless day the sky overhead will appear the bluest and will fade to a more and more pale color toward the horizon. The last reason has to do with our eyes. We have three major types of cones or color sensitive receptors in our eyes; red, blue and green. These are named because of the colors they are most sensitive to. Our blue receptors sense blues, indigos and violets. When we look at the sky our blue receptors are stimulated more than the red or green and so we perceive a blue sky.

And since I have forgotten to do this with past questions, from now on I will be citing sources. For this question it includes the following:

University of California
NASA's Space Place
How Stuff Works
Science Made Simple

Question #1: Exactly how much of the color spectrum do humans really see?

This question comes to us from Flexcia on FB.

The first thing we must examine is the question. There is a common misconception that the full electromagnetic spectrum and color spectrum are the same. The color spectrum is, by definition, the visible light or the colors we can normally see. The electromagnetic spectrum is the visible plus other parts of electromagnetic energy. (see the image here.)

Electromagnetic radiation occurs in waves. The nanometer (nm or 10^-9 meters) measurement is how far it is from one hump of the wave to the next. This is called wavelength. Higher numbers mean there is farther distance between the waves and a higher energy, slower moving wave. Higher energy has shorter wavelength.

Here is a way to think about wavelength.

So, imagine you are holding this rope and the other end is tied to a tree. If you want to do slow waves you can move your arm slowly but if you want more energy in the waves you have to really move your arm with a lot of energy. This can help remember whether short or long waves have more energy.


Humans vision is limited to the visible light between 400-700nm. However, animals can see in other wavelengths. For example, birds and bees can see the visible spectrum and ultraviolet (UV) portions of the scales. Snakes and some other animals can sense the thermal infrared (IR) portions of the spectrum. Spiders only see green and UV. So there are all types of vision out there among animals.

Back to Flexcia's question. What part can humans see? The visible spectrum is our limit but there are millions of colors within that spectrum. Every color you've ever witnessed in a movie, a video game clothing, nature, all those colors are all in the visible spectrum.

Within that spectrum of visible light some individuals have more visual acuity. This means that some people can distinguish more colors than others. At the extremes there are things like red-green colorblindness where individuals can not accurately distinguish between red and green light wave lengths. The other end is those who have hyper-acuity. Recently a woman was discovered/found that has tetrachromat. Tetrachromat is the ability of being able to distinguish between more colors than the average person. If you want to test your own abilities to distinguish colors you can give it a try here.

While we are on the subject of seeing we should take a moment here at the end to discuss how we are seeing those colors. Most color we see is reflected light. Well what does that mean? Here's the best description for the process.

Imagine you are looking at a green leaf of a tree. Sunlight (which contains all the visible light colors) comes down and hits the leaf. Trees absorb light energy for photosynthesis. Tree leaves absorb all colors except green light. This green light reflected from the leaf and enters our eye. Because only green light is entering our eye that is the color we see. Leaves are green. Well, what about in the Autumn? Tree leaves are other colors. This is because of the change in the light that a tree is using.. absorbing.. as it prepares to go dormant.

Now, let's apply this principle to other things. Look at your shirt or a nearby object. Whether you are outside (in the sunlight) or inside (under white artificial light) all the visible spectrum colors are being produced by the light source. The item you are looking for is absorbing all the light colors except the one you see. So if your shirt is blue... it is absorbing all the visible colors EXCEPT blue.

If we expand this idea we can relate this back to electromagnetic radiation. Imagine it's a very sunny day. You spread out two blankets in the sun.. a black one and a white one. We all recognize that the black one will be hotter to the touch after being in the sun. Why?

The light we see is white. That means in order to perceive an item as white it is reflecting all the visible colors. None of the energy from those light waves are being absorbed by the fabric. (remember energy often creates heat). The black fabric though is absorbing all the colors. Technically, none of the light waves are being reflected to our eyes. The black fabric is absorbing more energy and therefore is hotter to the touch.

What about our computer screens? This is not reflected light. Instead of reflecting light from our monitor the monitor is projecting a certain color of light produced by the mix of pixels and their colors. High definition TVs often have more pixels per inch on the screen of the TV and more pixels per inch on the tape or video taken. This allows for more colors that give us a feeling of a higher depth or definition to the image.

I hope this answers your question Flexcia.