Why Does Sunlight Become Blue In The Atmosphere?

On the last page we began answering the question on Why Is The Sky Blue? On this page, we want to understand a bit more about white light, so that we can understand how the atmosphere converts white light to blue light.

White Light

When you see white light, you might think you are just seeing the color white. However, you are not. White light is actually many colors added together. This means the white light from the sun is actually a combination of pure red light, orange light, yellow, green, blue, violet and indigo light all added together.

I don't want you to believe me though. I want you to see for yourself. So here are some experiments you can do on your own to see how the sun's light is actually a combination of different colors. If you have a shaped piece of glass (a prism), this will break up the white light into its individual colors:

prism breaking white light into its colors

Figure 1. Sunlight Passing Through Angled Glass Will Show It Is Made Up Of All Colors.

white light is made up of all colors in the rainbow

Figure 2. Holding White Paper Near The Prism Shows The Rainbow.

How does this work? Essentially light travels at a slower speed in glass than it does in air. This slow down forces the light to change directions slightly in the glass prism. The amount of the slowdown and change in direction depends on color. In this manner, when the white light travels into the glass, all of its components will come out at slightly different directions. Hence, we can see the rainbow color spectrum being produced from the white light. This is cool stuff! It means white light is really a combination of all the colors of the rainbow!

Now, if you don't have a prism, you can still do the experiment. Take a glass, and fill it with some water and take it outside around sunset. You will probably be able to see a rainbow come out of your glass:

using a glass with water in it late in the afternoon produces rainbows

Figure 3. Using a Wine Glass Filled With Water Produces Some Rainbows in the Sunlight.

In Figure 3, both the water and the glass slow down the sunlight. Hence we have the possibility of refraction (the bending of light) to produce rainbows in either the glass or the water. In the water, we see a kaleidoscope of colors from the light bouncing around in the water. Outside the glass, on the ground, we see a rainbow pattern as well.

Finally, you can even make this work even with a water hose in the daytime:

water through a hose on a sunny day will give you a rainbow

Figure 4. A water hose on a sunny day will break sunlight into its rainbow parts.

The Electromagnetic Spectrum

It turns out that the colors we see (red, blue, green, etc) are all light waves with different frequencies. This is illustrated in Figure 5:

red blue green light waves

Figure 5. A Plot Of Red, Blue and Green Light Waves Versus Time.

The shape of the waves in Figure 5 are known as sinusoids. All light waves travel at the same incredibly fast speed (the speed of light, about 186,000 miles per second). The difference between the waves is how fast they quickly they oscillate. From Figure 5, we see that the red wave changes the slowest. The green wave is faster and the blue wave oscillates the fastest. How quickly the wave oscillates tells us the frequency of the light wave.

The wavelength of light tells us how long each cycle of the wave is. From Figure 5, we see that the red wave has the longest wavelength, green the next longest, and blue has the shortest wavlength.

It turns out there are many frequencies of light waves, even more than we can see. Television signals, radio signals, cell phone waves are all light waves at different frequencies. They all move at the same speed, and have the same properties as visible light, we just can't see them. If we think about light in terms of frequencies (how quickly they bounce up and down), we have the electromangetic spectrum:
Frequency Band Name Frequency Range Wavelength (Meters) Application
Extremely Low Frequency (ELF)
3-30 Hz
10,000-100,000 km
Underwater Communication
Super Low Frequency (SLF)
30-300 Hz
1,000-10,000 km
AC Power (though not a transmitted wave)
Ultra Low Frequency (ULF)
300-3000 Hz
100-1,000 km
Very Low Frequency (VLF)
3-30 kHz
10-100 km
Navigational Beacons
Low Frequency (LF)
30-300 kHz
1-10 km
AM Radio
Medium Frequency (MF)
300-3000 kHz
100-1,000 m
Aviation and AM Radio
High Frequency (HF)
3-30 MHz
10-100 m
Shortwave Radio
Very High Frequency (VHF)
30-300 MHz
1-10 m
FM Radio
Ultra High Frequency (UHF)
300-3000 MHz
10-100 cm
TV, Cell Phones, GPS
Super High Frequency (SHF)
3-30 GHz
1-10 cm
Satellite Links, Wireless Communication
Extremely High Frequency (EHF)
30-300 GHz
1-10 mm
Astronomy, Remote Sensing
Red Light
4.6 THz (4.6*10^14 Hz)
650 nm (nanometers)
Visible to Human Eye
Orange Light
5.1 THz (5.1*10^14 Hz)
570 nm (nanometers)
Visible to Human Eye
Yellow Light
5.3 THz (5.3*10^14 Hz)
590 nm (nanometers)
Visible to Human Eye
Green Light
5.9 THz (5.9*10^14 Hz)
510 nm (nanometers)
Visible to Human Eye
Blue Light
6.3 THz (6.3*10^14 Hz)
475 nm (nanometers)
Visible to Human Eye
Violet Light
6.7 THz (6.7*10^14 Hz)
445 nm (nanometers)
Visible to Human Eye
X-rays
3*10^16 - 3*10^19 Hz
0.01-10 nm (nanometers)
Medical, Scanning inside Objects

Table 1. The Electromagnetic Spectrum

As you can see from Table 1, what we can see as visible light is only a very small section of the electromagnetic spectrum. There are many commonly used forms of light that we will never be able to see with our eye. This is just a brief tutorial, for a bit more information, see the frequency section of Antenna-Theory.com.

Summary

We are still interested in understanding why the sky is blue. Now we've learned a couple things:

  • The white sunlight is actually made up of all colors of light
  • When the sunlight hits the atmosphere, it scatters the light and we perceive the color blue

    In the next section, we'll look at understanding why the atmosphere scatters light, and why we then perceive the sky to be blue in the daytime. If you're ready, move on to the Earth's Atmosphere page.


    Back: Why is the Sky Blue?
    Top: White Light
    Next: Earth's Atmosphere

    Copyright - This explanation on "White Light" is copyrighted. This shouldn't be reproduced without permission from the author. Copyright www.whyistheskyblue.co, 2013.

  • 1. Why Is The Sky Blue?

    2. White Light

    3. Earth's Atmosphere

    4. Antennas

    5. Sunlight and the Eye