Sunlight and the Human Eye

We continue on our quest of understanding Why Is The Sky Blue? We've learned white light is the sum of all colors of light, the sunlight hits the atmosphere's molecules, which causes the electrons to move like electric current, and molecules excited in this manner will radiate best at higher frequencies. On this page, we will learn Why is the Sky Not Violet?


We have discussed how sunlight is made up of all the colors of the rainbow. However, the exact amount of light at each frequency can be measured, and the plot is known as the Solar Irradiance Spectrum, which is shown in Figure 1:

solar irradiance spectrum

Figure 1. The Solar Spectrum - The Peak Energy is At the Blue Wavelength.

The yellow curve in Figure 1 shows the spectrum of light versus wavelength (the inverse of frequency) at the atmosphere (i.e. in the sky). As you can see, the amount of energy is peak in the blue wavelengths, and drops dramatically for purple frequencies (and smaller wavelengths).

The red curve shows what the sunlight spectrum is at Sea Level (after atmosphere absorption). Note that the gaps are due to absorption by different molecules within the atmosphere.

This partly explains why the sky is blue, and not purple. However, there is another effect which has to do with the human eye.

The Human Eye

The human eye has 3 types of receptors known as cones. These cones are microsocopic structures within the eye responsible for observing blue, red and green light. The human eye has millions of cones for detecting light.

If you see purple, then your red and blue cones are excited equally (and the green cone not very well). To see yellow, the green cone is stimulated a bit more than the red cone. For a human to perceive a white color, this means all the cones are stimulated.

Let's look at the sensitivity of the 3 cones to light versus frequency. This is shown in Figure 2:

sensitivity of human eye cones

Figure 2. Sensitivity of Human Eye Cones.

Figure 2 shows how your 3 cones perceive color. The blue curve shows the response versus wavelength for your blue cone. The green curve shows the response for your green cone, and the red curve is the response for your red cone. The important thing is the blue curve in Figure 2. The blue curve is most sensitive at blue wavelengths, and drops off dramatically for the higher frequency (lower wavelength) violet light. Hence, the human eye itself has a natural sensitivity or preference for blue light as opposed to purple light.

Therefore, the two primary reasons we see the sky as blue and not purple is because:

  • The sun's light has more energy at blue than at purple light
  • The human eye is more sensitive to blue light than purple light

    I think we did it! If we are there we now know why the sky is not black, why it is not purple, and why it is not red. The sky is blue!


    The sky is blue is due to the following facts:

  • Sunlight is made up of all the colors of the rainbow. The amount of light from the sun is strongest at the blue frequencies, and less at purple and red frequencies.
  • If you see blue in the sky, there must be a light source in the sky away from the sun.
  • The Earth's atmosphere are made up of tiny molecules, mostly Oxygen (O2) and Nitrogen (N2).
  • When sunlight hits the atmospheric molecules, they induce an oscillating current at the same frequency as the light itself.
  • The excited molecules in the atmosphere become tiny antennas, which radiate with an efficiency that is proportional to the fourth power of frequency (). This means the molecules reradiate more energy at higher frequencies (i.e. a blue shift)
  • Small antennas radiate energy somewhat uniformly. This in addition to all the random orientations of the molecules makes it so the light hitting the atmosphere scatters in all directions. Hence, no matter where you look in the sky during the daytime, you will see blue.
  • The human eye is more sensitive to blue than to purple light, which helps to explain why the sky is blue and not purple.

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    More Information

    For more information, see the following topics:

  • Antennas - For a solid introduction to antenna theory, and a fascinating application of electromagnetics, check out the page.
  • Fourier Transform - You'll notice we talked a lot about frequency here. It turns out all of electronics is based on frequency and sinusoids and their combination. To learn more about this, along with "spectral analysis" using the Fourier Transform, check out this site.
  • Maxwell's Equations - All of electromagnetics is governed by Maxwell's Equations. To learn more about how all of that works in an intuitive manner, check out this page on Maxwell's Equations.
  • When Does The Time Change - This site is there so you can remember when to set your clocks back for daylight savings time!
  • Use the - This site provides an simple bitcoin explanation, along with the latest news on the bitcoin phenomenon.

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    These notes on sunlight spectrum and the human eye as relates to why is the sky not violet copyrighted. Copyright, 2013.

  • 1. Why Is The Sky Blue?

    2. White Light

    3. Earth's Atmosphere

    4. Antennas

    5. Sunlight and the Eye