Show Number 909


Why is the sky blue?

David finds out how the atmosphere makes different colors.


Why is the sky blue? People have been asking this question for centuries. The generally accepted scientific explanation for blue sky was first proposed by Lord Rayleigh, a British physicist and mathematician of the late 1800's.

Rayleigh's theory was unique. He agreed with other scientists of the time that dust and other large particles in the atmosphere could scatter light, and that when this occurred, the spectral colors red and blue were revealed. Rayleigh also agreed that no light was absorbed by large particles, but he took this concept of scattering in the atmosphere one step further.

Rayleigh concluded that as light traveled from the Sun to an observer, it encountered molecules, mostly of nitrogen and oxygen, in the atmosphere. Rayleigh then calculated a mathematical formula that demonstrated that, even in an atmosphere without smoke and dust, gas molecules like oxygen could redirect sunlight and scatter it in many directions. Sunlight is a form of visible light that contains all of the colors. When it is scattered, it is perceived by the human eye as having a specific color. When sunlight encounters gas molecules in the atmosphere, high frequency blue light is scattered out first. The most intense blues are usually seen between 10:00 a.m. and 3:00 p.m. on cloudless days, if we look at the sky about 45 degrees above the horizon with our with our backs to the Sun.

As the Sun begins to set, the sky at the horizon often appears to be red. Sunlight entering at the horizon level travels through more atmosphere than sunlight entering overhead. Most of the shorter wavelength light has been scattered out, allowing longer wavelengths of light to reach our eyes. When particles of dust provide additional opportunities for scattering, sunsets have a red glow. The brilliance is often enhanced in the sky by clouds.

Things to Talk About

  1. What kind of color do you see at night? What does the sky look like when the Moon is full?
  2. What happens to light and the color of the sky during an eclipse?

Wavelength--The distance from the top of a crest of a wave to the top of the following crest. This measurement applies to waves of all kind. The length of a wave is inversely proportional to the frequency of the wave.

Frequency--The number of wavelengths which pass a point during one second.

Spectrum--For sunlight and other white light, the spread of colors seen when the light is passed through a prism: red, orange, yellow, green, blue and violet. (Some definitions also list the color indigo between blue and violet).

Blue--A color whose hue is that of the clear sky or that of the portion of the color spectrum lying between green and violet.

White--Light, such as sunlight, that is a combination of all the colors.

Hue--A graduation of color; the attribute of colors that permits them to be classed as red, yellow, green, blue or an intermediate between any pair of these colors.


Additional Sources of Information

Activity Page

Weather or Not?

How does a change in weather affect the color of the sky?

Main Activity

By keeping daily records of weather conditions, you will be able to study how particles in the atmosphere scatter light and change the color of the sky.


1. Begin a two to four week log to record your daily observations of the sky where you live.

2. Choose two times per day that you will record your observations in the log. Always look at the sky with the sun behind you, and choose two areas of the sky to monitor. Your observation points should be at a 45 degree angle above the horizon. Never look directly into the sun!

3. Decide what words you will use to describe the different hues you may see. Pick a number to correspond to the intensity of the various hues. Include descriptions of the clouds and their brightness.

4. Write a brief description of what you see at both observation points each time you observe the sky. Include your notes about the weather at the bottom of each day's entry.


1. How did the sun's angle in relation to the horizon affect what you saw at each viewing point? How was it different on sunny and rainy days?

2. What did you notice about the brightness of the thick and thin clouds?

3. When and where did you see the most intense colors? How might the cloud cover have contributed to what you observed?

4. Did the hue and and the intensity compare to what you expected at the time you begin the activity?

Over the course of a month, look for examples of sunlight that has been scattered. Note the conditions that might have caused the sunlight to be redirected. Examples might include dew on a spider's web, oil on the ground after a rainstorm or light through a chandelier or glass ornament.

Make a variety of liquid suspensions. Add equal amounts of flour, baking soda, sugar and salt to 100 milliliters of water. Put a penny in several plastic cups and then pour equal amounts of the mixture into each cup. Shine a flashlight into the cup. What do you see? What happens if you vary the amount of the dry ingredients?

Find two pairs of polarizing sunglasses or pieces of polarizing filters. Between 10 a.m. and 3 p.m., look at the sky, away from the sun, through the glasses or filter. Place the second pair of glasses or piece of filter in front of the first. Rotate the polarizers and look through them again. How has the image been altered?

Build your own spectroscope to study the colors of the spectrum. You will need a covered shoebox or oatmeal box and a piece of diffraction grating (available through Edmund Scientific Company listed in the Resource section) that measures five centimeters by five centimeters. Cut a two and a half by two and a half centimeter square in one end of the box. Tape the diffraction grating over the hole. In the other end of the box, make a vertical slit about three centimeters long and no more than one centimeter wide. Look through the slit at the light of a lamp. Continue looking through the slit while rotating the box 90 degrees. Look through the slit at other light sources like florescent bulbs, halogen street lights, or at the sky AWAY FROM THE SUN. Compare your observations.

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Educational materials developed with the National Science Teachers Association.

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