Why is the Sky Blue?
On a clear sunny day, the sky above us looks bright blue. In the
evening, the sunset puts on a brilliant show of reds, pinks and oranges. Why is
the sky blue? What makes the sunset red?
To answer these questions, we must learn about light, and the
Earth's atmosphere.
Table of Contents
The Atmosphere
The atmosphere is the mixture of gas molecules and other materials
surrounding the earth. It is made mostly of the gases nitrogen (78%), and
oxygen (21%). Argon gas and water (in the form of vapor, droplets and ice
crystals) are the next most common things. There are also small amounts of
other gases, plus many small solid particles, like dust, soot and ashes,
pollen, and salt from the oceans.
The composition of the atmosphere varies, depending on your
location, the weather, and many other things. There may be more water in the
air after a rainstorm, or near the ocean. Volcanoes can put large amounts of
dust particles high into the atmosphere. Pollution can add different gases or
dust and soot.
The atmosphere is densest (thickest) at the bottom, near the Earth.
It gradually thins out as you go higher and higher up. There is no sharp break
between the atmosphere and space. Learn more about the atmosphere.
Light Waves
Light is a kind of energy that radiates, or travels, in waves.
Many different kinds of energy travel in waves. For example, sound is a wave of
vibrating air. Light is a wave of vibrating electric and magnetic fields.
It is one small part of a larger range of vibrating
electromagnetic fields. This range is called the electromagnetic spectrum.
Electromagnetic waves travel through space at 299,792 km/sec
(186,282 miles/sec). This is called the speed of light.
The energy of the radiation depends on its wavelength and
frequency. Wavelength is the distance between the tops (crests) of the waves.
Frequency is the number of waves that pass by each second. The longer the
wavelength of the light, the lower the frequency, and the less energy it
contains.
Colors of Light
Visible light is the part of the electromagnetic spectrum that our
eyes can see. Light from the sun or a light bulb may look white, but it is
actually a combination of many colors. We can see the different colors of the
spectrum by splitting the light with a prism.
The spectrum is also visible when you see a rainbow in the sky.
The colors blend continuously into one another. At one end of the
spectrum are the reds and oranges. These gradually shade into yellow, green,
blue, indigo and violet. The colors have different wavelengths, frequencies,
and energies. Violet has the shortest wavelength in the visible spectrum. That
means it has the highest frequency and energy. Red has the longest wavelength,
and lowest frequency and energy.
Light in the Air
Light travels through space in a straight line as long as nothing
disturbs it. As light moves through the atmosphere, it continues to go straight
until it bumps into a bit of dust or a gas molecule. Then what happens to the
light depends on its wave length and the size of the thing it hits.
Dust particles and water droplets are much larger than the
wavelength of visible light. When light hits these large particles, it gets
reflected, or bounced off, in different directions. The different colors of
light are all reflected by the particle in the same way. The reflected light
appears white because it still contains all of the same colors.
Gas molecules are smaller than the wavelength of visible light. If
light bumps into them, it acts differently. When light hits a gas molecule,
some of it may get absorbed. After awhile, the molecule radiates (releases, or
gives off) the light in a different direction. The color that is radiated is
the same color that was absorbed. The different colors of light are affected
differently. All of the colors can be absorbed. But the higher frequencies
(blues) are absorbed more often than the lower frequencies (reds). This process
is called Rayleigh scattering. (It is named after Lord John Rayleigh, an
English physicist, who first described it in the 1870's.)
Why is the Sky Blue?
The blue color of the sky is due to Rayleigh scattering.
As light moves through the atmosphere, most of the longer
wavelengths pass straight through. Little of the red, orange and yellow light
is affected by the air.
However, much of the shorter wavelength light is absorbed by the
gas molecules. The absorbed blue light is then radiated in different
directions. It gets scattered all around the sky. Whichever direction you look,
some of this scattered blue light reaches you. Since you see the blue light
from everywhere overhead, the sky looks blue.
As you look closer to the horizon, the sky appears much paler in
color. To reach you, the scattered blue light must pass through more air. Some
of it gets scattered away again in other directions. Less blue light reaches
your eyes. The color of the sky near the horizon appears paler or white.
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