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Why is the sky blue?
When transmitted light such as sunlight enters our atmosphere it collides with the
oxygen and nitrogen atoms. The color with the shorter wavelength is scattered more
by this collision. Because violet and blue are the shortest wavelengths the sky appears
to be violet / blue. But because our eyes are more sensitive to blue light than they are
violet light, we perceive the sky as blue.
Our eyes contain thousand of rods and cones, which are the receptors for light.
Whenever one of the 3 Stooges pokes you in the eye you see a giant blue spot. This is
because the blue receptors have been activated. Blue is one of the primary colors and
thus more easily activated and seen by our eyes.
Blue is also how I feel when my baby leaves and my hound dog dies. Also, how I feel
when the cops pull me over and I see their blue lights flashing in my rear view mirror.
Then, again, blue is the color of the K-mart special, so this color isn't all bad.
Why is the sky blue: Summary
So, why is the sky blue? It is because blue light from the sun strikes the air molecules
and scatters and our eyes perceive it as blue.
Why is the sky blue: Short Summary
Why is the sky blue, you ask? Blue in sunlight collides with air molecules and our
eyes see it as blue.
http://science.howstuffworks.com/question39.htm
Here is something interesting to think about: When you look at the sky at night, it is black,
with the stars and the moon forming points of light on that black background. So why is it
that, during the day, the sky does not remain black with the sun acting as another point of
light? Why does the daytime sky turn a bright blue and the stars disappear?
The first thing to recognize is that the sun is an extremely bright source of light -- much
brighter than the moon. The second thing to recognize is that the atoms of nitrogen and
oxygen in the atmosphere have an effect on the sunlight that passes through them.
There is a physical phenomenon called Rayleigh scattering that causes light to scatter
when it passes through particles that have a diameter one-tenth that of the wavelength
(color) of the light. Sunlight is made up of all different colors of light, but because of the
elements in the atmosphere the color blue is scattered much more efficiently than the
other colors.
So when you look at the sky on a clear day, you can see the sun as a bright disk. The
blueness you see everywhere else is all of the atoms in the atmosphere scattering blue
light toward you. (Because red light, yellow light, green light and the other colors aren't
scattered nearly as well, you see the sky as blue.)
Here are some interesting links:
How Light Works
How the Sun Works
How Stars Work
The Physics Classroom: Blue Skies and Red Sunsets
Blue Sky and Rayleigh Scattering
Weather World 2010: Sunsets
Why isn't the Martian sky blue like the Earth's?
http://www.sky-watch.com/articles/skyblue.html
Home Articles Guide to Observing Tour of the Constellations Book Reviews
Why Is The Sky Blue?
High Noon
“Why is the sky blue?” may seem to be a simple question, but it actually touches
some of the deepest aspects of astronomy and skygazing because it deals with light.
“Seeing” something whether it’s a table across the room or a galaxy millions of light
years away consists of light entering our eyes and the brain taking the light and
creating a picture within our brain. So before asking the question “Why is the sky
blue?”, it’s necessary to know the answer to the question, “What is light?”
What Is Light?
Light consists of weightless particles (that contain no mass) traveling at about
186,000 miles or 300,000 kilometers per second. These particles of light are called
photons. But light also exhibits the characteristics of a wave. Traditionally, wave
activity is explained by imagining a pebble being dropped into a lake, but that
imaginary lake has been filled with imaginary pebbles years ago. So instead, imagine
a long rope tied to the side of a wall and a girl shaking rhythmically one end of the
rope up and down. Each up and down shake of the rope is a cycle and the number of
cycles per second is the wave’s frequency. If you observe the rope from the side, you
will see a wave pattern of alternating crests and troughs. The distance from one crest
to another is the wavelength. When the girl shakes the ropes more rapidly, the
frequency will naturally increase, but the wavelength will shorten.
Light acts in many ways similarly to the shaking rope. It also has a frequency and a
wavelength. As the frequency increases, the wavelength decreases and as the
frequency decreases, the wavelength increases. The frequency of light determines its
color. Blue light has a higher frequency and a shorter wavelength than red light. Our
eyes are sensitive to only a small part of the spectrum of frequencies. There is ultra-
violet and infra-red light just beyond our eyes’ range, radio waves with much lower
frequencies than light, and gamma rays with much higher frequencies. Sunlight
contains many different colors which can be separated through a prism or a rainbow
into different colors.
In short, keep in mind the following three facts:
Light must enter our eyes in order for us to see it
The Sun emits light with different colors
Different colored light has different wavelengths
Sunlight and the Atmosphere
The atmosphere around the Earth is largely made up of two colorless gases: oxygen
and nitrogen. Red and blue light reacts very different from each other to oxygen.
Because the wavelength of blue light is roughly the size of an atom of oxygen, blue
light interacts with the oxygen and is scattered by it, while red light, with its longer
wavelength, goes right pass the oxygen atoms. If the Earth had no atmosphere, the
sun’s light would travel directly from the Sun in a straight line towards our eyes and
we would see the Sun as a very bright star in sea of blackness. But because the Sun’s
blue light is scattered by the oxygen in the atmosphere, blue light from the Sun enters
our eyes from all sorts of different angles and we see the entire sky as blue. The
atmosphere scatters violet light even more effectively, but our eyes are more sensitive
to blue. Wherever we look towards the sky, some light is bouncing off an oxygen
atom and entering our eyes, making the sky appear to be blue.
Sunrise, Sunset
Who hasn’t enjoyed watching a sunset as the Sun’s red disk sinks below the horizon.
Why does the Sun now appear red? During sunrise and sunset, the Sun’s light must
pass a greater distance through the atmosphere in order to reach our eyes because
instead of dropping directly through the atmosphere, it reaches the Earth at an angle.
The same scattering effect on the blue light, also takes place, but the blue light is
unable to pass through the extra distance and reach our eyes. This leaves only the red
light which passes, unhindered through the atmosphere and reaches our eyes in a
direct line with little or no scattering. We see the Sun’s disk red because its blue light
has been blocked by the atmosphere. We don’t see the entire sky red because there is
no scattering and the red light reaches us in a direct line.
http://www.csiro.au/resources/ps1ti.html
Why is the sky blue?
Follow these instructions to find out why the sky is
blue and sunsets are red.
What you need
What to do
What's happening
On a clear day, the sky looks bright blue. At dusk, a
sunset can show reds, pinks and oranges. Why is the
sky blue? What makes the sunset red? Try this activity
to find out.
YO U WIL L NE E D
To do this activity you need to collect:
a large glass jar or vase
water
milk or powdered milk
a torch
a dark room
WH AT T O DO
1. Fill the jar with water.
2. Add a few drops of milk or half a teaspoon of
powdered milk.
3. Switch on the torch and shine down into the
water - it should appear blue.
4. Shine the torch through one side of the jar. The
torch should be near the middle.
5. Go to the other side and look at the light
through the water.
6. The water should appear pink, while the area
directly in line with the torch should appear a
yellow-orange colour.
WH AT ' S H APPE NI NG?
When you look up in the sky it is actually scattered
blue light that you are seeing.
The sun produces white light, which is made up of light
of all colours: red, orange, yellow, green, blue, indigo
and violet - these are the colours you find in a rainbow.
Light is a wave and each of these colours corresponds
to a different frequency, and therefore wavelength, of
light.
The seven colours of the spectrum all have different
wavelengths and they are arranged accordingly. Violet,
indigo and blue light have a higher frequency - which
means shorter wavelength - than red, orange, and
yellow light.
When the white light from the sun shines through the
Earth's atmosphere, it collides with gas molecules. It is
these molecules that scatter the light. Blue light has a
high frequency, and it is scattered ten times more than
red light.
But why does the sky become pink and red at sunset?
The sky changes colour because the atmosphere
spreads the light in a different way according to the
position of the sun.
The red light that is not scattered continues on in its
original direction. When the sun is on the horizon, its
light takes a longer path through the atmosphere to
your eyes than when the sun is directly overhead. By
the time the light of the setting sun reaches your eyes,
most of the blue light has been scattered out. The light
you finally see is reddish orange, the colour of white
light minus blue.
In this activity you changed the colour you saw by
changing the position of the beam of light. Similarly,
the sky appears a different colour depending on the
position of the sun. The water containing milk
molecules causes the light to scatter in the same way
the molecules in the atmosphere do.
The atmosphere is the mixture of gas molecules and
other materials surrounding the earth. It is made mostly
of the gases nitrogen and oxygen. Argon gas and water
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. It is these molecules and particles that cause
the scattering of light.
The most brilliant sunsets occur when there are lots
dust and smoke particles in the air - so beautiful sunsets
often occur over polluted cities.
This experiment was featured in Science by Email.
