It is a beautiful summer night and you are standing
outside waiting for the sun to set. The wind blows across your
skin leaving ripples of little hairs sticking straight up. The
smell of fresh cut grass wakens your spirit as you lie on a
hill beneath a large oak tree. Suddenly the sky becomes a vivid
neon orange and the clouds, brighter than the sky, look like
mirrors. Hues of pink and red abruptly break into the orange
making the sky a plethora of warm colours. The cumulus clouds
change colour and become almost iridescent. You wonder, "Why
does this happen? Why are sunsets so colourful and clouds so
The sky behaves this way because of the light
rays from the sun and the earthís atmosphere. Firstly, the sun
is made up of pure white light. On earth we see the sun as yellow
during the day and orange or red during sunset because of scattering,
but if you were in space the sun would be white because there
is no atmosphere to block itís rayís paths. Scattering occurs
because the earth has an atmosphere.
Earthís atmosphere is made up of seventy-eight
percent nitrogen gas and twenty-one percent oxygen gas.
Argon and water, either in the form of vapor, droplets, or ice
crystals, make up the rest of the atmosphere. In addition, many
small particles inhabit the atmosphere such as dust, ashes and
soot, pollen, and salt. The earthís atmosphere gets increasingly
denser the closer one gets to earth. It is because of these
pollutants and the density of the atmosphere that make the sky
Pure white light, which the sun is made of,
consists of the colours: red, orange, yellow, green, blue, indigo,
and violet. Light travels in wavelengths at a certain frequency.
Wavelengths are the lengths between the crests of each the waves
and frequencies are the numbers of waves per second. Red
travels at the lowest frequency and has the longest wavelength,
where as blue has the fastest frequency and the shortest wavelength.
Light, in space, travels in a straight line.
When light from the sun hits the earthís atmosphere it continues
to go straight until it bumps into something, such as dust or
water particles. All different colours are affected differently
and all can be absorbed, but higher frequencies, such as blue,
are absorbed more often than lower frequencies, such as red.
This is called Rayleigh scattering, named after Lord
John Rayleigh, an English physicist who first described
it in the 1870ís.
The sky is blue during the daytime because of
Rayleigh scattering. The blue, because of itís shorter wavelength,
bumps into gas molecules that are very tiny, and are then absorbed
by them. The blue light is then radiated everywhere, and the
sky is blue. Red and orange light is not absorbed because their
wavelengths are long and tend not to bump into small gas particles.
As one looks at the horizon during the day time, it looks white.
That is because it takes a longer time for the blue light to
hit your eye, thereby passing through more air and the light
gets scattered in more directions.