Why is the sky blue?
Notes: April 26th 2011
Something to ponder…?
• 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
• 1. Sun is an extremely
bright source of light.
• 2. We have an
atoms of oxygen and
nitrogen in the
• Think of ringing bells!
• Physical phenomenon that causes light to
scatter when it passes through particles.
• Tiny particles bend high frequency light.
(oxygen, nitrogen)- blue mountain skies
• Large particles bend low frequency light.
(methane, sulfur)- sunsets in LA!
Blue sky… why not violet?
• This scattering of the higher frequencies of light
illuminates the skies with light on the BIV end of
the visible spectrum.
• Violet is scattered most easily, so why isn’t the
• Our eyes are more sensitive to light
with blue frequencies.
• Three primary color cones in our eye:
Blue, green, and red.
Why is the sun yellow?
• The lower frequencies
of sunlight (ROY)
tend to reach our eyes
unscattered as we
look directly at the
midday sun. Although
sunlight is a combo of
all colors, yellow is
Appearance of sun
• Appearance of the sun changes with the time of
• As the path which the light takes through the
atmosphere increases ROYGBIV encounters
more atmospheric particles, and scatters more
• During sunset hours, the light passing through
our atmosphere to our eyes tends to be
concentrated with red and orange frequencies.
This is why sunsets have a reddish-orange hue.
What effects a sunset?
• The effect of a red sunset becomes more
pronounced if the atmosphere contains more
and more sulfur aerosols (industrial pollutant)
• Some mountain ranges are famous for their blue
hazes which result from aerosols from the
vegetation reacting with ozone to form small
particles (200 nm) which scatter blue light.
• John Tyndall (1859)
discovered that when
light passes through
clear fluid holding
small particles in
suspension, the shorter
blue wavelengths are
strongly than red.
• People with blue eyes,
really don’t have blue
eyes. The fluid in our
eyes scatters or
reflects the blue light
from our eyes.
• In the cells of a blue-
• Blue gem stones.
Why is Mars sky red?
• Images from Viking Mars landers (1977), and
Pathfinder (1997) show a red sky from the
Martian surface. This is due to a red iron rich
dusts (magnetite) thrown up in dust storms,
which absorbs blue wavelengths and reflects red.
Why are clouds white?
• Clouds appear
white because they
consist of water
droplets that are
many sizes and
• (Mie scattering)
Why is sea water greenish blue?
• Water absorbs
• Absorption of red
light by water.
• What is left is the
complement of red,
which is cyan.
Why are sea creatures red?
• In deep sea water,
since there is no red
light coming in to be
reflected, the lobsters
and other sea creatures
look black, and
have avoided being
eaten by their
• The sky is blue because blue from sunlight
is scattered in all directions by molecules in
• Sunsets are red because all the other color
frequencies are filtered out.
• Water looks greenish blue because water
absorbs red light.
• The color of things depend on what colors
are reflected or absorbed by molecules.