Both the sky and the ocean are blue, but for
two very different reasons.
The sky is blue because of something called
Rayleigh scattering. For very small
particles such as individual molecules, the
wavelength of light that is reflected is a
function of the particle size and type of
molecular bonds between atoms. It turns out that
air molecules are just the right size and the
bonds vibrate in such a way that they scatter
blue light the most. This causes the sky to be
overwhelmed with blue light.
When you watch the sky turn different colors at
sunset, you are watching light become increasingly
scattered by more atmosphere and larger particles
(dust) as the sun descends toward the horizon.
On cloudy or foggy days, the water droplets in
the atmosphere both absorb the light and scatter
all wavelengths equally, causing a grey or white
The ocean is not blue because of
scattering. Instead, the ocean is blue (as is
pure water) because of selective
absorption. Liquid water absorbs
preferentially both short wavelength light (UV)
and longer wavelength visible and infrared light.
If you drop a light sensor that measures the
amount of light at different wavelengths into the
ocean or into a lake, what you find is that almost
all of the UV (280-400nm) and infrared (heat)
wavelengths are absorbed.
Of the visible light, the longest visible
wavelengths are absorbed most (red-orange,
600-700nm), then the medium-length visible
wavelengths (yellow-green, 500-600nm) and last the
short visible wavelengths (blue-violet,
400-500nm). This explains why very pure water,
without many particles, appears deep blue. It
also explains why, as you descend with depth in
the ocean and the light has to travel further and
further though the water, light will weaken and
appear more and more blue (you may notice this
diving or when looking at underwater video or
Particles and dissolved substances, depending
on their size and physical properties (including
the type of molecular bonds), will scatter or
absorb visible light at different wavelengths
and actually change the color of the water.
Tropical water, which has few particles and
very little color- absorbing dissolved substances,
appears deep blue.
Water in very productive areas of the ocean
with lots of algae will appear green. (James
Joyce described the Irish Sea as "snot green" in
Ulysses.) Water with a lot of sand or suspended
particles will appear brown, and often river
water, which has lots of color-absorbing dissolved
compounds and suspended mud, will appear
Scientists actually use this information to
study the ocean from space-- by measuring the
wavelengths of light reflected from the surface of
the ocean using satellites. In this way they can
measure temperature and estimate the amount and
kind of particles suspended in the water.
Answer to why the grass is green?:
The simplest answer to why plants are green is
that the parts that are green (e.g., leaves and
stems) are reflecting green light. Objects
appear certain colors to us because they reflect
those colors and absorb every other color.
For example, red things reflect red but absorb
everything else. (What colors do you think
white objects reflect and absorb? What about black
objects? ) Molecules that absorb colors are
usually called pigments. We have pigments in
our skin, as do most animals. Plants have many
different kinds of pigments. The most common
pigment in plants is called chlorophyll-a,
which happens to absorb red light and reflect
green. Plants need to absorb light because they
use energy from light to turn carbon dioxide into
proteins and carbohydrates. This is called
photosynthesis. (When we make proteins and
carbohydrates, we have to rebuild them from the
digested proteins and carbohydrates in our food.
This is called anabolism. )
Plants also need water for
photosynthesis. Water is made up of two
hydrogen atoms and one oxygen atom. When plants
use water to turn carbon dioxide into proteins and
carbohydrates, water becomes split apart into its
separate hydrogen and oxygen atoms. One molecule
of oxygen gas (which is made of two oxygen atoms)
is generated for every two molecules of water that
are split. The oxygen gas is not needed by the
plant and so it is released into the atmosphere.
The hydrogen atoms are conserved within the plant
and are used to make energy (ATP).
Why do you think many land plants would
evolve to absorb one color of light in particular
(in this case, red)?
Not all plants are green. For example, some of
the single-celled plants that float in the surface
waters of the ocean are brown or orange. Why do
you think this is? (As a hint, water absorbs
some colors more than others, so sunlight
actually changes once it enters the ocean).
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