WHY
IS THE SKY
BLUE?
If
waves are made at the
side of a
lake and a rock is
sticking out of the
water, a little off shore,
when they reach it, do the ripples "see" the rock -
do they
bounce off it,
bend round it - or do they pass it by, as if
nothing was there?
If the
wavelength of the waves (the
distance between crests) is smaller than, or equal
too (roughly) the size of the rock, the waves "see"
it. If they are longer than the rock they pass by
unchanged. For this reason a light microscope may
only discriminate detail of a size equal to the
wavelength of light. Finer detail requires an
electron microscope - electrons have a shorter
wavelength than
photons because of their high
momentum. (See:
Quantum Mechanics
Basic Idea.)
White light contains blue wavelengths. If
there are the appropriate sized "rocks" in the air
the problem is close to being solved.
SUNLIGHT JUST MISSING THE EARTH
If a
searchlight beam was looked at
from the side nothing would be seen, unless
there was some sideways
scatter of the
light.
Some
light from the Sun
just misses the
Earth. If it were not
scattered (some of it downward) by the
atmosphere
none of it would ever illuminate the surface. If
the blue light is scattered - if there is something
in the atmosphere blue waves can "see" - then, when
someone looks up at the sky, they will
observe blue.
The problem is there are no
particles in the atmosphere of the appropriate
size.
ATMOSPHERE PARTICLES
All
molecules and
atomic
scale
particles are too small. Neither are the various
dust particles appropriately sized; they would scatter all wavelengths, not pick out the blue. Indeed, if a
glass tube is filled with perfectly dustless, clean
air and white light shone along it, blue light
scatters out of the sides. It is clear therefore
that it is the air itself and not any
sophistications which is responsible for
dissipating blue light.
RESOLUTION OF THE "PARADOX," HOLES IN THE
SKY
There is no particle in the atmosphere
of an appropriate size to scatter blue light; we
appear to be up against a brick wall.
The - rather elegant, perhaps -
resolution of this metaphorical paradox is gained by
imagining the air to consist in many particles
flying about at random. By pure
chance there will exist holes in the air where,
simply by chance, no particles happen to be. These
holes are the desired size, turning the sky blue.
PREDICTION
Should the Earth's covering of gas get
thinner, for any reason, it is easy to see that the
holes in the sky would grow larger and the sky would
turn red.
The Sun is red when low in the sky
because its light has traveled through a large
thickness of atmosphere, before reaching the eye and
the scattering of the blue sideways, throughout its
journey, has left only the longer, redder
wavelengths.
SHEPHERD'S WARNING
Bugger off or I'll shove a sheep up
your tradesman's entrance.
[Note of caution: This frequently
asked question was addressed in New Scientist
magazine; the above explanation was not among
those printed.]
Quote from Kung 'No offence Sporus but the reason it wasn't printed is probably because it's piffle.' This raises an interesting point. I got the theory from a book by Jeans. Jeans and Rayleigh worked together (recall the Jeans-Rayleigh Law). Perhaps they fell out over the sky blue question. I'd back Jeans, he was unquestionably a genius.