The famous "green flash" at sunset is a scientific
phenomenon that has to do with the refraction and
dispersion of light. It is rarely seen because it
requires specific conditions but it is
scientifically possible. It may also be possible
to see similar flashes during sunrise as
To explain the most common cause of the
green flash we need to think about the dispersion
of light. White light is made up of light in the
colors of the rainbow. These colors have
different energies and so pass through matter at
slightly different speeds. Shorter wavelengths of
light (those with higher energies -blue light) are
more efficiently scattered out of the beam of
light as they pass through air particles (why the
sky is blue). Dispersion of light also depends on
the density of the matter it is passing through.
The light is also refracted - it bends towards
more dense matter. The atmosphere is more dense
closer to the surface of the earth. And so the
light bends down towards the surface of the earth.
Now, the part more difficult to understand is
that when the sun nears the horizon some of the
light you see is coming from below the horizon. I
like to think of this as a mirror image - a
mirage, sort of like what you see when driving on
a hot day and the road surface seems to shimmer in
heat waves reflecting the sky. Under the right
conditions -if the air temperature near the
earth's surface is higher than the air above it,
the hot air will act like a mirror. As the sun
dips below the horizon there will be a few seconds
of time when the image of the tip of the sun going
down and the reflection is magnified. Then due to
the dispersion of light and the angles of
refraction, the green color of light can be seen.
The best bet for observing this phenomenon?
Observe the sunset from lower altitudes (sea
level)on a very clear day where the sun is setting
on a very flat horizon. Another tip is to use
binoculars or a camera and wait while the sun is
setting until the last moments before staring at it.
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