Sundogs and Rainbows

By Jennifer Lyons and Marcus Kienlen
Waves and Optics, Dr. Dawes
What are they?
-An atmospheric phenomenon that creates bright spots of light in the
sky, often on a luminous ring on either side of the sun.
-Sundogs may appear as a colored patch of light to the left or right
of the sun, 22° distant and at the same distance above the horizon
as the sun, and in ice halos.
-They can be seen anywhere in the world during any season, but
they are not always obvious or bright. Sundogs are best seen and are
most conspicuous when the sun is low.
How do they form?
- Sundogs form as sunlight is refracted by hexagonal
plate-like ice crystals. The crystals drift and float
gently downwards with their large hexagonal faces
almost horizontal.
--They are visible when the sun is near the horizon
and on the same horizontal plane as the observer and
the ice crystals.
-As sunlight passes through the crystals, it is bent by
22 degrees before reaching our eyes. This bending of
light results in the formation of a sundog.
Red light is refracted less strongly than blue and the
inner, sunward, edges of sundogs are therefore red
hued. However, it is often so meshed together that
it just appears as white light.
The difference between sundogs and halos is the
orientation of the ice crystals through which the light
passes before reaching our eyes. If the hexagonal
crystals are oriented with their flat faces horizontal,
a sundog is observed. If the hexagonal crystals are
randomly oriented, a halo is observed
How they are formed: The light enters the raindrop and is
initially refracted. The most intense light is then reflected off
of the back of the raindrop. The light then exits the raindrop
and is refracted again. The angle of incident light, to exiting
light is between 40-42 degrees. Blue light is refracted at a
greater angle than red light,
but due to the reflection of light rays from
the back of the droplet, the blue light
emerges from the droplet at a smaller
angle to the original incident white light
ray than the red light. Due to this
misalignment of colors we are able to see
a rainbow.
With the sun is low and behind you. All the sunbeams head in, strike the
cloud of water droplets ahead of you and bounce back at an angle of
40-42 degrees.
Naturally the beams can bounce 40 degrees any which way--up, down,
and sideways. But the only ones you see are the one that lie on a cone
with a side-to-axis angle of 40 degrees and your eyes at the tip.
The position of a rainbow depends on the observer's location and the
position of the Sun, it is not an actual location in the sky. This makes
finding that pot of gold impossible . The light that reaches the
observer makes up the rainbow for that observer. Thus every observer
sees a different rainbow.
•The inside of the rainbow appears brighter
because more light emerges from the rain
drop at a smaller angle than the rainbow
•If the Sun is higher than 42 degrees,
rainbows generally can’t be seen unless the
observer is high off the ground (in a plane or
at a waterfall)
•Double rainbows are formed when the light
in the raindrop gets reflected inside the
raindrop again, with a total reflected angle
of 50 degrees, rather than the 42 degrees of
the primary rainbow. The colors of the
double rainbow appear in reverse order
since the blue light is bent more.
•Reflective glass beads, used in
road paint, scattered on ground
act as water droplets to produce
a “glory.”
•A glory is created in essentially
the same way a rainbow is
except with a much smaller (520 degrees) total reflected
•It can only be seen when the
observer is directly between the
Sun and the water droplets (or
in this case, reflective glass

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