Moon Corona
As you walk home late at night, and wispy clouds glide across the
moon, you notice a sort of soft glow around the moon. This is called a \"corona.\"
Sometimes you can see a nice corona around a streetlamp in the fog, or looking
through a frosty window pane. If you look very carefully, you\'ll notice the corona is
colored; it is bluish near the center (i.e. right next to the moon), whitish in the center,
and yellowish on the outside. Under very special conditions, the corona is strongly
colored, with blue at the center, followed by green, yellow, and red on the outside.
a) Can you explain this phenomenon in terms of diffraction by water droplets in the
cloud ? (Hint: you’ll need to invoke Babinet’s Principle.)
b) Suppose the average droplet size is 20?m. At what angle relative to the center of
the moon would you expect to see the greenish color in the corona?
c) Can you explain under what conditions the corona will be strongly colored, and
when it will be essentially just white?
Solution
Diffraction means bending of light around some particles . In atmosphere , light generally bends around water droplets found in clouds. Water droplets scatter light wave and act as source of outgoing waves .The outgoing waves mutually interfere and when their crest coincide , they form constructive interference (bright spot) and when crest and trough coincide they form destructive interference (dark spot) . scattered light from all droplet surfaces and few reflected and transmitted waves combine to form a diffraction pattern called corona .
C) When all the coronas of individual spectral colours superimpose each other , it becomes white .
.
Moon Corona As you walk home late at night- and wispy clouds glide acr.docx
1. Moon Corona
As you walk home late at night, and wispy clouds glide across the
moon, you notice a sort of soft glow around the moon. This is called a "corona."
Sometimes you can see a nice corona around a streetlamp in the fog, or looking
through a frosty window pane. If you look very carefully, you'll notice the corona is
colored; it is bluish near the center (i.e. right next to the moon), whitish in the center,
and yellowish on the outside. Under very special conditions, the corona is strongly
colored, with blue at the center, followed by green, yellow, and red on the outside.
a) Can you explain this phenomenon in terms of diffraction by water droplets in the
cloud ? (Hint: you’ll need to invoke Babinet’s Principle.)
b) Suppose the average droplet size is 20?m. At what angle relative to the center of
the moon would you expect to see the greenish color in the corona?
c) Can you explain under what conditions the corona will be strongly colored, and
when it will be essentially just white?
Solution
Diffraction means bending of light around some particles . In atmosphere , light generally bends
around water droplets found in clouds. Water droplets scatter light wave and act as source of
outgoing waves .The outgoing waves mutually interfere and when their crest coincide , they form
constructive interference (bright spot) and when crest and trough coincide they form destructive
interference (dark spot) . scattered light from all droplet surfaces and few reflected and
transmitted waves combine to form a diffraction pattern called corona .
2. C) When all the coronas of individual spectral colours superimpose each other , it becomes white
.
