atmospheric optical phenomena (rainbows, halos, sundogs and sun pillars!) lecture 22
TRANSCRIPT
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Atmospheric Optical
Phenomena
(Rainbows, Halos, Sundogs and Sun Pillars!)
Lecture 22
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Nature of Light - Reflection
• The Law of Reflection– Light rays always bounce off the reflecting
surface at the same angle at which the meet at that surface.
On a ROUGHROUGH surface light will strike (and reflect) at different
angles.
On a SMOOTHSMOOTH surface you can easily see that the angles are the
same.
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Cloud drops scatter white light
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Rainbows
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Rainbows
• Clarity of color varies
• Outermost band is always REDRED
• Outermost band is always VIOLETVIOLET
• Usually see SIX Colors– Red, Orange, Yellow,
Green, Blue, Violet
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Rainbows
• Usually seen when the observer has the Sun on one side and a rain shower on the opposite side.– i.e. Sun at your back,
facing the rain
• Fine mists from waterfalls and sprinklers can generate mini-rainbows.
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Double Rainbows• Dimmer
• Visible above the primary bow
• Make a larger arch (by 8 deg)
• Narrow band
• COLORS are REVERSED!– Outermost = Violet– Innermost = Red
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Rainbow Formation
• Need three things:– 1) Sunlight– 2) Water droplets– 3) An observer in between the rain and the sun
• Refraction– As light travels through water it is bent– Different colors travel at different speed in water
• Each color is then bent at a different angle• Violet is refracted and bent the most• Red is refracted and bent the least
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Rainbow Formation
• Dispersion in a Prism– The separation of
colors by refraction.
• For a rainbow the rain drops act as a prism!
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Rainbow Formation
• The angle between sunlight and the dispersed color is always:– 42° for red– 40° for violet
• The curved shape results from the fact that the light always travels at 42° from the path of sunlight
• An observer will only see one color from each raindrop
• Each observer sees their OWN rainbow!
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When don’t you see rainbows?
• If the sun is higher than 42° above the horizon– If you’re an “earthbound” observer
• If living in the Mid-latitudes (us) during mid-day!!– Sun is too high in the sky
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Secondary Rainbow
• Forms the same way EXCEPT the dispersed light is refracted twice– Reverses the colors!
– Results in a 50° angle for the color red…. 8° above the primary rainbow’s red.
– Extra refraction also makes it dimmer
PRIMARY RAINBOWPRIMARY RAINBOW
SECONDARY RAINBOWSECONDARY RAINBOW
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Halos
• Narrow whitish ring around the sun.
• Look for halos on days when the sky is covered with a thin layer of cirrus clouds.
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Halos
• Two different Types!
• 22° halo– Most Common type– Subtends an angle of 22°
from the observer
• 46° halo– Less frequently observed– Larger
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Halo Formation
• Similar to a rainbow
• Formed by dispersion of light
• ICE CRYSTALSICE CRYSTALS instead of water drops– Plate– Column– Capped Column– Bullet
Column
Plate
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Paths taken by light to produce 22° halo
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Differences between 22° and 46° halos
• The path the light takes differs– For 22° halo light strikes
one side of the crystal and passes through the other side.
– For 46° halo light strikes the side of a crystal and then passes out either the top or bottom
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Why are halos white?
• Raindrops are almost always spherical.
• Ice crystals vary in shape and size– Thus, they are “imperfect”– The colors overlap and “wash” each other out
• If you do see a color, it’s usually reddish reddish on the inside of the halo
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Sun Dogs or Parhelia
• Two bright regions on either side of the sun
• Usually associated with a 22° halo
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Sun Pillars
• Usually seen at sunrise or sunset
• Sunlight is reflected from the lower sides of falling plate crystals and capped columns
• Usually reddish in color– Direct sunlight at sunrise and
sunset is usually reddish in color
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