Atmospheric Optical

Phenomena

(Rainbows, Halos, Sundogs and Sun Pillars!)

Lecture 22

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.

Cloud drops scatter white light

Rainbows

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

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.

Double Rainbows• Dimmer

• Visible above the primary bow

• Make a larger arch (by 8 deg)

• Narrow band

• COLORS are REVERSED!– Outermost = Violet– Innermost = Red

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

Rainbow Formation

• Dispersion in a Prism– The separation of

colors by refraction.

• For a rainbow the rain drops act as a prism!

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!

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

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

Halos

• Narrow whitish ring around the sun.

• Look for halos on days when the sky is covered with a thin layer of cirrus clouds.

Halos

• Two different Types!

• 22° halo– Most Common type– Subtends an angle of 22°

from the observer

• 46° halo– Less frequently observed– Larger

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

Paths taken by light to produce 22° halo

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

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

Sun Dogs or Parhelia

• Two bright regions on either side of the sun

• Usually associated with a 22° halo

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