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. ROUGH On a ROUGH surface light will strike (and reflect) at different angles. SMOOTH On a SMOOTH surface you can easily see that the angles are the same.

Cloud drops scatter white light

Rainbows

Clarity of color varies REDOutermost band is always RED VIOLETOutermost band is always VIOLET 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 RAINBOW SECONDARY 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 reddishIf you do see a color, it’s usually 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