Optics, Light and Color This chapter (19) discusses: White light and colors Bending of light Mirages Optical phenomena

How we perceive light Rods and Cones Rods respond to all visible wavelengths of light and allow us to distinguish between light and dark Cones respond to specific wavelengths allowing us to see color White light is perceived when all visible wavelengths strike the cones with nearly equal intensity

Sunlight As sunlight enters the atmosphere it is either: Absorbed Reflected Scattered Transmitted

Clouds Why are clouds white? Why do some of them turn dark?

Reflection When sunlight bounces off the surface at the same angle at which it strikes it, the light is reflected (The phenomena itself is called reflection)

Scattering Atmospheric constituents and contaminants tend to deflect light from its path and send it out in all directions in a process known as Scattering. Another term for scattering is ? Cloud droplets around 10 μm in diameter can effectively scatter all wavelengths of visible radiation equally in a phenomena known as geometric scattering.

Clouds Optically thick – effective ‘scatterers’ of light, poor absorbers. Hence clouds tend to appear white Taller, larger clouds become more effective at reflecting. Cloud bases tend to appear dark

Blue Skies Oxygen and Nitrogen are selective scatterers. Scatter shorter waves more effectively than longer waves Green/Blue/Purple scattered more than red/yellow/orange Selective scattering is known as Rayleigh scattering.

Sky Color Why do we see the sky as blue? Other atmospheric constituents in various concentrations can cause different wavelengths to scatter more, changing the color of the sky

Crepuscular Rays Light beams from the sun seen at dawn and dusk Light beams shining through a break in the clouds

Crepuscular Rays

Anti-Crepuscular Rays

Red Suns At sunset, most shorter wavelength light is scattered away leaving only the longer wavelengths, hence the reds and oranges The more scattering in the atmosphere, the more ‘red’ the sunset Volcanoes

Blue Moons and Suns Size of atmospheric particles must be similar to the wavelength of visible light, making them more effective scatters of red light – Red sky, blue sun Same happens for the blue moon Without scattering, sun and moon would be white against a constant black sky

Transmission Light passing through a substance is ‘transmitted’ Denser materials slow the light and ‘bend’ it

Refraction Term for bending of light Depends on Material Density Angle light enters the medium

Scintillation Starlight passing through the atmosphere encounters air of different densities, constantly changing the path of the light Causes the star to appear to twinkle or flicker – known as Scintillation Planets don’t twinkle unless they are near the horizon

Setting Sun and Moon Refraction causes light rays from the lower part of the sun or moon to bend more than the upper part, making them appear to ‘flatten out’. Since light is bent most at the horizon, they appear brighter than they actually are

Twilight Time immediately after sunset (or before sunrise) when the sky remain illuminated Civil Twilight Astronomical Twilight Length depends on latitude and season Duration increases with increasing latitude

Civil Twilight Lasts from Sunset till the sun is 6° below the horizon

Astronomical Twilight Lasts from sunset till the sky is completely dark and observation of the faintest astronomical stars is possible

Green Flash

Mirage When an object appears to be displaced from its true position Created by light passing through layers of air differing in density (usually caused by sharp changes in temperature)

Inferior Mirage Objects appear lower than they are and are inverted (upside down)

Superior Mirage Objects that appear to be shifted upward and remain right side up

Fata Morgana Meaning “Fairy Castles” Complex Superior Mirage Distorts the objects to be unrecognizable Can distort any type of object Why are objects unrecognizable?

Fata Morgana Image

Halos A ring of light encircling and extending outward from the sun or moon

Halos Produced when the sun- or moonlight is refracted as it passes through ice crystals Indicates cirriform clouds are present 22° halo is most common, 46° halo less common 22° formed by crystals less than 20μm 46° formed by crystals 15 – 25 μm

Tangent Arcs Bright arcs seen at the top or bottom of a halo Forms due to large hexagonal columnar crystals fall with their long axes horizontal to the ground

Targent Arcs

Sundogs “Parhelia” Small ice crystals act as prisms

Sun Pillars Sun dogs, tangent arcs, halos created by refraction through ice crystals Sun pillars created by reflection off ice crystals Appear most often at sunrise and sunset

Sun Pillar

Rainbows Primary and secondary can only be seen when facing the rain with the sun BEHIND you Caused by light entering rain drops Reflected and dispersed Red is always at top and purple at bottom Secondary bow caused by multiple reflections in rain drop, colors reversed

Rainbows Triple rainbow rare Would be seen facing the sun Each progressive rainbow gets dimmer due to reflection and dispersion

Coronas Bright ring of light surrounding the moon (or sun) Due to diffraction (bending of light as it passes around objects)

Corona

Iridescence Caused when the corona distorts due to the presence or different sized cloud droplets Usually seen within 20° of sun Associated with cirro- and altocumulus clouds

Iridescence

Glory Diffraction process, seen from aircraft Formed by water droplets less than 50 μm in diameter Forms around the shadow of the aircraft

Glory