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

2. 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

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

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

5. 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)

6. 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.

7. 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

8. 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.

9. 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

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

11. Crepuscular Rays

12. Anti-Crepuscular Rays

13. 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

14. 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

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

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

17. 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

18. 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

19. 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

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

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

22. Green Flash

23. 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)

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

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

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

27. Fata Morgana Image

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

29. 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

30. 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

31. Targent Arcs

32. Sundogs
“Parhelia”
Small ice crystals act as prisms

33. 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

34. Sun Pillar

35. 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

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

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

38. Corona

39. 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

40. Iridescence

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

42. Glory