Color optics CH 4: Color, light and atmospheric optics Prof. Leedi

What is an “optical” “phenomena”? Optical:Optical: –of or related to the science of optics; –of or relating to vision; –of or relating to or utilizing light. Phenomenon:Phenomenon: –an observable fact or event; –a fact or event of scientific interest susceptible of scientific description and explanation; –a rare event; –an exceptional thing or occurrence.

Optical phenomena process + atmospheric constituent optical phenomena atmospheric structure What is missing from this diagram?

Optical phenomena process + atmospheric constituent optical phenomena atmospheric structure light

Processes: transmissionreflectionscatteringabsorptionrefractiondispersiondiffraction

transmission the passage of electromagnetic radiation through a mediumthe passage of electromagnetic radiation through a medium transmission is a part of every optical phenomena (otherwise, the phenomena would never have occurred in the first place!)transmission is a part of every optical phenomena (otherwise, the phenomena would never have occurred in the first place!)

reflection the process whereby a surface of discontinuity turns back a portion of the incident radiation into the medium through which the radiation approached; the reflected radiation is at the same angle as the incident radiation.the process whereby a surface of discontinuity turns back a portion of the incident radiation into the medium through which the radiation approached; the reflected radiation is at the same angle as the incident radiation.

reflection from smooth surface angle of incidence angle of reflection light ray

scattering the process by which small particles suspended in a medium of a different index of refraction diffuse a portion of the incident radiation in all directions. No energy transformation results, only a change in the spatial distribution of the radiation.the process by which small particles suspended in a medium of a different index of refraction diffuse a portion of the incident radiation in all directions. No energy transformation results, only a change in the spatial distribution of the radiation.

molecular scattering (or other particles)

the process in which incident radiant energy is retained by a substance.the process in which incident radiant energy is retained by a substance. –A further process always results from absorption: the irreversible conversion of the absorbed radiation goes into some other form of energy (usually heat) within the absorbing medium.the irreversible conversion of the absorbed radiation goes into some other form of energy (usually heat) within the absorbing medium. absorption (attenuation)

substance (air, water, ice, smog, etc.) incident radiation absorption transmitted radiation

refraction the process in which the direction of energy propagation is changed as a result of:the process in which the direction of energy propagation is changed as a result of: –a change in density within the propagation medium, or –as energy passes through the interface representing a density discontinuity between two media.

refraction in two different media less dense medium more dense medium

refraction in two different media less dense medium more dense medium tt tt

gradually changing medium ray wave fronts low density high density

dispersion the process in which radiation is separated into its component wavelengths (colors).the process in which radiation is separated into its component wavelengths (colors).

the “classic” example white light prism

diffraction the process by which the direction of radiation is changed so that it spreads into the geometric shadow region of an opaque or refractive object that lies in a radiation field.the process by which the direction of radiation is changed so that it spreads into the geometric shadow region of an opaque or refractive object that lies in a radiation field.

light Solid object shadow region

Optical phenomena process + atmospheric constituent optical phenomena atmospheric structure light

Atmospheric Constituents: empty space molecules dust and pollutants salt particles volcanic materials cloud droplets rain drops ice crystals

Optical phenomena process + atmospheric constituent optical phenomena atmospheric structure light

Atmospheric Structure temperature gradient humidity gradient clouds layers of stuff - pollutants, clouds

Optical phenomena process + atmospheric constituent optical phenomena atmospheric structure light

scattering off cloud droplets ~ 20  mscattering off cloud droplets ~ 20  m white clouds

dark clouds scattering and attenuation from larger cloud droplets and raindropsscattering and attenuation from larger cloud droplets and raindrops

Courtesy, Dr. J. M. Pike, Oregon, 1996

scattering from O 2 and N 2 molecules, dustscattering from O 2 and N 2 molecules, dust –violet light is scattered 16 times more than red blue skies

molecular scattering (nitrogen and oxygen) [blue scatters more than red]

hazy (milky white) sky scattering from tiny particlesscattering from tiny particles –terpenes (hydrocarbons) and ozone

Courtesy, Dr. J. M. Pike, Oregon, 1996

crepuscular rays scattering and attenuation from shadow- casting objects (clouds and mountains)scattering and attenuation from shadow- casting objects (clouds and mountains) crepuscular rays attenuation ( blocking of some sunlight by clouds) rays made visible by scattering from molecules and stuff in the atmosphere 어둑어둑한

Courtesy, Dr. J. M. Pike, Oregon, 1996

anticrepuscular rays side view front view anti-solar point horizon anticrepuscular rays

orange sun (as at sunset or sunrise) scattering from moleculesscattering from molecules –this is the normal sunset we see frequently

red sun (as at sunset or sunrise) scattering from molecules, dust, salt particles, volcanic materialscattering from molecules, dust, salt particles, volcanic material –at 4° elevation angle, sun light passes through 12 times as much atmosphere as when directly overhead

green or blue sun scattering from volcanic ash, dust, smokescattering from volcanic ash, dust, smoke –uniform-sized particles

blue moon scattering from volcanic ash, dust or smokescattering from volcanic ash, dust or smoke the 2nd full moon in the same calander monththe 2nd full moon in the same calander month –Could you ever have a blue moon in February? –1999 has two blue moons: 1 and 31 January and 2 and 31 March1 and 31 January and 2 and 31 March –Notice that February 1999 will not have a full moon at all! The next year that has two blue moons is 2018.The next year that has two blue moons is 2018.

twinkling (scintilation) refraction by small-scale temperature and relative humidity fluctuationsrefraction by small-scale temperature and relative humidity fluctuations 번쩍임

twilight scattering and refraction by molecules and refractive index changes (air density decreases with altitude)scattering and refraction by molecules and refractive index changes (air density decreases with altitude) 황혼

green flash refraction and scattering by molecules and refractive index changesrefraction and scattering by molecules and refractive index changes –occurs right at sunrise or sunset. For a few seconds there may be some green light visible when the sun is almost completely below the horizon

inferior mirage refraction from hot surface with cooler air aloftrefraction from hot surface with cooler air aloft Height Temperature

superior mirage refraction from temperature inversionrefraction from temperature inversion Height Temperature cold air warm air

Fata Morgana Italian for “Fairy Morgan”Italian for “Fairy Morgan” refraction when temperature increases with height slowly, then rapidly, then slowlyrefraction when temperature increases with height slowly, then rapidly, then slowly Occurs frequently in Artic regions over ice fields (pg 98)Occurs frequently in Artic regions over ice fields (pg 98) Height Temperature

halo refraction from ice crystals ~ 20  m diameter which are randomly orientedrefraction from ice crystals ~ 20  m diameter which are randomly oriented 46°

Halo 22°

Circumzenithal Arc Halo Sun Dogs About 8 am Saturday 17 February 1996 Fargo, North Dakota

Circumzenithal Arc: An arc of the halo type, usually brightly colored and about 90° in arc length. Its center is located at the zenith and the arc is found about 46° above the sun.An arc of the halo type, usually brightly colored and about 90° in arc length. Its center is located at the zenith and the arc is found about 46° above the sun. It is produced by refraction with a prism angle of 90° when light enters the tops of tubular ice crystals (principle axes vertical) and leaves by some prism face. This halo can occur only for elevations under about 32°; it is typically short- lived but also very brilliant.It is produced by refraction with a prism angle of 90° when light enters the tops of tubular ice crystals (principle axes vertical) and leaves by some prism face. This halo can occur only for elevations under about 32°; it is typically short- lived but also very brilliant.

sundog refraction from flat ice crystals (usually > 30  m)refraction from flat ice crystals (usually > 30  m) –low sun –ice crystals horizontally oriented as they fall

Sundog formation white light Ice crystal (hexagonal)

Sundog in sky 22°

sun pillar reflection from flat ice crystals with low sunreflection from flat ice crystals with low sun

Sun pillar Side view Front view

Rainbow caused by refraction, reflection and dispersion within liquid raindropscaused by refraction, reflection and dispersion within liquid raindrops requires sunshinerequires sunshine sun must be behind the viewer and relatively low in the skysun must be behind the viewer and relatively low in the sky

rainbow (primary) 42° 40°

Combining two raindrops

rainbow (secondary) Can also get two reflections within a single raindrop, producing secondary bowCan also get two reflections within a single raindrop, producing secondary bow Colors of secondary bow are “upside down” compared to firstColors of secondary bow are “upside down” compared to first Secondary bow is higher in the skySecondary bow is higher in the sky –see page 129, Figure 5.28

Courtesy, Dr. J. M. Pike, Oregon, 1996

corona caused by diffraction - the bending of light as it passes around objectscaused by diffraction - the bending of light as it passes around objects light waves form constructive and destructive interference pattern - colorslight waves form constructive and destructive interference pattern - colors requires uniform-sized particlesrequires uniform-sized particles –volcanic dust –small water droplets Pages 105, Figure 4.29 and 4.30Pages 105, Figure 4.29 and 4.30

glory (Brocken Bow, if seen from ground) caused by refraction, reflection, refraction and surface wave from water dropletscaused by refraction, reflection, refraction and surface wave from water droplets a set of colored rings which appear around the shadow of an observer when the sun is behind and the light falls on a cloud composed of water droplets (<50  m)a set of colored rings which appear around the shadow of an observer when the sun is behind and the light falls on a cloud composed of water droplets (<50  m) also observable from mountainsalso observable from mountains

Courtesy, Dr. J. M. Pike, Oregon, 1996

Heiligenschein 1) reflection and retroreflection from dew drops 2) shadows from objects such as grass

iridescence caused by diffraction from different sized small cloud dropletscaused by diffraction from different sized small cloud droplets

Courtesy, Dr. J. M. Pike, Oregon, 1996

The End