CH 4: CCoolloorr, light and atmospheric opticsoptics

Prof. LeediProf. Leedi

What is an “optical” “phenomena”?

• Optical: Optical: – of or related to the science of optics; of or related to the science of optics;

– of or relating to vision; of or relating to vision;

– of or relating to or utilizing light.of or relating to or utilizing light.

• Phenomenon:Phenomenon:– an observable fact or event;an observable fact or event;

– a fact or event of scientific interest susceptible of scientific a fact or event of scientific interest susceptible of scientific description and explanation;description and explanation;

– a rare event;a rare event;

– an exceptional thing or occurrence.an exceptional thing or occurrence.

Optical phenomena

process + atmosphericconstituent

opticalphenomena

atmosphericstructure

light

Processes:

transmissiontransmission

reflectionreflection

scatteringscattering

absorptionabsorption

refractionrefraction

dispersiondispersion

diffractiondiffraction

transmission

• the passage of electromagnetic radiation the passage of electromagnetic radiation through a mediumthrough a medium

• transmission is a part of transmission is a part of everyevery optical optical phenomena (otherwise, the phenomena phenomena (otherwise, the phenomena would never have occurred in the first would never have occurred in the first place!)place!)

reflection

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

reflection from smooth surface

angle of incidence

angle ofreflection

light ray

scattering

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

molecular scattering (or other particles)

• the process in which incident radiant energy the process in which incident radiant energy is is retainedretained by a substance. by a substance. – A further process always results from A further process always results from

absorption:absorption:• the irreversible conversion of the absorbed radiation the irreversible conversion of the absorbed radiation

goes into some other form of energy (usually heat) goes into some other form of energy (usually heat) within the absorbing medium.within the absorbing medium.

absorption (attenuation)

substance (air, water, ice, smog, etc.)

incidentradiation

absorption

transmittedradiation

refraction

• the process in which the the process in which the direction direction of energy of energy propagation is propagation is changedchanged as a result of: as a result of: – a a change in densitychange in density within the propagation within the propagation

medium, ormedium, or– as energy passes through the interface as energy passes through the interface

representing a representing a density discontinuitydensity discontinuity between between two media.two media.

refraction in two different media

less densemedium

more densemedium

refraction in two different media

less densemedium

more densemedium

t

t

gradually changing medium

ray

wavefronts

low density

high density

dispersion

• the process in which radiation is separated the process in which radiation is separated into its component wavelengths (into its component wavelengths (ccoolloorrss).).

the “classic” example

white light

prism

diffraction

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

light

Solid object

shadowregion

Optical phenomena

process + atmosphericconstituent

opticalphenomena

atmosphericstructure

light

Atmospheric Constituents:

empty spaceempty space

moleculesmolecules

dust and pollutantsdust and pollutants

salt particlessalt particles

volcanic materialsvolcanic materials

cloud dropletscloud droplets

rain dropsrain drops

ice crystalsice crystals

Optical phenomena

process + atmosphericconstituent

opticalphenomena

atmosphericstructure

light

Atmospheric Structure

temperature gradienttemperature gradient

humidity gradienthumidity gradient

cloudsclouds

layers of layers of stuffstuff - pollutants, clouds - pollutants, clouds

Optical phenomena

process + atmosphericconstituent

opticalphenomena

atmosphericstructure

light

• scatteringscattering off off cloud droplets cloud droplets ~ 20 ~ 20 mm

white clouds

dark clouds

• scattering and attenuation scattering and attenuation from from larger cloud larger cloud droplets and raindropsdroplets and raindrops

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

• scatteringscattering from from OO22 and N and N22 molecules, dust molecules, dust

– violet light is scattered 16 times more than redviolet light is scattered 16 times more than red

blue skies

molecular scattering (nitrogen and oxygen)

[[blue blue scatters more than scatters more than redred]]

hazy (milky white) sky

• scattering scattering from from tiny particles tiny particles – terpenes (hydrocarbons) and ozoneterpenes (hydrocarbons) and ozone

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

crepuscular rays

• scatteringscattering and and attenuationattenuation from from shadow-shadow-casting objectscasting objects (clouds and mountains) (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)

• scatteringscattering from from moleculesmolecules– this is the normal sunset we see frequentlythis is the normal sunset we see frequently

red sun (as at sunset or sunrise)

• scatteringscattering from from molecules, dust, salt molecules, dust, salt particles, volcanic materialparticles, volcanic material– at 4° elevation angle, sun light passes through at 4° elevation angle, sun light passes through

12 times as much atmosphere as when directly 12 times as much atmosphere as when directly overheadoverhead

green or blue sun

• scatteringscattering from from volcanic ash, dust, smokevolcanic ash, dust, smoke– uniform-sized particlesuniform-sized particles

blue moon

• scatteringscattering from from volcanic ash, dust or smokevolcanic ash, dust or smoke

• the the 2nd2nd full moon full moon in the same calander monthin the same calander month– Could you ever have a blue moon in February?Could you ever have a blue moon in February?– 1999 has two blue moons:1999 has two blue moons:

• 1 and 1 and 3131 January and 2 and January and 2 and 3131 March March– Notice that February 1999 will not have a full moon at all!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)

• refractionrefraction by by small-scale temperature and small-scale temperature and relative humidity fluctuationsrelative humidity fluctuations

번쩍임

twilight

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

황혼

green flash

• refraction and scattering refraction and scattering by by molecules and molecules and refractive index changesrefractive index changes– occurs right at sunrise or sunset. For a few occurs right at sunrise or sunset. For a few

seconds there may be some seconds there may be some green light green light visible visible when the sun is almost completely below the when the sun is almost completely below the horizonhorizon

inferior mirage

• refractionrefraction from from hot surface with cooler air hot surface with cooler air aloftaloft

Hei

ght

Temperature

superior mirage

• refractionrefraction from from temperature inversiontemperature inversion

Hei

ght

Temperature

cold air

warm air

Fata Morgana• Italian for “Fairy Morgan”Italian for “Fairy Morgan”

• refraction refraction when when temperature increases with height slowly, temperature increases with height slowly, then rapidly, then slowlythen rapidly, then slowly

• Occurs frequently in Artic regions over ice fields (pg 98)Occurs frequently in Artic regions over ice fields (pg 98)H

eigh

t

Temperature

halo

• refraction refraction from from ice crystals ice crystals ~ 20 ~ 20 m m diameter which are diameter which are randomlyrandomly oriented oriented

46°

Halo

22°

Circumzenithal ArcCircumzenithal Arc

HaloHalo

Sun DogsSun Dogs

About 8 amAbout 8 am

SaturdaySaturday

17 February 199617 February 1996

Fargo, North DakotaFargo, North Dakota

Circumzenithal Arc:

• An arc of the halo type, usually brightly colored and An arc of the halo type, usually brightly colored and about 90° in arc length. Its center is located at the about 90° in arc length. Its center is located at the zenith and the arc is found about 46° above the sun.zenith and the arc is found about 46° above the sun.

• It is produced by refraction with a prism angle of It is produced by refraction with a prism angle of 90° when light enters the tops of tubular ice crystals 90° when light enters the tops of tubular ice crystals (principle axes vertical) and leaves by some prism (principle axes vertical) and leaves by some prism face. This halo can occur only for elevations under face. This halo can occur only for elevations under about 32°; it is typically short-lived but also very about 32°; it is typically short-lived but also very brilliant.brilliant.

sundog

• refractionrefraction from from flat ice crystals flat ice crystals (usually > (usually > 30 30 m)m)– low sunlow sun– ice crystals horizontally oriented as they fallice crystals horizontally oriented as they fall

Sundog formation

white light

Ice crystalIce crystal(hexagonal)(hexagonal)

Sundog in sky

22°22°

sun pillar

• reflectionreflection from from flat ice crystals flat ice crystals with low sunwith low sun

Sun pillar

Side view

Front view

Rainbow

• caused by caused by refraction, reflection and refraction, reflection and dispersion dispersion within within liquid raindropsliquid raindrops

• requires requires sunshinesunshine

• sun must be behind the viewer and sun must be behind the viewer and relatively low in the skyrelatively low in the sky

rainbow (primary)

42°40°

Combining two raindrops

rainbow (secondary)

• Can also get Can also get twotwo reflections within a single reflections within a single raindrop, producing secondary bowraindrop, producing secondary bow

• Colors of secondary bow are “upside down” Colors of secondary bow are “upside down” compared to firstcompared to first

• Secondary bow is higher in the skySecondary bow is higher in the sky– see page 129, Figure 5.28see page 129, Figure 5.28

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

corona

• caused by caused by diffractiondiffraction - the bending of light - the bending of light as it passes around as it passes around objectsobjects

• light waves form light waves form constructive and constructive and destructive interference destructive interference pattern - colorspattern - colors

• requires requires uniform-sized particlesuniform-sized particles– volcanic dustvolcanic dust– small water dropletssmall 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 caused by refraction, reflection, refraction refraction, reflection, refraction

and surface wave and surface wave from from water dropletswater droplets

• a set of colored rings which appear around a set of colored rings which appear around the shadow of an observer when the sun is the shadow of an observer when the sun is behind and the light falls on a cloud behind and the light falls on a cloud composed of water droplets (<50 composed of water droplets (<50 m)m)

• also observable from mountainsalso observable from mountains

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

Heiligenschein

1) 1) reflection and retroreflection reflection and retroreflection from from dew dew dropsdrops

2) 2) shadowsshadows from from objectsobjects such as grass such as grass

iridescence

• caused by caused by diffractiondiffraction from from different sized different sized small cloud dropletssmall cloud droplets

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

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

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

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

The End