Natural Light Phenomena 6/02/08Lecture 21 1UCSDPhysics 10Natural LightNatural LightThe Natural Appearance of ThingsThe Natural Appearance of ThingsSpring 2008 2UCSDPhysics 10Why is the sky blue?Why is the sky blue?••Blue light more readily Blue light more readily ““scatteredscattered”” by air molecules by air molecules– called Rayleigh Scattering; strong function of wavelength– blue light in sky has been diverted from some other path– with some blue light missing, sun looks yellow/orangeSpring 2008 3UCSDPhysics 10The spectrum of the blue skyThe spectrum of the blue skyThe sky (blue curve)has a spectrum thatgets steeper and steeper towards theblue/violet end of thevisible spectrum.Shown on top of theblue curve is a modelthat goes accordingto theory: 1/4The orange curveis the spectrum ofa white piece of paperin the sunSpring 2008 4UCSDPhysics 10Is the night sky blue too?Is the night sky blue too?••You bet! Just too dim to perceiveYou bet! Just too dim to perceive– time exposure at night under moonlight shows thisYou can find blue from scattering in other circumstances as well:water, glaciers, astrophysical reflection nebulae…Natural Light Phenomena 6/02/08Lecture 21 2Spring 2008 5UCSDPhysics 10Rainbows, Halos, Sun-dogs, and MoreRainbows, Halos, Sun-dogs, and More……••Rainbows come from the interaction of sunlight with roundRainbows come from the interaction of sunlight with roundwater dropletswater droplets– preferred single-reflection path with ~42° deflection angle– see http://mysite.verizon.net/vzeoacw1/rainbow.html• drag incoming ray, and you get a stationary behavior at 42°– rainbow arc always centered on anti-solar point– different colors refract at slightly different angles• owes to differences in refractive index for different colorsred appears higher in sky than bluesingle bounce; red & blue paths differentSpring 2008 6UCSDPhysics 10Rainbows come in pairsRainbows come in pairs……Secondary rainbow has tworeflections. Red now appearslower than blue in the sky.Beautiful double rainbow in Zion National Park.The primary is brighter, and the color sequenceis reversed from that seen in fainter secondary.Area between rainbows oftenseen to be darker than elsewhere.Note: rainbow can exist in foreground.Spring 2008 7UCSDPhysics 10QuestionsQuestions••Which general direction will a rainbow be foundWhich general direction will a rainbow be foundin the evening?in the evening?••Why donWhy don’’t you see rainbows during the middle oft you see rainbows during the middle ofthe day?the day?Spring 2008 8UCSDPhysics 10The halo, and sun-dogsThe halo, and sun-dogs••2222° halo around sun due to hexagonal ice crystals° halo around sun due to hexagonal ice crystals– often more noticeable around moon at night (less glare)••Sun-dogs (Sun-dogs (parheliaparhelia) join halo, level with sun) join halo, level with sun– from horizontally situated ice crystals• akin to leaves falling in stable horizontal orientation– colored due to refractive dispersion through ice crystalNatural Light Phenomena 6/02/08Lecture 21 3Spring 2008 9UCSDPhysics 10Sun-dog geometrySun-dog geometryAntarctic skies: lots of ice…Spring 2008 10UCSDPhysics 10Glorys Glorys and and Heiligenschein Heiligenschein (shadow-hiding)(shadow-hiding)••A circular rainbow about theA circular rainbow about theanti-solar direction is called aanti-solar direction is called agloryglory– Sometimes 2–3 colored rings– often see shadow in middle– water droplet phenomenon••The anti-solar point may alsoThe anti-solar point may alsoget bright due to shadow-hidingget bright due to shadow-hiding– called heiligenschein– often see from airplane overtextured terrain– no, the person in the photo is notan angelSpring 2008 11UCSDPhysics 10The Green FlashThe Green Flash••The atmosphere acts like aThe atmosphere acts like a mildmildprism: the refractive index variesprism: the refractive index variesslightly with wavelengthslightly with wavelength••Exaggerated low on horizonExaggerated low on horizon••Different colors bent differentDifferent colors bent differentamounts by atmosphereamounts by atmosphere– the whole sun is bent 0.6° at thehorizon– it has actually set before itsrefracted image sets!••RedRed image sets first, followed byimage sets first, followed bygreengreen– the blue has long been scatteredawaySpring 2008 12UCSDPhysics 10References and AssignmentsReferences and Assignments••ReferencesReferences– Lynch & Livingston’s Color and Light in Nature– Minnaert’s Light and Color in the Outdoors••AssignmentsAssignments– Read Chap. 27 pp. 515–526– Read Chap. 28 pp. 544–547– Read Chap. 34 pp. 671–674; skim rest as needed/interested– HW8, due 6/06: 30.E.42, 27.E.10, 27.E.11, 27.E.15,27.E.20, 27.E.29, 28.E.31, 28.E.33, plus four more requiredproblems posted on website– Last Q/O due Friday 6/06 by