WHY IS THE SKY BLUE ? - The Burns Home Page

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WHY IS THE SKY BLUE ? - The Burns Home Page

Natural Light Phenomena 03/10/2006SPH4UI10 Rainbow Questions1. Is the inside red?2. Radius in degrees?3. Width in degrees?4. Comparison of light intensity insideoutside.5. Time or Day6. Direction7. If 2, where is the second bow?8. If 2, Is the inside red?9. If 2, Radius of Second?10.Are rainbows polarized?SPH4UI Blue light more readily “scattered” 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/orange/red2SPH4UISPH4UIWHY IS THE SKY BLUE ?INCOMING SOLARRADIATIONNITROGENMOLECULE3Lecture 21 1

Natural Light Phenomena 03/10/2006SPH4UIWHY IS THE SKY BLUE ?SPH4UIBlue Haze & Selective ScatterSCATTERING OF BLUE LIGHTPlants exude terpenes that react with ozone and can also selectively scatter bluewavelengths, creating a blue haze.Mountains allow for enough viewing distance to create this needed scatter.SPH4UIAtmospheric Dust & RaysSPH4UIIs the night sky blue too?• You bet! Just too dim to perceive– time exposure at night under moonlight showsthisDust and salts are large enough to cause geometric scattering, and change blueskies into hazy white skies.Concentrated dust or salts beneath clouds can create white crepuscular rays ofsunlight.You can find blue from scattering in other circumstances as well:water, glaciers, astrophysical reflection nebulae…8Lecture 21 2

Natural Light Phenomena 03/10/2006SPH4UIRed at Sunrise and SunsetSPH4UILow on the horizon, the sun's lightpasses through nearly 12 times moreatmospheric gas and aerosol, whichhas scattered out most short (blue)wavelengths. Longer oranges andreds then comprise the sunlight, whichreflects from clouds and water.10SPH4UISPH4UIThe RainbowWater Droplet1112Lecture 21 3

Natural Light Phenomena 03/10/2006SPH4UISPH4UIRainbow ColoursThe Angle’s51541314SPH4UISPH4UIThe AngleRainbows come in pairs…If sun is higher than 42 degrees,then alas no rainbow (it is belowthe horizon).42rainbowBeautiful double rainbow in Zion National Park.The primary is brighter, and the color sequenceis reversed from that seen in fainter secondary.Secondary rainbow has tworeflections. Red now appearslower than blue in the sky.Area between rainbows oftenseen to be darker than elsewhere.Note: rainbow can exist in foreground.1516Lecture 21 4

Natural Light Phenomena 03/10/2006SPH4UISPH4UIPrimary1718SPH4UISPH4UISecondaryThen we have Two1920Lecture 21 5

Natural Light Phenomena 03/10/2006SPH4UIQuestions• Which general direction will a rainbow be foundin the evening? Look East• Why don’t you see rainbows during the middle ofthe day?SPH4UIThe halo, and sun-dogs45 degreesup fromyour toes isstill at theground.• 22° halo around sun due to hexagonal ice crystals– often more noticeable around moon at night (less glare)• Sun-dogs (parhelia) 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 crystal2122SPH4UISPH4UIAre Rainbows Polarized?Any light that is reflected is polarized in the plane. Thereforerainbows are somewhat polarized, about 96%.2324Lecture 21 6

Natural Light Phenomena 03/10/2006SPH4UISun-dog geometrySPH4UIGlorys and Heiligenschein (shadow-hiding)• A circular rainbow about theanti-solar direction is called aglory– Sometimes 2–3 colored rings– often see shadow in middle– water droplet phenomenon• The anti-solar point may alsoget bright due to shadow-hiding– called heiligenschein– often see from airplane overtextured terrain– no, the person in the photo is notan angel2526SPH4UICool Rainbow EffectsSPH4UIThe pot of Gold?Primary,Secondary, andReflected RainbowReflectedSupernumeraryPrimary andReflected Rainbow27Rainbow at Night28Lecture 21 7

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