Atmospheric Optics: Rainbows, Sundogs, Halos, Glories
•A bit of history •How rainbows are made •Physics: dispersion & geometric optics •Beautiful stuff
April 6, 2005
2005
PHYS 401 - Lecture 33
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Administrative stuff
FINAL exam is scheduled for APRIL 14 noon in LSK 460 (3 hour exam see http://students.ubc.ca/current/exams.cfm
Format of final exam: 6 questions, all equal weight (but not all equally challenging). You do any 5 of them. If you want to do all 6, I will mark all and the best 5 will count. One question will either be from a worked example in the text, or from an assigned homework problem. I’ll be available April 11-13 and can set up an office hour each day, or you can email me to make an appointment to drop by. Your report on your presentation is due Monday April 11 If you hand in earlier, I will be more generous in marking! Please also sign a note at the end giving me permission to include your write up in packet I will make for class and have ready for you to pickup at 9am Tuesday April 12. (if you don’t give me permission, I won’t hand out your writeup)
April 6, 2005 PHYS 401 - Lecture 33 2
Historical look at Rainbows
Aristotle is often attributed with performing the first quantitative analyses of the rainbow. He explained the circular shape.
Rene Descartes performed early modern analyses of the rainbow in the early 1600’s. He noticed that rainbows appear when drops of water are illuminated by the sun and the light is refracted
In late 1600’s Newton (with his newly formalized calculus!) made very accurate measurements and calculations for rainbows
April 6, 2005 PHYS 401 - Lecture 33 3
Rainbows
Necessary ingredients: falling raindrops, sunlight. Refraction of sunlight by the water droplets makes rainbow: It’s all geometric optics! (Snell’s law)
•Rainbow always appears as a circular arc centered on the point exactly opposite the sun (“anti-solar point”) •First rainbow appears 42° from anti-solar point •When a secondary rainbow appears, it is 51° from anti-solar point
April 6, 2005 PHYS 401 - Lecture 33 4
Rainbows
Each raindrop refracts sunlight into all colours, but only one colour from each drop is at angle to reach your eye. .Full rainbow of many colours results from individual refracted rays from many raindrops.
April 6, 2005 PHYS 401 - Lecture 33 5
Secondary Rainbow
When light has two (rather than one) internal reflection in raindrop, we see a secondary rainbow at 51° from anti-solar point
Rainbow colour spectrum is reversed in secondary bow. (Primary Bow has red at top, violet at bottom. Secondary is reversed, due to 2 internal reflections. 3 reflections (supernumerary bow has been observed, but 4 has not in the sky (but has been observed in the lab with lasers and water droplets!)
April 6, 2005 PHYS 401 - Lecture 33 6
Polarization
Rainbows are linearly polarized, with E field aligned tangent to the plane of incidence. This is due to Brewster effect: the first internal reflection takes place near the Brewster angle; 53° for water, so that almost all the light reflected is polarized perpendicular to the incidence plane
Particle
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Sundogs
22° halo around sun or moon.. 6 sided ice crystals
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Sundogs
22° halo around sun or moon: refraction thru ice crystal
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Sundogs
These
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Halos and Glories
Gloris are caused by scattering of light from small drops of water Can see it in the shadow of your airplane in clouds.
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Halos and Glories
Glories are caused by scattering of light from small drops of water - fog Can see it in the shadow of your airplane in clouds.
Nasa is looking for the 22 degree halo in the ice cover of Jupiter's moon Europa, using images from the space probe Galileo. If it finds the halo, it would be proof that the ice is in the form of hexagonal crystals, which only form at temperatures above 170K. Scientist suspect that a huge liquid ocean (biggest of the solar system) lies beneath Europa's ice crust, heated by tidal forces from Jupiter. If that were true, it would be an excellent place to search for extraterrestrial life. April 6, 2005 PHYS 401 - Lecture 33 12
Next Class
Review: on the board!
I’ll go over my review notes You bring any questions
Light interacting with water drops, dust or ice crystals in the atmosphere produces a host of beautiful spectacles - rainbows, halos, glories, sundogs, coronas among others. Select and explain one or more and describe how they form and the physics behind them. For example, rainbows are electromagnetic phenomena that rely upon the refraction of light. Explain the physics of rainbows and some of the subtle effects seen, for example primary and secondary bows, supernumerary bows, spokes and other interesting features.
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