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Where Do Rainbows Come From? A Tour of Differential Calculus.

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Presentation on theme: "Where Do Rainbows Come From? A Tour of Differential Calculus."— Presentation transcript:

1 Where Do Rainbows Come From? A Tour of Differential Calculus

2 Where would you rather be?

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5 Supernumerary Rainbows

6 Rainbows in Mythology Greek: Path to heaven made by Iris. Chinese: Goddess Nuwa uses 5 stones of different colors to patch a slit in the sky Indian: Bow of Indra, God of thunder Irish: Leprechauns hide their pot of gold at the end of rainbow Persian: Reflection of light off of holy sites

7 Rainbows in Bible The Deluge (Genesis ): The Deluge (Genesis ): I have set my bow in the clouds, and it shall be a sign of the covenant between me and the earth. When I bring clouds over the earth and the bow is seen in the clouds, I will remember my covenant that is between me and you and every living creature of all flesh; and the waters shall never again become a flood to destroy all flesh. I have set my bow in the clouds, and it shall be a sign of the covenant between me and the earth. When I bring clouds over the earth and the bow is seen in the clouds, I will remember my covenant that is between me and you and every living creature of all flesh; and the waters shall never again become a flood to destroy all flesh.

8 Rainbows in Religion

9 Where do rainbows come from? Attempts by the scientists Ibn Sahl, Al-Biruni ~ 990 Ibn Sahl, Al-Biruni ~ 990 Al-Hacen, Al-Shirazi, Al-Farisi ~ 1280 Al-Hacen, Al-Shirazi, Al-Farisi ~ 1280 Freiberg ~ 1300 Freiberg ~ 1300 Snell, Descartes, Huygens, Newton, Fermat ~ 1650 Snell, Descartes, Huygens, Newton, Fermat ~ 1650 Young, Airy ~ 1830 Young, Airy ~ 1830 Mie ~ 1910 Mie ~ 1910 You ~ 2020 You ~ 2020

10 Unweaving the Rainbow Do not all charms fly Do not all charms fly At the mere touch of cold philosophy? There was an awful rainbow once in heaven: We know her woof, her texture; she is given In the dull catalogue of common things. Philosophy will clip an Angel's wings, Conquer all mysteries by rule and line, Empty the haunted air, and gnomed mine - Unweave a rainbow … Keats, 1820 Lamia

11 Where Do Rainbows Come From? Basics: Reflection and Refraction

12 Let Us Unweave the Rainbow Q- How does light choose a path to go from point A to point B? Q- How does light choose a path to go from point A to point B? A- Light is like an intelligent prescient being. A- Light is like an intelligent prescient being. Light chooses a path that takes the Light chooses a path that takes the least time to go from A to B! least time to go from A to B! This is Fermats Principle. This is Fermats Principle. Depending on your point of view it unweaves or camouflages some of the mysteries of rainbows. Depending on your point of view it unweaves or camouflages some of the mysteries of rainbows. Lets see how it explains reflection and refraction. Lets see how it explains reflection and refraction.

13 How does light reflect? Suppose you are to run from A to B and touch the wall L on the way. What path is the shortest?

14 Geometry to the rescue! Angle of Incidence=Angle of Reflection

15 So we understand the reflection part. How about refraction?

16 Refraction: Shallow deep pools, broken straight lines

17 How Does Light Refract?

18 Calculus to the Rescue!

19 Draw a Picture

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21 Time Minimization

22 Enter the Dragon

23 But where do rainbows come from? Different colors of light travel at different speeds in water, so they refract differently. This gives the spectrum of colors. Intensity is highest when deflection is stationary. So we need to find angle of deflection and differentiate it. Now Enters the Real Dragon.

24 Which ray causes the rainbow?

25 Angle of Deviation

26 Where do rainbows come from?

27 42 degrees above suns line

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29 References James Stewart, Calculus Richard Dawkins, Unweaving the Rainbow Wikipedia (for pictures)

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