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Today: Total Internal Reflection, Refraction, Dispersion, Rainbows <ul><li>Next exam one week from Thursday! </li></ul><ul><li>Quiz posted, due Thursday </li></ul>Spectacular rainbow, late afternoon Sweden, 1999. http://www.atoptics.co.uk/index.htm

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HW Question Tom Decaro—Sunlight reflections through a diffraction grating

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Tom Decaro—Sunlight reflections through a diffraction grating

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Tom Decaro -- Fluorescent lights through a diffraction grating

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Analisa Goodman—Fluorescent lights through a diffraction grating

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Homework problem <ul><li>Incandescent – Blackbody radiation Color is yellow (cooler than the sun) Most photons are infrared </li></ul><ul><li>Fluorescent – Electrons excite mercury, which emits UV photons. UV photons absorbed by phosphors, which fluoresce in visible </li></ul><ul><li>Incandescent “wastes” lots of photons in the IR. </li></ul>Visible range Images:wikipedia

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Conservation of energy in lighting Electrical Energy Heat flow Invisible photons Visible photons In winter, these are beneficial for heating the room Lighting Efficiency Visible photons Electrical Energy

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First off, let’s experiment with refraction Key Concept Compared with speed in a vacuum (300 million meters per second) Light travels SLOWER inside matter

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Clicker Question--Refraction <ul><li>Which of the following diagrams best represents what happens when a red laser hits a glass / air interface? </li></ul>A B C Air Glass

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Clicker Question -- Refraction <ul><li>Which of the following diagrams best represents what happens when a red light wave encounters an air / diamond interface? </li></ul>Reflection: Angle of reflection = angle of incidence Refraction: Imagine the wave as a two wheeled cart encountering a boundary A B C Air Glass FAST SLOW

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Total internal reflection (TIR) <ul><li>Imagine what happens when a ray of light is in a higher index of refraction material… </li></ul>Air glass “ Evanescent” Wave Total internal reflection Let’s try a laser demo again

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TIR is a key for fiber optic communication Air glass http://www.fiberoptic-connectors.com/images/reflection.gif

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TIR enables a special surface microscopy technique Air glass “ Evanescent” Wave Excite fluorescence in cell with evanescent wave www.olympusamerica.com Red = TIRF Green = regular

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<ul><ul><li>Yes </li></ul></ul><ul><ul><li>No </li></ul></ul>Is it possible to have a total internal reflection prevent someone from seeing a coin at the bottom of a pool?

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<ul><ul><li>Yes </li></ul></ul><ul><ul><li>No </li></ul></ul>Is it possible to have a total internal reflection prevent someone from seeing a coin at the bottom of a pool?

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Clicker Question—Speed of light <ul><li>Do all colors of light travel at the same speed inside of glass? </li></ul><ul><li>Yes </li></ul><ul><li>No </li></ul>

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Clicker Question—Speed of light <ul><li>Do all colors of light travel at the same speed inside of glass? </li></ul><ul><li>Yes </li></ul><ul><li>No </li></ul>No! This is called dispersion and because of this, different colors have different angles of refraction! Let’s see if we can do this with sunlight…

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Demos Prisms Acrylic sphere rainbow http://www.youtube.com/watch?v=rQukmSPctks&feature=related

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Rainbows are the result of dispersion inside spherical raindrops (close to 1 millimeter in size) NOT THIS SHAPE! Higher frequencies Refract MORE Each raindrop emits it’s own “fan” For a given raindrop, your eye will only see rays of particular angle. http://www.atoptics.co.uk/ http://www.tpwd.state.tx.us

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These special angles form a 3-D cone… which you see as a portion of a CIRCLE “ Primary rainbow” = one reflection http://www.atoptics.co.uk/rainbows/primrays.htm

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Two reflections allow light to go in other directions And produce the secondary rainbow Two reflections http://www.atoptics.co.uk/rainbows/orders.htm

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Rainbow facts <ul><li>Rainbows are perceived as circular </li></ul><ul><li>The center of the circle is on a line continuing the sun through your eyes (antisolar point) </li></ul><ul><li>Rainbows result from large, but spherical raindrops </li></ul><ul><li>Refraction, dispersion, and reflection are the keys to raindrops. </li></ul><ul><li>Rainbows are not quite the same colors as light from a prism </li></ul><ul><li>Bright inside primary bow, dark between </li></ul>

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Rainbow colors are not exactly the same as through a prism (or diffraction grating)…what differences do you notice? Rainbow Prism Diffraction Grating

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Rainbows differ from regular prism spectra, because colors overlap Website demo http://www.sundog.clara.co.uk/rainbows/primrays.htm

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Why is it dark between the primary and secondary rainbow? http://www.atoptics.co.uk/rainbows/adband.htm Alexander’s dark band Spectacular rainbow, late afternoon Sweden, 1999. http://www.atoptics.co.uk/index.htm

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Reflection Bows

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Clicker…which bow is formed by the reflection? A B

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Go to this website for many fascinating atmospheric phenomena! http://www.atoptics.co.uk/