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

Next exam one week from Thursday!

Quiz posted, due Thursday

HW Question Tom Decaro—Sunlight reflections through a diffraction grating

Tom Decaro—Sunlight reflections through a diffraction grating

Tom Decaro -- Fluorescent lights through a diffraction grating

Analisa Goodman—Fluorescent lights through a diffraction grating

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

Incandescent – Blackbody radiation Color is yellow (cooler than the sun) Most photons are infrared

Fluorescent – Electrons excite mercury, which emits UV photons. UV photons absorbed by phosphors, which fluoresce in visible

Incandescent “wastes” lots of photons in the IR.

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

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

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

Which of the following diagrams best represents what happens when a red laser hits a glass / air interface?

Clicker Question -- Refraction Which of the following diagrams best represents what happens when a red light wave encounters an air / diamond interface? 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

Which of the following diagrams best represents what happens when a red light wave encounters an air / diamond interface?

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

Imagine what happens when a ray of light is in a higher index of refraction material…

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

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

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

Yes

No

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

Yes

No

Clicker Question—Speed of light Do all colors of light travel at the same speed inside of glass? Yes No

Do all colors of light travel at the same speed inside of glass?

Yes

No

Clicker Question—Speed of light Do all colors of light travel at the same speed inside of glass? Yes No 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…

Do all colors of light travel at the same speed inside of glass?

Yes

No

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

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

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

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

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

Rainbows are perceived as circular

The center of the circle is on a line continuing the sun through your eyes (antisolar point)

Rainbows result from large, but spherical raindrops

Refraction, dispersion, and reflection are the keys to raindrops.

Rainbows are not quite the same colors as light from a prism

Bright inside primary bow, dark between

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

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

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

Reflection Bows

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

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