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

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

1. Incandescent – Blackbody radiationColor is yellow (cooler than the sun)Most photons are infrared

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

3. Incandescent “wastes” lots of photons in the IR.

Visible range

Images:wikipedia

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 ConceptCompared 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

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 incidenceRefraction: Imagine the wave as a two wheeled

cart encountering a boundary

A B C

Air

Glass

FAST

SLOW

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

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” WaveExcite fluorescence in cell with evanescent wave

www.olympusamerica.com

Red = TIRFGreen = 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

Is it possible to have a total internal reflection prevent someone from seeing

a coin at the bottom of a pool?

Clicker Question—Speed of light

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

A. YesB. No

Clicker Question—Speed of light

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

A. Yes

B. 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…

Demos

Prisms

Acrylic sphere rainbow

http://www.youtube.com/watch?v=rQukmSPctks&feature=related

Rainbows are the result of dispersion insidespherical raindrops (close to 1 millimeter in size) NOT

THISSHAPE!

Higher frequenciesRefract MORE

Each raindrop emits it’sown “fan”

For a given raindrop, your eye willonly 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 directionsAnd 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

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

RainbowPrism

Diffraction Grating

Rainbows differ from regular prism spectra, because colors overlap

Website demohttp://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 fascinatingatmospheric phenomena!

http://www.atoptics.co.uk/