slow fast Light passing from a medium in which it is ________ to one in which it is __________________ : 11 22 refracted reflected Now _______________  1 : slow fast 11 22 less refracted more reflected slow faster increase

Keep increasing  1 until_______________________. At that point,  1 is called the _________________angle,  c. slow fast  1 =  c  2 = 90 0 even less refracted even more reflected For any angle  1 ______: slow fast no refracted all light is reflected  1 >  c  2 equals 90 0 critical >  c

This is called________________________________________  It can only occur when light passes into a medium ____________________________. To find  c : n 1 = slow n 2 = fast cc  2 = 90 0 Snell’s Law: n 1 sin  c = n 2 sin   If fast medium is air: sin  c = 1/n 1  c = sin -1 (1/n 1 ) Total Internal Reflection (TIR). in which it is faster n 1 sin  c = 1·1 n 1 sin  1 = n 2 sin  2

slow medium n1n1 1/n 1 cc water Lucite diamond Here are some typical critical angles for slow media with air as the fast medium:

incident ray A beam of light enters Lucite:  1 > 42 0 no refracted ray here Totally internally reflected ray

Of what use is TIR? Fiber optics.

Why fiber optics instead of electrical wire? 1. Less _______________________(dying out) of signal… 2. …means less ____________________are needed 3. Light has a high f  ___________________can be sent 4. Each fiber can carry __________________because _____________________ (color) can be sent on one fiber. 5. More signals per wire  ____________________needed 6.Light is immune to electrical __________________ from other fibers and_______________________________ 7. Can be used where _____________________are present attenuation “repeaters” more info >1 signal > 1 frequency less wires interference environmental noise explosive fumes

Other uses of TIR… binoculars:

periscopes: prismsmirrors

Ex: Underwater, you can only see a 97 0 “cone” of light above you that represents the entire sky. All light incident on the surface from below within the blue area is totally reflected back into the water. 97 0

This is called "Snell's Window."

What keeps polar bears warm? 1. Their fur traps air. 2. The fur hairs act like fiber optic light pipes that channel UV down to their skin. 3. Guess what color their skin is! Warning…. next photo shows …

In a vacuum, all frequencies (colors) of light travel ______________________. This is ____________in any other media. Why? 1. Higher f  ___________interactions  _____________ 2. Higher f  closer to ________________f of molecules  more ______________  _____________ Media in which the speed of a wave depends on its ___________________ are said to be______________________. wave non-dispersive medium dispersive media soundairliquids and solids lightvacuumgases, liquids and solids at the speed c false more slower natural resonance slower dispersive frequency

The Dark Side of the dispersion of light: ROYGBIV the visible spectrum

_______________________is the ____________________of a wave into its different _______________________. incident white light Blue bends ___________ because_________________________ Blue has a ___________________ index of refraction. Dispersion separation frequencies red blue glass block it changes v more more higher Notice: The rays leaving the block are parallel because the block sides are parallel.

Dispersion with _____________________: Note : the rays are NOT parallel to the incident ray because the sides of the glass prism are not parallel incident white light a glass prism  Blue bends ______________ going in, and it bends ________________ coming out. more red blue

Newton's Experiment: Before Newton: White light is “pure.” The prism “stained” the white light. After Newton: White light contains all colors. board with hole prism 2 nd prism white light

Rainbows are caused by dispersion. 1.incident white 2.some reflected 3.1 st refraction 4.dispersion 5.total internal reflection 6. 2 nd refraction 7. rainbow

For details about how the raindrop does it, press here.

Why the arc? What time of day is best for rainbows?

Why is the secondary rainbow fainter???

A lens is basically _________________ separated by a rectangular block: white light focus for ______ light focus for ______ light Not all colors focus _____________________. This is called___________________________________ two prisms chromatic (color) aberration. at the same point white light lens blue red

Newton did not think that lenses could be cured of chromatic aberration. So he invented a telescope that was based on__________________________ parabolic mirror  All colors incident at the same spot will reflect with _______________________ focus All colors focus at the same spot  __________________. the same angle: no aberration white light the Law of Reflection.

Later, the achromatic doublet was discovered: two lenses with different indices of refraction that mostly corrects for chromatic aberration. The first achromatic refracting lens was invented in 1733 by an English barrister named Chester Moore Hall although it was independently invented and patented by John Dollond.barristerChester Moore HallJohn Dollond