Total Internal Reflection, Dispersion, Polarization section 26-8 Physics 1161: Lecture 18

Water on the Road Mirage

Palm Tree Mirage

Mirage Near Dana – Home of Ernie Pyle

Fata Morgana The fata morgana mirage is one that can occur only where there are alternating warm and cold layers of air near the ground or water surface. Instead of traveling straight through these layers, light is bent towards the colder, more optically dense, air.

Total Internal Reflection normal 22 11 n2n2 n1n1 Recall Snell’s Law: n 1 sin(  1 )= n 2 sin(  2 ) (n 1 > n 2   2 >  1 )  1 = sin -1 (n 2 /n 1 ) then  2 = 90 cc Light incident at a larger angle will only have reflection (  i =  r ) ii rr “critical angle” For water/air: n 1 =1.33, n 2 =1  1 = sin -1 (n 2 /n 1 ) =

Can the person standing on the edge of the pool be prevented from seeing the light by total internal reflection ? 1) Yes 2) No Preflight % 42% “There are millions of light ’rays’ coming from the light. Some of the rays will be totally reflected back into the water, but most of them will not.” 8

Fiber Optics

Indices of Refraction

What is the critical angle of a diamond-air boundary?

Internal Reflection in Diamond The critical angle for diamond in air is 24.5 o. Any ray which strikes the inside surface at an angle greater than 24.5 o will be totally internally reflected.

The diagrams show incident rays approaching a boundary with a second medium. The relative indices of refraction of the two media are indicated. In which diagram will total internal reflection occur, providing the angle of incidence exceeds the critical angle? 1.Diagram A 2.Diagram B 3.Diagram C 4.None of these

The diagrams show incident rays approaching a boundary with a second medium. The relative indices of refraction of the two media are indicated. In which diagram will total internal reflection occur, providing the angle of incidence exceeds the critical angle? 1.Diagram A 2.Diagram B 3.Diagram C 4.None of these

Dispersion prism White light Blue light gets deflected more n blue > n red The index of refraction n depends on color! In glass: n blue = 1.53n red =

Skier sees blue coming up from the bottom (1), and red coming down from the top (2) of the rainbow. Rainbow: Preflight 18.5 Wow look at the variation in index of refraction! Which is red? Which is blue? Blue light is deflected more!

Green Flash

Rainbow

Rainbow Zoom

Double Rainbow

Double Rainbow on Way to School

Alexander’s Dark Band Sky is light inside primary rainbow Dark between primary and secondary bows Light beyond the secondary rainbow Dark region between is called Alexander’s Dark Band

Does moon halo predict bad weather?

22 o Halo A halo is a ring of light surrounding the sun or moon. Most halos appear as bright white rings but in some instances, the dispersion of light as it passes through ice crystals found in upper level cirrus clouds can cause a halo to have color.dispersioncirrus Randomly oriented hexagonal ice crystals Diameter less than 20.5 micrometers

Sundogs Sundogs or parhelia on right and left of sun Flat faces horizontally oriented Hexagonal ice crystals Diameter greater than 30 micrometers

Sundog

Polarization Transverse waves have a polarization – (Direction of oscillation of E field for light) Types of Polarization – Linear (Direction of E is constant) – Circular (Direction of E rotates with time) ** – Unpolarized (Direction of E changes randomly) x z y

Linear Polarizers Linear Polarizers absorb all electric fields perpendicular to their transmission axis. Molecular ViewMolecular View (link)

Unpolarized Light on Linear Polarizer Most light comes from electrons accelerating in random directions and is unpolarized. Averaging over all directions: S transmitted = ½ S incident Always true for unpolarized light!

Polarizing sunglasses are often considered to be better than tinted glasses because they… Preflight 18.3, 18.4 block more light block more glare are safer for your eyes are cheaper When glare is around  B, it’s mostly horiz. polarized! 21 27% 62% 9% 1% Polarizing sunglasses (when worn by someone standing up) work by absorbing light polarized in which direction? horizontal vertical 60% 40%

Brewster’s angle …when angle between reflected beam and refracted beam is exactly 90 degrees, reflected beam is 100% horizontally polarized ! Reflected light is partially polarized (more horizontal than vertical). But… n 1 sin  B = n 2 sin (90-  B ) n 1 sin  B = n 2 cos (  B ) horiz. and vert. polarized BB BB 90º-  B 90º horiz. polarized only! n1n1 n2n2