Total Internal Reflection, Dispersion, Polarization

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

2. Water on the Road Mirage

3. Palm Tree Mirage

4. Mirage Near Dana – Home of Ernie Pyle

6. 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.

7. Total Internal Reflection q2 “critical angle” qr qc qi q1 Recall Snell’s Law: n1 sin(q1)= n2 sin(q2) (n1 > n2 q2 > q1 ) q1 = sin-1(n2/n1) then q2 = 90 Light incident at a larger angle will only have reflection (qi = qr) n2 n1 For water/air: n1=1.33, n2=1 q1 = sin-1(n2/n1) = 48.80 normal

8. Preflight 18.1 Can the person standing on the edge of the pool be prevented from seeing the light by total internal reflection ? “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.” 1) Yes 2) No 10 58% 42% 8

9. Fiber Optics

10. Fiber Optics

11. Indices of Refraction

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

13. Internal Reflection in Diamond • The critical angle fordiamond in air is24.5 o. • Any ray which strikes the inside surface at an angle greater than 24.5o will be totally internally reflected.

14. 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? • Diagram A • Diagram B • Diagram C • None of these

15. 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? • Diagram A • Diagram B • Diagram C • None of these

16. White light Dispersion The index of refraction n depends on color! In glass: nblue = 1.53nred = 1.52 nblue > nred prism Blue light gets deflected more 23

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

18. Green Flash http://www.faqs.org/faqs/astronomy/faq/part3/section-13.html

19. Rainbow

20. Rainbow Zoom

21. Double Rainbow

22. Double Rainbow on Way to School

23. 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

24. Does moon halo predict bad weather?

25. 22o 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. Diameter less than 20.5 micrometers Randomly oriented hexagonal ice crystals

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

27. Sundog

28. y x z 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)

29. Linear Polarizers • Linear Polarizers absorb all electric fields perpendicular to their transmission axis. http://www.colorado.edu/physics/2000/index.pl?Page=polarization/blocking_light.html Molecular View (link)

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

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

32. Brewster’s angle horiz. polarized only! qB qB horiz. and vert. polarized 90º 90º-qB Reflected light is partially polarized (more horizontal than vertical). But… n1 n2 …when angle between reflected beam and refracted beam is exactly 90 degrees, reflected beam is 100% horizontally polarized ! n1 sin qB = n2 sin (90-qB) n1 sin qB = n2 cos (qB)