2. Physics 102: Lecture 18, Slide 1 Snell’s Law, Total Internal Reflection, Brewster’s Angle, Dispersion, Lenses Physics 102: Lecture 18

3. Physics 102: Lecture 18, Slide 2 Snell’s Law: A Quick Review n1n1 n2n2 When light travels from one medium to another the speed changes v=c/n, but the frequency is constant. So the light bends: n 1 sin(  1 )= n 2 sin(  2 ) 11 22

4. Physics 102: Lecture 18, Slide 3 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 ) = 48.8 0

5. Physics 102: Lecture 18, Slide 4

6. Physics 102: Lecture 18, Slide 5 Fiber Optics Telecommunications Arthoscopy Laser surgery Total Internal Reflection only works if n outside < n inside At each contact w/ the glass air interface, if the light hits at greater than the critical angle, it undergoes total internal reflection and stays in the fiber. n inside n outside

7. Physics 102: Lecture 18, Slide 6 Can the person standing on the edge of the pool be prevented from seeing the light by total internal reflection ? 1) Yes2) No Preflight 18.1

8. Physics 102: Lecture 18, Slide 7 ACT: Refraction As we pour more water into bucket, what will happen to the number of people who can see the ball? 1) Increase2) Same3) Decrease

9. Physics 102: Lecture 18, Slide 8

10. Physics 102: Lecture 18, Slide 9 Brewster’s angle When angle between reflected beam and refracted beam is exactly 90 degrees, reflected beam is 100% horizontally polarized ! Reflected light is usually unpolarized (mixture of horizontally and vertically polarized). 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

11. Physics 102: Lecture 18, Slide 10 ACT: Brewster’s Angle When a polarizer is placed between the light source and the surface with transmission axis aligned as shown, the intensity of the reflected light: (1) Increases(2) Unchanged(3) Decreases T.A.

12. Physics 102: Lecture 18, Slide 11 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 Polarizing sunglasses (when worn by someone standing up) work by absorbing light polarized in which direction? horizontal vertical

13. Physics 102: Lecture 18, Slide 12

14. Physics 102: Lecture 18, Slide 13 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 = 1.52

15. Physics 102: Lecture 18, Slide 14 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!

16. Physics 102: Lecture 18, Slide 15 LIKE SO!In second rainbow pattern is reversed

17. Physics 102: Lecture 18, Slide 16

18. Physics 102: Lecture 18, Slide 17 Flat Lens (Window) n1n1 n2n2 Incident ray is displaced, but its direction is not changed. tt 11 11 If  1 is not large, and if t is small, the displacement, d, will be quite small. d

19. Physics 102: Lecture 18, Slide 18 1) Rays parallel to principal axis pass through focal point. 2) Rays through center of lens are not refracted. 3) Rays through F emerge parallel to principal axis. Assumptions: monochromatic light incident on a thin lens. rays are all “near” the principal axis. Converging Lens Principal Rays F F Object P.A. Image is: real, inverted and enlarged (in this case). Image

20. Physics 102: Lecture 18, Slide 19 Converging Lens All rays parallel to principal axis pass through focal point F. Double Convex P.A. F n lens > n outside F At F Inside F Outside F P.A. F F Preflight 18.6 A beacon in a lighthouse produces a parallel beam of light. The beacon consists of a bulb and a converging lens. Where should the bulb be placed?

21. Physics 102: Lecture 18, Slide 20

22. Physics 102: Lecture 18, Slide 21 3 Cases for Converging Lenses Object Image This could be used in a camera. Big object on small film Inverted Reduced Real Past 2F Image Object This could be used as a projector. Small slide on big screen Inverted Enlarged Real Between F & 2F ImageObject This is a magnifying glass Upright Enlarged Virtual Inside F

23. Physics 102: Lecture 18, Slide 22 ACT: Converging Lens Which way should you move object so image is real and diminished? (1)Closer to lens (2)Further from lens (3)Converging lens can’t create real diminished image. F F Object P.A.

24. Physics 102: Lecture 18, Slide 23 1) Rays parallel to principal axis pass through focal point. 2) Rays through center of lens are not refracted. 3) Rays toward F emerge parallel to principal axis. Diverging Lens Principal Rays F F Object P.A. Only 1 case for diverging lens: Image is always virtual, upright, and reduced. Image

25. Physics 102: Lecture 18, Slide 24

26. Physics 102: Lecture 18, Slide 25 Which way should you move object so image is real? 1)Closer to lens 2)Further from lens 3)Diverging lens can’t create real image. ACT: Diverging Lenses F F Object P.A.

27. Physics 102: Lecture 18, Slide 26