light – what is it?

1. light – what is it? Physics 2C Lecture 15

2. What is light? • Wave? • Particle? • Does it go in straight lines? • sizes compared to λ ? Physics 2C Lecture 15 Fig 37-1a, p.1178

3. ? Physics 2C Lecture 15

4. Physics 2C Lecture 15 p.1178

5. emwelt • http://www.phy.ntnu.edu.tw/java/emWave/emWave.html Physics 2C Lecture 15

6. Physics 2C Lecture 15 Fig 37-3, p.1179

7. Wave demo http://www.falstad.com/ripple/ Physics 2C Lecture 15

8. Thomas Young’s Double Slit Experiment in 1801 & the Wave Nature of Light • Need Coherent • Source of Waves • for Interference • Realm of Geometric Optics; all sizes >>l • (next week) • Now use Physical • Optics, Sizes~l • Huygens’ Principle Interference Fringe Pattern Physics 2C Lecture 15

9. what is light • It’s a wave • later we will find out its also a particle Physics 2C Lecture 15

10. Physics 2C Lecture 15 Fig 37-4, p.1179

11. Path Difference for Two Parallel Rays: d =r1 - r2 = d sinq Constructive Interference d sinq = lm (All m, +/-) Destructive Interference d sinq = (m+1/2)l • L>>d sinq ~ q = ym/L = ml/d • (constructive) • ym mL/d Bright Fringes • ym = (m+½)lL/d Dark Fringes m = “order” of fringe m = 0 zeroth order, maximum, central bright fringe m = +/- 1 first order maxima Physics 2C Lecture 15

12. 1 2 3 4 1 2 5 3 6 ½ 4 5 ½ Physics 2C Lecture 15 Fig 37-4b, p.1179

13. 1 2 3 4 5 1 2 3 6 4 5 ½ Physics 2C Lecture 15 Fig 37-4c, p.1179

14. Example • = 546 nm d = 0.12 mm L = 2 m Find separation Dy of adjacent maxima near Central Maximum ym = lLm/d ym+1 = lL(m+1)/d Dy = ym+1 - ym = lL/d Dy = (546 x 10-9 m)(2 m)/(0.12 x 10-3 m) = 9.1 mm ~ 1cm Physics 2C Lecture 15

15. Sound – air waves Waves in ocean – water Music – waves in a string So for light What waves? Michelson Interferometer Albert Michelson 1869 US Navel Academy 1878 Measure speed of light 1882 Michelson-Morley Experiment at Case Tech in Cleveland: measure speed of earth’s motion in “Ether”. Null result led to Einstein’s Theory of Special Relativity 1907 First American to win Noble Prize (Physics) 1918 Caltech So what waves? Ether ? Physics 2C Lecture 15

16. Physics 2C Lecture 15 Fig 37-22, p.1194

17. Moving through the ether shockwave applet – from publisher No Ether! Physics 2C Lecture 15

18. Destructive interference between waves from two sources occurs when the path difference is • a. λ • b. 2λ • c. 3λ • d. 4λ • e. λ/2 Physics 2C Lecture 15

19. Rope with total mass m = 2 kg, L = 80 m and mass M = 20 kg at end If end of rope driven in SHM with f=0.056 Hz then wavelength of traveling wave up rope is ? L = 80 m Tension in rope F and wave speed v M = 20 kg If end of rope driven in SHM with f=0.056 Hz then wavelength of traveling wave up rope is Physics 2C Lecture 15

20. Consider a common mirage formed by super-heated air just above a roadway. A truck driver whose eyes are 2.00 m above the road, where n = 1.000 3, looks forward. She perceives the illusion of a patch of water ahead on the road, where her line of sight makes an angle of 1.20° below the horizontal. Find the index of refraction of the air just above the road surface. Physics 2C Lecture 15

21. traveling waves ^ ^ positive direction is when one is moving toward the other damped SHO p=E/c driven and damped SHO I=I0 cos2 Constructive Interference d sinq = lm Physics 2C Lecture 15 L>>d sinq ~ q = ym/L speed of light=3x108 m/s

22. +p +p +q +q +f +R +R +f Convex -f Concave Refraction at curved surface Object:p Image: q Focal length : f Magnification :M Real : light at image Virtual : no light Mirrors Lenses Ray1- || to axis, then thru f Ray2-thru f then || Ray3-thru R and back (Ray-4- to center of mirror then reflect at same angle) Ray1- || to axis, then thru f on back Ray2-thru f on front then || Ray3-thru center of lens Physics 2C Lecture 15