Lecture 26: TUE 27 APR 2010

1. Physics 2102 Jonathan Dowling Lecture 26: TUE 27 APR 2010 Ch. 36: Diffraction

2. AVG = 60 STD = 15 A=100–85 B=84–75 C=74–45 D=44–40 F=39–0 EXAM III

3. Michelson Interferometers: As we saw in the previous example, interference is a spectacular way of measuring small distances (like the thickness of a soap bubble), since we are able to resolve distances of the order of the wavelength of the light (for instance, for yellow light, we are talking about 0.5 of a millionth of a meter, 500nm). This has therefore technological applications. In the Michelson interferometer, light from a source (at the left, in the picture) hits a semi-plated mirror. Half of it goes through to the right and half goes upwards. The two halves are bounced back towards the half plated mirror, interfere, and the interference can be seen by the observer at the bottom. The observer will see light if the two distances travelled d1 and d2 are equal, and will see darkness if they differ by half a wavelength.

4. Michelson-Morley Experiment Michelson won the Nobel prize in 1907, "for his optical precision instruments and the spectroscopic and metrological investigations carried out with their aid" "The interpretation of these results is that there is no displacement of the interference bands. ... The result of the hypothesis of a stationary ether is thus shown to be incorrect." (A. A. Michelson, Am. J. Sci, 122, 120 (1881))

5. The largest Michelson interferometer in the world is in Livingston, LA, in LSU owned land (it is operated by a project funded by the NationalScience Foundation run by Caltech and MIT, and LSU collaborates in the project). Mirrors are suspended with wires and will move detecting ripples in the gravitational field due to astronomical events. http://www.ligo-la.caltech.edu

6. TAMA (Japan)Mitaka VIRGO (French-Italian) Cascina, Italy AIGO (Australia), Wallingup Plain, 85km north of Perth Gravitational Waves Interferometry:an International Dream GEO600 (British-German) Hannover, Germany LIGO (USA) Hanford, WA and Livingston, LA

7. Things You Should Learn from This Lecture • When light passes through a small slit, is spreads out and produces a diffraction pattern, showing a principal peak with subsidiary maxima and minima of decreasing intensity. The primary diffraction maximum is twice as wide as the secondary maxima. • We can use Huygens’ Principle to find the positions of the diffraction minima by subdividing the aperture, giving qmin= ±p l/a, p = 1, 2, 3, ... . • Calculating the complete diffraction pattern takes more algebra, and gives Iq=I0[sin(a)/a]2, where a = p a sin(q)/l. • To predict the interference pattern of a multi-slit system, we must combine interference and diffraction effects.

8. Single Slit Diffraction When light goes through a narrow slit, it spreads out to form a diffraction pattern.

9. Analyzing Single Slit Diffraction For an open slit of width a, subdivide the opening into segments and imagine a Hyugen wavelet originating from the center of each segment. The wavelets going forward (q=0) all travel the same distance to the screen and interfere constructively to produce the central maximum. Now consider the wavelets going at an angle such that l = a sin q @a q. The wavelet pair (1, 2) has a path length difference Dr12 = l/2, and therefore will cancel. The same is true of wavelet pairs (3,4), (5,6), etc. Moreover, if the aperture is divided into p sub-parts, this procedure can be applied to each sub-part. This procedure locates all of the dark fringes.

10. Conditions for Diffraction Minima

11. Pairing and Interference Can the same technique be used to find the maxima, by choosing pairs of wavelets with path lengths that differ by l? No. Pair-wise destructive interference works, but pair-wise constructive interference does not necessarily lead to maximum constructive interference. Below is an example demonstrating this.

12. Calculating theDiffraction Pattern We can represent the light through the aperture as a chain of phasors that “bends” and “curls” as the phase Db between adjacent phasors increases. b is the angle between the first and the last phasor.

13. Calculating theDiffraction Pattern (2)

14. l = 633 nm a = 0.25 mm 0.5 mm 1 mm 2 mm Blowup q (radians) Diffraction Patterns The wider the slit opening a, or the smaller the wavelength , the narrower the diffraction pattern.

15. Radar: The Smaller The Wavelength the Better The Targeting Resolution Laser: =1 m  Ka-band: =1cm X-band: =10cm K-band: =2cm

16. Angles of the Secondary Maxima The diffraction minima are precisely at the angles wheresin q = p l/a and a = pp(so that sin a=0). However, the diffraction maxima are not quite at the angles where sin q= (p+½) l/aand a= (p+½)p (so that |sin a|=1). l = 633 nm a = 0.2 mm 1 2 3 4 5 q(radians) To find the maxima, one must look near sin q= (p+½) l/a, for places where the slope of the diffraction pattern goes to zero, i.e., whered[(sin a/a)2]/dq = 0. This is a transcendental equation that must be solved numerically. The table gives the qMax solutions. Note that qMax < (p+½)l/a.