Cost ~ 620 million dollars Length ~ 4 kmVIRGO@ Cascina, ItalyCost ~ 100 million euros Length ~ 3 kmPHYS 1493/1494/2699: Exp. 6 – Interferometers
19Michelson Interferometer (on steroids…)●As you probably heard from the news the LIGO interferometer was a huge success! ●On September 14th, 2015 at 09:50:45 UTC they recorded the very first signal coming from a gravitational wave going through the Earth●This signal most likely came from two black holes with ~30 solar masses each merging together 1.3 billions years ago! ●Quite remarkable for ‘just’ an interferometer…PHYS 1493/1494/2699: Exp. 6 – Interferometers
20Fabry-Perot Interferometer●Same idea: split beam and then recombine. ●However, instead of splitting the beam just once, it splits many timesusing mirrored cavity.●Each reflection adds a small difference in path length, ●The final result is a circular interference pattern ●Main advantage:the fringes are brighter and well distinguishable PHYS 1493/1494/2699: Exp. 6 – Interferometers
21Condition for bright spots (constructive)●The condition for the presence of bright spots is the same for both the previous interferometers ●Suppose we start with the two beams in phase and look at the position of a bright spot, then: 1.Modify one of the two path lengths bright spots will not be bright spots anymore 2.They will be bright spots again when the total path length difference is λ3.This will happen again at 2λ, 3λ, etc. ●In general, if we change one of the paths by dm, in order to have a bright spot back in its original position we must have:PHYS 1493/1494/2699: Exp. 6 – Interferometers
22The ExperimentPHYS 1493/1494/2699: Exp. 6 – Interferometers
23Goals●In this experiment you will use both the Michelson and the Fabry-Perot interferometers ●Fabry-Perot Interferometer: ●Measure the wavelength of the He-Ne laser●Michelson Interferometer: ●Measure the wavelength of the He-Ne laser●Measure the index of refraction of air (nair). ●Measure the index of refraction of glass (nglass).PHYS 1493/1494/2699: Exp. 6 – Interferometers
24Fabry-Perot (FP) interferometer: setupLASERConverging lensAdjustable mirrorMovable mirrorViewing ScreenMicrometer knobMove this guy = change in path length = change interference patternCircular fringes appear hereAllows you to move the mirror by very small fractions of a mmPHYS 1493/1494/2699: Exp. 6 – Interferometers
25How to read a micrometer●The micrometer will allow you to move one of the two mirrors by very small distance. ●This will shift the interference patterEach of these marks is 100μmEach of these marks is 1μmExample: In the picture you can read the 500 μm mark, the 25 μm mark and the knob reads 7 μm. Position = 532 μmPHYS 1493/1494/2699: Exp. 6 – Interferometers
26FP interferometer: procedure●Follow directions for FP setup on the lab manual ●A few tips: ●Turning the micrometer knob to move mirrors will cause the fringe pattern to change.
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