need help with this The atomic force microscope allows scientists to probe extre
ID: 2123492 • Letter: N
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need help with this
The atomic force microscope allows scientists to probe extremely tiny forces between single atoms. The principle of operation depends on the deflection of a cantilever by atomic forces. The degree of deflection is detected by bouncing a beam of laser light off the surface of the cantilever, which is usually coated with gold or aluminum to be reflective. The reflected beam is usually directed to a photodetector (typically a 4-quadrant type) and the displacement of the beam is measured. This gives us a way to measure very small forces on the order of nN and smaller. Assume that we have the same setup as the one shown in 1). What if we measured the displacement using an interferometer? Thus, 2Deltay = L where L is the path length difference in one arm of the interferometer. Two students contemplate on how to set this up. The wavelength of the laser used was 532nm. Student A: Suppose that the interferometer is adjusted so that a dark fringe is seen when Delta y = 0. How far does the cantilever move until we have a bright fringe? What is the force on the cantilever that corresponds to this displacement? Suppose that a bright fringe corresponds to full power at 100%, and that you are able to resolve 0.1% of full power (meaning this is the smallest power difference you can measure.) What is the smallest force you can measure? Student B suggests that the interferometer should be set so that the slope of intensity versus path length difference is the highest. Where is this setting relative to a bright or dark fringe? What is the smallest force you can measure using student B's setting? Which is a better scheme? To calculate the intensity versus phase, see equation 35.10 and use the appropriate expression for the phases.Explanation / Answer
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