• Repeat measurement 10 times for different d m ! PHYS 1494: Experiment 6 - Interferometer
FP Interferometer: He-Ne Wavelength • Plot d m vs. m (number of fringes): • For each measurement, extract wavelength λ using above equation. • Report average: • Compare to accepted value of 632.8 nm PHYS 1494: Experiment 6 - Interferometer
Michelson Interferometer: Setup PHYS 1494: Experiment 6 - Interferometer LASER Beam splitter Movable mirror Adjustable mirror Lens Viewing screen
Michelson Interferometer: Procedure • Remove lens in front of the laser. • Follow procedures as in lab manual. • Tips for setup: • For best results, make sure the two beams are almost exactly on top of each other before placing lens in front of laser! • Same tips as in FP apply to Michelson. • Measurement of He-Ne laser wavelength: • Same procedure: Measure fringes vs distance of movable mirror. Take 10 trials. • Report • Compare to accepted value and Fabry-Perot Interferometer. PHYS 1494: Experiment 6 - Interferometer
Michelson Interferometer: Measure n air • Change the optical path length of the beam by changing the pressure: • A change in pressure causes a change in index of refraction. • The schematic setup for the Michelson interferometer is going to be: PHYS 1494: Experiment 6 - Interferometer Change in pressure = change in index of refraction = different wavelength
Vacuum Cell and Pump PHYS 1494: Experiment 6 - Interferometer Vacuum cell mounted on rotational pointer Vacuum hand pump Release toggle Pressure gauge
Michelson Interferometer: Measure n air • The vacuum cell will be at P atm initially. • You will slowly pump out the air to change the pressure and count the number fringes that pass through a certain point • Constrain the rotational pointer when pumping the air to prevent the vacuum cell from oscillating/rotating • Pressure gauge reads cm Hg (not inches Hg) • Unit conversion not important since we'll be only needing pressure differences (as long as you are consistent). PHYS 1494: Experiment 6 - Interferometer
Calculate the Index of Refrection • We already saw that the number of fringes passing a point when the index of refraction is changed is given by: • Problem: neither n i nor n f are known. • Solution: divide everything by the change in pressure: • However: Δn/ΔP = α = constant • Therefore: PHYS 1494: Experiment 6 - Interferometer Linear relation between number of fringes passing through a point ( m ) and change in pressure
Michelson Interferometer: Compute n air • Make at least 7 measurements of m and ΔP • Convert m to Δn through: • Note: Use λ 0 = 632.8 nm • Plot ΔP vs. Δn: • Determine slope α • Now use P atm =76 cm Hg, P vac = 0 , n vac = 1 to compute n air : PHYS 1494: Experiment 6 - Interferometer
Measuring the Index of Refraction of Glass PHYS 1494: Experiment 6 - Interferometer LASER Glass mounted on rotational pointer
Measuring the Index of Refraction of Glass • Similar principle as n air : change in OPT = change in pattern • Due to the angle of the glass with respect to the axis of the laser, the beam will refract according to Snell's Law : PHYS 1494: Experiment 6 - Interferometer • The incidence angle θ is related to
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