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MICHELSON INTERFEROMETER
Aim: -
To determine the wavelength of laser using Michelson Interferometer.
Principle:-
When light from laser is allowed to fall on the beam splitter, one portion is transmitted through beam splitter to M1 and other is reflected by beam splitter to M2. A partially silvered glass plate, Beam splitter is installed between the Laser and Mirror M2 such that it is inclined at an angle of 45 degrees. Role of beam splitter is to direct half-light to mirror M1 by reflecting and let the other half go straight to mirror M2. Both these beams superimpose back at beam splitter and the interference pattern can be observed on the screen.
Wavelength is calculated by the formula: λ = (2d/N) ∆
Material required:-
Laser mount, beam splitter with mount, mirrors with mount , rotation stage with glass slide, pressure cells, thumb screen
Procedure:-
1 )Align the laser and interferon-meter in the Michelson mode.
Place the rotation stage between the beam splitter and movable mirror, perpendicular to the optical path.
Mount the glass plate on the rotation stage
Position the stage & glass such that glass slide is perpendicular to the optical path.
When glass plate is introduced in the optical path of Michelson interferometer, the fringe will be shifted & will become blur. To make the fringe sharpen again, move mirror mount to & fro till the clear set of fringes is achieved on tie viewing screen.
Slowly rotate the rotation stage. Count the number of fringe translations that occur as you rotate the table to an angle (at least 10 degrees.)
20 fringes were counted moving past the fixed spot (both inwards and outwards). After completing 20 fringes, micrometer reading was noted down. It gave the distance by which the mirror moved.
At least three measurements of each quantity are to be taken and then the average of that quantity is calculated.
The wavelength of laser is calculated by: (2D/N) where d is the change in position
Step 8 was repeated number of times for obtaining the calibration constant and then, for getting the final value of the wavelength.
Calibrating the micrometer
a) For more accurate measurements of the mirror movement, we use a laser to Calibrate the micrometer.
b) The interferometer is set up in the Michelson mode. The micrometer knob is turned on as we count at least 20 fringes.
c) The change in the micrometer reading is recorded and this value is used as d'. The actual mirror movement (N*lambda)/2, where lambda is the known wave length of the laser and N is the number of fringes that were counted.
Observations
Part 1
Part 2
S.no
No. of fringes
Vernier Scale Reading(Divisions)
Distance moved by the mirror(in mm) 1 2 3 4 5 6 7 8 9
Calculations:
Part 1
Mean value of d'=0.29 mm
d=n λ/2=20*650 nm/2= xxxx mm
S.no. No. of
fringes
Initial
reading(mm)
Final
reading(mm)
Distance(mm)
Δ=d/d'=0.xxxx
Part 2
Mean value of d=0.00xx
λ = (2d/N)*Δ= = xxx nm
Conclusions :-
As d decreases interference rings shrink and each time d decreases by λ/2 a ring disappears at the center. As d increases interference rings expand and each time d increases by λ/2 a ring grows out from the center. This provides a basic method for determining the wavelength of monochromatic light. If N rings appear or disappear at the center as d changes by D, the wavelength of light will be given by λ =2D/N.
Precautions:
- Direct exposure to laser shall be avoided.
- The apparatus should be arranged carefully.
