Michelson's interferometer: To measure the wavelength of light from the given source using Michelson's interferometer.

 To measure the wavelength of light from the given source using Michelson's interferometer. 

Apparatus: Michelson's interferometer, source of light (sodium vapour lamp or a Laser source). a tin sheet with a fine hole, a convex lens etc. 

Theory:

The Michelson's interferometer is adjusted to obtain a circular fringe in the telescope's field of view. If mirrors  M1 and  M1 are equidistant from the glass plate G1, the field of view will be black. The position of the  M1 mirror is adjusted so that certain bright fringes appear in the telescope's field of view with its center coincident with the crosswire. When the mirror  M1 is moved backward or forwards, each fringes in the telescope's focal plane is moved parallel to itself. When the mirror  M1 is moved through the distance λ/ 2, the path difference changes λ, and the position of certain bright fringes is taken by the next bright fringes. If n is the number of fringes that move across the field of view when the mirror is moved a distance l, then

n λ/ 2 = l

 λ = 2l/n

Procedure:

• Find the least count of Michelson's interferometer.
• Get clear, bright circular fringes in the telescope's field of view.
• When the center of the crosswire (as seen through the telescope) lies on the bright fringe, record the main (or linear) scale reading, the R.M.S. reading, and F.M.S. reading.
• Now move the Fine Micrometer Screw (F.M.S.) slowly and continue counting the number of bright fringes that shift across the field of view. (The edge will go up or down). When a large number of fringes say 25 has shifted again, make the center of the crosswire (as seen through the telescope) rest on the bright fringes. Record readings on the main (linear) scale, R.M.S., and F.M.S.
• Similarly, repeat the process after shifting another 25 fringes and so on. Take about readings. 

Observations: 

• Least reading on the main (linear scale) = mm = cm
• Least count of Rough Micrometer Screw (R.M.S.) = mm = cm
• Least count of Fine Micrometer Screw (F.M.S.) = mm = cm

  

 

		Position of Mirror M1				Difference 100 Fringes
S.NO.	No. Of Fringes shifted	Main Scale reading cm	R.M.S Reading  cm	F.M.S Reading cm	Total in cm
1  2  3  ...  ...  19  20	0  25  50  ..  ...  475  500

 Mean difference for 100 fringes = l = ...... cm

Wavelength of light  λ = 2l/n = .... cm = Å

     Where n = 100

Precautions:

• Glass plates G1 G2  and mirrors M1 M2 are front silvered. These should not
• The screw behind the  M2 mirror must be turned at a very small angle.
• The micrometer screw must always be driven in the same direction to avoid errors due to backlash.
• There should be no movement near the experiment because the fringes tend to be disturbed.
• There should be no linear or lateral displacement of fringes.

Let me know if you have more questions or if there is a specific topic that you would like to know more about.