Let D_m and D_m+k be the diameters of the m^th and (m+k)^th dark ring respectively. Then,

D²_m  =  4 mRλ

D²_m+k  =  4 (m+k) Rλ

D²_m+k  -  D²_m  =  4 kRλ

λ  =  [D²_m+k  -  D²_m] / 4kR

D²_m  =  4 mRλ

where λ is the wavelength used and R is the radius of curvature of the lens.

Refractive index of liquid,

( n  =  mλR / r^2 )

where, m order of the fringe, R Radius of curvature of the lens

λ wave length of light, r radius of the ring

To find the diameter of thin wire or thickness of a thin strip of paper using air wedge method.

The diameter or thickness of the wire,

d  =  lλ / 2β

λ is the wavelength of the light

β is the bandwidth

l is the diameter of the wire or thickness of a thin strip of paper from the line of contact of the two plates of the air wedge.

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