In this apparatus, light from a sodium lamp falls on the glass plate, inclined at 45 degree to the horizontal, get reflected, and then falls normally on the convex lens placed over the glass plate. A system of bright and dark concentric circular rings are observed through a microscope, arranged vertically above the glass plate. The microscope is properly focused so that alternate bright and dark concentric circular rings are observed more clearly. Measurements are taken from a micrometer driven traveling microscope, which is integrated with this apparatus.
The two interfering beams, derived from a monochromatic source satisfy the coherence condition for interference. Ring shaped fringes are produced by the air film existing between the convex surface of a long focus planoconvex lens and the plane of glass plate.
Let Dm and Dm+k be the diameters of the mth and (m+k)th dark ring respectively. Then,
D2m = 4 mRλ
D2 m+k = 4 (m+k) Rλ
D2 m+k - D2 m = 4 kRλ
λ = [D2 m+k - D2 m] / 4kR
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
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.
Fig: Newton’s Rings Setup