Engineering Physics

by

BhaskarDepartment of Physics

K L University

Lecture 07 (25 Aug)Interference in Thin Films

• Another method for viewing interference is to place a planoconvex lens (L) on top of a flat glass (G) surface.

• The air film between the glass surfaces varies in thickness from zero at the point of contact to some thickness “d”.

• A pattern of light and dark rings is observed. These rings are called Newton’s Rings.

• Newton’s Rings can be used to test optical lenses.

Newton Rings

Newton Rings…..

Newton Rings…• Ray 1 undergoes a phase change of 180

on reflection, whereas Ray 2 undergoes no phase change

R= radius of curvature of lens

r=radius of Newton’s ring

Newton Rings…How are these rings are formed?

Let us consider a particular point which is at a distance of r from the point of contact of the lens with the glass plate.All points having the same thickness – Destructive Interference.

Why are they circular?The locus of all the points from a circular dark ring with ‘r’ as radius.Centre is dark because of phase change during reflection at glass plate.

For destructive interference

For constructive interference

Newton Rings…

Newton Rings…..

Conclusion: Fringe width decreases with the order of the fringe and fringes get closer with increases in their order.

D16 -D9=2√Rλ ------------->7 Fringes

D9-D4 = 2√Rλ -------------->5 fringes

D4-D1= 2√Rλ ---------------->3 Fringes

Newton Rings…..

Out Comes:

Determination of wavelength of monochromatic source.

Determination of refractive index of liquid.

Determination of wavelength of monochromatic source:

Measure the radii or diameter of rings formedMeasure the radius of curvature of the lens (R).

λ = Dn , Dm

are the diameters of the mth and nth dark rings

m

D2

D 2 vs m

R = l²/6h+h/2Where l is the distance between

two legs of spherometer. h is height of convex lens.

Spherometer

Newton Rings…Determination of refractive index of liquid:

Liquid

Determination of refractive index ( µ ) of liquid

liquidglass plate

lens

airglass plate

lens

= 4(m-n) λ R =

µ

Newton Rings…

For a air film between the plane convex lens and a flat glass

• For a liquid medium (whose refractive index is to be measured) between the plane convex lens and a flat glass

• Resultantly the RI ofliquid medium can be written as

1) Optical Testing : Surface Quality a) Flat Surfaces

b) Spherical Surfacesc) Surface roughness

2) Direct phase measurementsa) phase shiftingb) phase stepping

3) Aligning very high quality lenses

4) Space applicationsa) Radio Astronomyb) Measuring light intensityc) used in retrieving images from telescope

Primary Application's of Interferometer

Newton Rings… In white light

light :a wave leading to our future

The end

1) In Newton’s ring experiment , the diameter of the 10th dark ring is 0.433cm . Find the wavelength of incident light, if the radius of curvature of the lens is 70cm .

2) In Newton’s ring experiment , the diameter of the 5th dark ring was 0.3 cm and the diameter of 25th ring was 0.8 cm. If the radius of curvature of the plano convex lens is 100cm. Find the wavelength of light used.

Problems