The figure below shows a lens with radius of curvature R lying on a flat glass p

Question

The figure below shows a lens with radius of curvature R lying on a flat glass plate and illuminated from above by light with wavelength lambda. This photo, taken from above the lens, shows that circular interference fringes (called "Newton's rings") appear, associated with the variable thickness d of the air film between the lens and the plate. The radius of curvature R of the lens is 5.0 m and the lens diameter is 20 mm. (Ignore the interference between the top and bottom surfaces of the lens.) How many bright rings are produced? Assume that lamba = 579 nm. How many bright rings would be produced if the arrangement were immersed in water (n = 1.33)?

Explanation / Answer

a) For number of bright rings

d = (2 n -1)* l /4

Here, d = 10-5 m

=> 10-5 = (2n-1)*579*10-9/4

=> n = 35 bright rings

b) if arrangement immersed in water

wavelength = 579/1.66 = 348.795 nm

=> n = 58 bright rings

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