The convex surface of a plano-convex glass lens comes into contact with a glass plate. The curvature radius of the lens's convex surface is `R`, the wavelength o flight is equal to `lambda`. Find the width `Delta r` of a Newton ring as a function of its radius `r` in the region where `Delta r lt lt r`.

Explanation

We have
`r^(2) = (1)/(2) k lambda R`
So for `k` differing by `1 (Delta k = 1)`
`2r Delta r = (1)/(2) Delta k lambdaR = (1)/(2) lambda R`
or `Delta r = (lambda R)/(4r)`.