Newton's rings can be seen when a planoconvex lens is placed on a flat glass surface. For a particular lens with an index of refraction of n= 1.51 and a glass plate with an index of n= 1.78, the diameter of the third bright ring is 0.760 mm.
If water (n= 1.33) now fills the space between the lens and the plate, what is the new diameter of this ring?
rN = [(N-0.5)λR]1/2
r=radius of Nth bright ring
λ=wavelength of light that passes through the glass
R=radius of curvature of the lens
The Attempt at a Solution
If I was given a value for λ for the wavelength of light, I know I could simply plug in everything to find the radius of curvature of the planoconvex lens, then divide λ by 1.33 and solve for the new r3, but with the given data I am not sure how to find the wavelength of light, or if there is some other way to go about solving this problem.