- #1

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## Homework Statement

Here is a worked problem:

A Newton’s ring apparatus is illuminated by light with two wavelength components. One of

these wavelengths is 546 nm. If the 11th bright ring of the 546 nm fringe coincides with the

10th ring of the other wavelength, what is the second wavelength? The spherical surface

has a radius of 1 m.

Solution:

Radius of m^{th}bright ring is:

##x = ((m-\frac{1}{2})\lambda R)^{1/2}##

So

##(10.5 \times 546 \times 10^{-9} \times 1)^{1/2} = (9.5 \times \lambda \times 1)^{1/2}##

##\lambda = 603.5 \ nm##

I don't see why they've used "m-1/2" instead of "m+1/2"?

## Homework Equations

According to my textbook the radius of m

^{th}bright fringe is:

##x = ((m+\frac{1}{2})\lambda R)^{1/2}##

Where R is the radius of curvature of the convex lens.

## The Attempt at a Solution

My textbook always uses "m+1/2" whereas my lecture notes always use "m-1/2". I'm confused because id I use the first one I get an entirely different answer.

I would appreciate it if any one could explain why we need to use the minus for this particular problem, and not the plus.