# Quality of Lenses Using Newton's rings

wilsonge

## Homework Statement

Basically I'm writing up a formal report on our experiment on newton's rings - one of the things in our aims was in investigate the quality of the lenses we were using. Whilst many sites also say newton's rings can be used to investigate the quality of the lenses not much further information was given and I couldn't find anything in our lab sheet.

So far I think i've gathered from my research that its to do with the width of the rings produced (i.e. equal widths of the fringes means a perfect lens). However I am unclear as to what the exact process is to getting a numerical value of the quality of the lens - is there a equation somewhere that I'm missing? If there is no equation can I express this numerically with some kind of units?

## Homework Equations

Good Question :) Are there any?

## The Attempt at a Solution

Not really applicable but see above - cause my question is littered with the limited research I've found

Sorry this isn't really layed out in quite the way you want it - but it isn't really algebra either :)

Thanks in advance for any help given,
George

P.S. Not sure if this should have gone in the undergraduate section - sorry if its in the wrong place!!!

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## Answers and Replies

wilsonge
Been thinking a bit more about this and I'm wondering could I use the lens maker's equation and then use the difference in the refractive index - only problem is I only used one lens for each set of Newton's Rings formed so not sure if that gets me anywhere or not

George

Mentor
The radius of the nth dark ring is given by

$$r_n = \sqrt{n\;R\;\lambda}$$

where $$\lambda$$ is the wavelength of light and R the lens radius of curvature. I suppose you could calculate R for each measured rn and then provide a standard deviation figure or some such.