Ask Me Help Desk - Physic-Optics, finding refractive index

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Focal length - Wikipedia, the free encyclopedia

The formula is :

$\frac1f = (n-1) \[\frac{1}{R_1} - \frac{1}{R_2} + \frac{(n-1)d}{nR_1R_2} \]$

Since your lens is a planoconvex one, I guess you'll have to take the other 'radius' to be infinite. Let the 'other radius', which is the flat part be R_2

$\frac1f = (n-1) \[\frac{1}{R_1} - \frac{1}{\infty} + \frac{(n-1)d}{n R_1\infty} \]$

$\frac1f = (n-1) \[ \frac{1}{R_1} - 0 + 0 \]$

$\frac1f =\frac{ (n-1)}{R_1}$

I'm not sure for that one though. :o