CBSE Class 12-science Answered
if for a perfectly symmertical lens, r1=r2, then how will we find focal length??
Asked by Dark | 05 Jan, 2022, 11:54: AM
We have Lens maker's formula
![begin mathsize 14px style 1 over f equals space open parentheses mu space minus space 1 close parentheses open parentheses 1 over r subscript 1 space minus space 1 over r subscript 2 close parentheses end style](https://images.topperlearning.com/topper/tinymce/cache/c2a3e940230eee5d55039557f45191c1.png)
where f is focal length , μ is refractive index of materilal of lens , r1 is radius of curvature of left side
spherical surface and r2 is radius of curvature of right side spherical surface.
numerically r1 = r2 = r, by sign convention r1 is positive and r2 is negative .
Hence we get from eqn.(1)
![begin mathsize 14px style f space equals space fraction numerator r over denominator 2 open parentheses mu minus 1 close parentheses end fraction end style](https://images.topperlearning.com/topper/tinymce/cache/0487d84251a55b852b0f4d2e99c3b4c8.png)
From the known value of radius of curvature and refractive index of material ,
we get focal length f from above expression
Answered by Thiyagarajan K | 05 Jan, 2022, 13:22: PM
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