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CBSE Class 10 Answered

how to find focal length of a lens in another medium
Asked by krishnasajeevan4601 | 03 May, 2019, 06:43: AM
answered-by-expert Expert Answer
(1) Experimentally measuring focal length in another medium if possible.
        we try to get image of long distance object so that incident rays are considered parallel and
        we get the image at focal point.    Alternatively we can use the lens equation (1/v) - (1/u) = 1/f ,
        by forming image of an given object and  measuring the object-to-lens distance u and image-to-lens distance v.
 
(2) if experiments are difficult and not possible, we use lens-makers formula (1/f) = (μ -1 ) [ (1/R1) + (1/R2) ] ,
      where R's are radii of curvature and μ is refractive index of medium. 
      Let us consider we know focal length fa of lens in air is known. Then focal lenth fm in another medium is obtained
      from lens-makers formula as
                                                              begin mathsize 16px style f subscript m space equals space f subscript a fraction numerator open parentheses mu subscript a minus 1 close parentheses over denominator open parentheses mu subscript m minus 1 close parentheses end fraction end style
Hence from known values of fa and respective refractive index of air and medium, 
one can get focal length of lens in another medium.
Answered by Thiyagarajan K | 03 May, 2019, 07:23: AM

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