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ICSE Class 10 Answered

If the lens is placed in water instead of air its focal length increases why?
Asked by aditisinha952003 | 02 Mar, 2019, 07:23: PM
answered-by-expert Expert Answer
Lens makers formula :  begin mathsize 12px style 1 over f space equals space open parentheses mu minus 1 close parentheses open parentheses 1 over R subscript 1 space plus space 1 over R subscript 2 close parentheses end style
where f is focal length, μ is refractive index of material of lens, R's are radii of curvature of lens surfaces .
 
If radii of curvature is constant, then focal length is inversely proportional to (μ-1).
 
If lens,  made of glass,  is in air aμg = 3/2  or (μ-1) = 0.5
 
If lens,  made of glass,  is in water  begin mathsize 16px style mu presuperscript w subscript g space equals space fraction numerator mu presuperscript a subscript g over denominator mu presuperscript a subscript w end fraction space equals space fraction numerator begin display style bevelled 3 over 2 end style over denominator begin display style bevelled 4 over 3 end style end fraction space equals 9 over 8 end style   or  (μ-1) = 0.125
focal length is increased in water because (μ-1) is decreased

Answered by Thiyagarajan K | 02 Mar, 2019, 07:50: PM

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