Question

A concave lens made of glass has a focal length 10 cm in air. Find its focal length when immersed in water.
[ ang = 1.5, allw = 4/3]
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Answer 1

Answer:

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$From \: lens \: makers \: formula, fa1=(aμg−1)(R11−R21)         ......... (1)</p><p></p><p> \\ \\ \\ where \: R1  \: and \: R2 are \: radius \: of \: curvature \: of \: two \: sides \: of \: lens. \\ \\ </p><p></p><p>fw1=(wμg−1)(R11−R21)        ............ (2) \\ \\ Dividing,</p><p></p><p></p><p>fafw=wμg−1aμg−1              ............. (3)</p><p></p><p>Now, aμg=23</p><p> \\ \\ aμw=34</p><p> \\ \\∴aμwaμg=μgμa×μaμw=μaμw=wμg</p><p></p><p></p><p>=23×43=89 \\ \\ </p><p></p><p></p><p> From 3,  fw=89−123−1×10=8121×10=40cm</p><p></p><p></p><p></p><p>$