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NEET-XII-Physics

exam-2 year:2016

with Solutions -
Qstn# 101 Prvs-QstnNext-Qstn
  • #101
    Two identical glass (``\mu_g``= 3/2) equiconvex lenses of
    focal length f each are kept in contact. The space
    between the two lenses is filled with water
    (``\mu_w`` = 4 / 3). The focal length of the combination
    is :-
    (1) ``\frac {4f}{3}``
    (2) ``\frac {3f}{4}``
    (3) ``\frac {f}{3}``
    (4) f
    digAnsr:   2
    Ans : (2)
    Sol. 3
    2
    3
    2
    µ =3
    4
    3
    f1
    f2
    f3
    µ =1
    µ =2
    = =
     
     
     
    1 3
    R
    f f
    3
    2 -1
    2
    = R = f (given)
    = = =
     
     
     
    2
    -R -3 3
    f R - f
    4 2 2
    2 -1
    3
    = + +
    eq 1 2 3
    1 1 1 1
    f f f f
     
    = + + 
     
    1 2 1
    -
    f 3f f
    &implies; =
    eq
    1 4
    f 3f &implies;
    =eq
    3f
    f
    4