A converging lens of focal length 30 ,cm is p | vedclass

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(A) The power of a lens is given by $P = 1/f$. For a converging lens,the power is positive.When two converging lenses are placed in contact,the total

Vedclass · Accepted Answer

$15$

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(A) The power of a lens is given by $P = 1/f$. For a converging lens,the power is positive.When two converging lenses are placed in contact,the total power of the combination is $P_{eq} = P_1 + P_2$.Since both lenses are converging,$P_1 > 0$ and $P_2 > 0$,therefore $P_{eq} > P_1$.Since $P_{eq} = 1/f_{eq}$ and $P_1 = 1/f_1$,the inequality $P_{eq} > P_1$ implies $1/f_{eq} > 1/f_1$,which means $f_{eq} Given $f_1 = 30 \,cm$,the focal length of the combination must be less than $30 \,cm$.Among the given options,only $15 \,cm$ is less than $30 \,cm$ and positive (as the combination of two converging lenses is also a converging lens).Thus,the correct option is $A$.

$A$ converging lens of focal length $30 \,cm$ is placed in contact with another converging lens of unknown focal length. Then,the possible value for the focal length of the combination is ......... $cm$.

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