JEE MAIN - Physics (2007 - No. 6)
Two lenses of power $$-15$$ $$D$$ and $$+5$$ $$D$$ are in contact with each other. The focal length of the combination is
$$ + 10\,cm$$
$$ - 20\,cm$$
$$ - 10\,cm$$
$$ + 20\,cm$$
Explanation
Power of combination is given by
$$P = {P_1} + {P_2} = \left( { - 15 + 5} \right)D$$ $$\,\,\,\,\,\,\,\,\, = - 10D.$$
Now, $$P = {1 \over f} \Rightarrow f = {1 \over P} = {1 \over { - 10}}$$ metre
$$\therefore$$ $$f = - \left( {{1 \over {10}} \times 100} \right)cm = - 10\,cm.$$
$$P = {P_1} + {P_2} = \left( { - 15 + 5} \right)D$$ $$\,\,\,\,\,\,\,\,\, = - 10D.$$
Now, $$P = {1 \over f} \Rightarrow f = {1 \over P} = {1 \over { - 10}}$$ metre
$$\therefore$$ $$f = - \left( {{1 \over {10}} \times 100} \right)cm = - 10\,cm.$$
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