Question 202: Ray Optics

Question

Two lenses of power $-15$ $D$ and $+5$ $D$ are in contact with each other. The focal length of the combination is

Accepted Answer

Power of combination is given by
$P = {P_1} + {P_2} = \left( { - 15 + 5} ight)D$  $\,\,\,\,\,\,\,\,\, =  - 10D.$
Now, $P = {1 \over f} \Rightarrow f = {1 \over P} = {1 \over { - 10}}$ metre
$	herefore$ $f =  - \left( {{1 \over {10}} 	imes 100} ight)cm =  - 10\,cm.$

Answer

$ + 10\,cm$

Answer

$ - 20\,cm$

Answer

$ - 10\,cm$

Answer

$ + 20\,cm$

Ray Optics

Two lenses of power -15 D and +5 D are in contact with each other. The focal length of the combination is

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