Lenses and mirrors

The power of a diverging lens is 10D and that of a converging lens is 6D. When these two lenses are placed in contact with each other. The power of their combination will be

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The focal length of diverging lens is
${ F }_{ 1 }=\dfrac { 1 }{ P } =-\dfrac { 1 }{ 10 }$
The focal length of converging lens is
${ F }_{ 2 }=\dfrac { 1 }{ P } =+\dfrac { 1 }{ 6 }$
The focal length of combined lens is
$\dfrac { 1 }{ f } =\dfrac { 1 }{ { f }_{ 1 } } +\dfrac { 1 }{ { f }_{ 2 } }$
$P=\dfrac { 1 }{ f } =-10+6=-4D$
The combined lens will act as diverging (concave ) lens.
Option C is correct.

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