Question

A converging lens of power 8d is combined with a diverging lens of power 10 d calculate the focal length of the combination

Answer 1

power of converging lens ⇒positive

so, power of converging lens, $P_C=+8D$

power of diverging lens ⇒ negative

so, power of diverging lens, $P_D=-10D$

we know, power of combination of lenses is given by, $P=P_C+P_D$

or, P = +8D -10D = -2D

power of lens is reciprocal of focal length.

so, P = 1/f = -2D

or, f = -1/2 m = -50cm

hence, focal length of combination is -50cm [ here negative sign indicates that combination of lens system is diverging ]

Answer 2

Hey !

Given,

we know that,

Converging lens =  +ve

Now,

Power of diverging lens is -ve

Power of combination of lenses

P = +8 D -10 D = -2 D

P = 1/f = -2 D

f = -1/2m = -50 cm

Good luck !