Question

A convex lens of Focal length of 40cm is in contact

with a concave lens of focal length 25cm. The power

of the combination is.

(1) -1.5 D (2) -6.5 D

(3) +6.5 D (4) +6.67 D

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Answer 1

Heya,

Thanks for the question

CONCEPT USED

1. Power of two lens of power P1 and P2 is given by P1 + P2.
2. Focal Length of a convex lens is positive.
3. Focal length of a concave lens is negative.
4. Power of a lens of focal length f (in cm) $\frac{100}{f}$

WORKING ON STEPS

Power of convex lens = $\frac{100}{40} = 2.5$

Power of concave lens = $\frac{100}{25} = 4$

hence, power of the combination is

= 4 + 2.5

= 6.5 D .

Regards.

Kshitij

Answer 2

Answer:

(1)-1.5D

Explanation:

Focal length 40 then

power=100/f

power=100/40

power=2.5D for convex lens

Focal length 25 then

power=100/f

power=100/25

power=-4D for concave lens

The combination of both is 2.5-4=-1.5D