Question

A convergent lens of power 8D is combined with a divergent lens of power - 10 D. calculate the focal length of combination.

Answer 1

Answer:   power=8-10=-2D

focal length=   -500mm

Explanation:

In this case, the two lenses have +8 and -10 dioptres of power, so their combination will have -2 dioptres of power. That means the combination has a focal length of -500 mm (i.e. it is diverging)

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Answer 2

Given That

Power of convergent lens= 8D ( let p1)

Power of divergent lens= -10D ( let p2)

Therefore Total power of resultant lens= p1+p2

                                                                 =8D+(-10D)

                                                                 =8D-10D

                                                                   =-2D

Since we know that

power =1/focal length of lens

or vice versa therefore

focal length of lens = -1/2

                                 =-0.5m or -50cm

Negative sign means it is a concave lens.

Hope you understand....