A convex lens of focal length 20 cm is placed coaxially in contact with a concave lens of focal length 25 cm. Determine the power of the combination. Will the system be converging or diverging in nature?
focal length of convex lens, f1 = +20cm = +0.20 m
focal length of convex lens, f2 = - 25cm = +0.25 m
Equivalent focal length, `F = 1/(f_1)+1/(f_1)=1/20+1/(-25)=1/(100)`
∴ F = 100cm
Power of convex lens, `P_1 = 1/f_1=1/0.20`
Power of concave lens, `P_2 = 1/f_2 = 1/(-0.25)`
Power of the combination, p = p1 + p2
`= 1/0.20 +1/-0.25`
=`(500 - 400)/100`
= 1 D
The focal length of the combination = 1 m = 100 cm
As the focal length is in positive, the system will be converging in nature.
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