Correct Answer - Option 1 : -5D
Concept:
-
Power of Lens (P): Power of Lens is the reciprocal of focal length (f).
\(P = \frac{1}{f}\)
- Unit of Power of Lens is diopter (D).
- Power of lens signifies, the converging or diverging capability.
- When two or more lenses are kept in Contact, then the power of the combination of the lens is the sum of the Power of all lenses.
Power of n number of the lens in contact is given as
P = P1 + P2 + P3 + ... Pn
- Power and focal length of the concave lens are negative.
- Power and focal length of the convex lens are positive.
Calculation:
Focal length of (convex) lens 1 f1 = + 20 cm = 0.2 m
Power of lens 1
\(P_1 = \frac{1}{0.2} = 5 D\) -- (1)
Focal length of (concave) lens 2 f1 = - 10 cm = - 0.1 m
Power
\(P_2 = \frac{1}{0.1} = - 10 D\) -- (2)
Power of combination
P = P1 + P2
⇒ P = 5 D - 10 D (From Eq 1 and 2)
So, the power is - 5 D