Two lenses of focal lengths, 20 cm and - 25 cm, respectively are placed in contact with each other. The power of the combination of lenses is

1 D

The correct answer is Option (1) → 1 D

Power of a lens: $P = \frac{100}{f (\text{cm})} \, \text{D}$

Given: $f_1 = 20 \, \text{cm} \Rightarrow P_1 = \frac{100}{20} = 5 \, \text{D}$

$f_2 = -25 \, \text{cm} \Rightarrow P_2 = \frac{100}{-25} = -4 \, \text{D}$

Power of combination (lenses in contact): $P_\text{total} = P_1 + P_2 = 5 - 4 = 1 \, \text{D}$

Power of combination = 1 D