Two lenses of power -15 D and + 5 D are in contact with each other. The focal length of the combination is

Question

Two lenses of power -15 D and + 5 D are in contact with each other. The focal length of the combination is (A) −20 cm (B) −10 cm (C) +20 cm (D) +10 cm

Tardigrade Admin · Accepted Answer

P=P1+P2=-10 t=(1/P) ⇒ 0.1 m ⇒ -10 cm

Q. Two lenses of power $- 15 D$ and $+ 5 D$ are in contact with each other. The focal length of the combination is

$- 20 c m$

$- 10 c m$

$+ 20 c m$

$+ 10 c m$

$P = P_{1} + P_{2} = - 10$
$t = \frac{1}{P} \Rightarrow 0.1 m \Rightarrow - 10 c m$

Q. Two lenses of power −15D and +5D are in contact with each other. The focal length of the combination is

−20cm

−10cm

+20cm

+10cm

P=P1​+P2​=−10 t=P1​⇒0.1m⇒−10cm

Q. Two lenses of power $- 15 D$ and $+ 5 D$ are in contact with each other. The focal length of the combination is

$- 20 c m$

$- 10 c m$

$+ 20 c m$

$+ 10 c m$

$P = P_{1} + P_{2} = - 10$
$t = \frac{1}{P} \Rightarrow 0.1 m \Rightarrow - 10 c m$