Question

Two thin lenses of power +3.5 D and -2.5 D are placed in contact. Find the focal length of the lens combination.
a) 10cm
b) -1cm
c) 100cm
d) 0.001cm​

Answer 1

Answer:

option c) 100 cm

Explanation:

Given :

Two thin lenses of power +3.5 D and -2.5 D are placed in contact.

To find :

the focal length of the lens combination

Solution :

Let

• P₁ = +3.5 D
• P₂ = -2.5 D

The total power of the lens combination is their sum.

P = P₁ + P₂

P = 3.5 D + (-2.5 D)

P = 3.5 D - 2.5 D

P = 1 D

Hence, the power of the lens combination is 1 D.

We know,

The power of the lens is the reciprocal of the focal length.

$\longmapsto \bf P=\dfrac{1}{f(in \ m)}$

The focal length of the lens combination,

f = 1/P

f = 1/1

f = 1 m

f = 100 cm

Therefore, the required focal length is 100 cm.