Question

A convex lens of focal length 10 cm is placed in contact with a concave lens of focal length 20 cm. The focal length of this combination of lenses will be
(a) +10 cm
(b) +20 cm
(c) −10 cm
(d) −20 cm

Answer 1

Answer:

-10 cm

Explanation:

because the focal length of convex lens is 10 cm

and focal length of concave lens is always negative so it will be -20 cm

now, adding both focal lengths we get

{ 10 + (-20 ) } cm = -10 cm

please mark me BRAINLIEST ANSWER !!!

Answer 2

A convex lens of focal length 10 cm is placed in contact with a concave lens of focal length 20 cm. The focal length of this combination of lenses will be  +20 cm

Explanation:

In the question, it is given that,

Convex lens focal length, f = +10 cm

Concave lens has the focal length, f' = -20 cm

Let the power of the convex and concave lens is given as P and P', respectively. So,

P = 0.1 D  And P' = -0.05 D

Let the combination of lenses power be shown as p, p = 0.05 D.

Let the focal length of the combination of lenses be shown as F. This is denoted as f = 20 cm.

Thus, this combination of lenses will have the focal length of +20 cm.  

Therefore the correct answer is Option(b) +20 cm