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

Focal length of combinations of lenses = -10

Explanation:

Given,

Focal length of convex lens = +10

Focal length of concave length lens = -20

then,

Combination of focal length = 10 - 20

concave lens of focal length = -10

Focal length of combinations of lenses = -10

Answer 2

Net Focal Length is 20 cm.

Explanation:

As we already know,

• Power (P) = $\frac {1}{Focal\ Length}$

Hence,

• Power of Convex Lens, $P_1 = \frac {1}{10 cm}$

= $\frac {1}{0.1 m}$ = + 10 D

• Power of Concave Lens, $P_2 = \frac {1}{- 20 cm}$

= $\frac {- 1}{0.2 m}$ = - 5 D

• Also, When lenses comes into contact then the Len's power adds up.

So, we get:

Net Power (P) = $P_1 + P_2$

Net Power (P) = 10 D - 5 D = 5 D

Net Focal Length = $\frac {1}{Power}$

= $\frac {1}{5}$

= 0.2 m

= 20 cm