Question

Arrange the following combinations in the increasing order of focal length :
a) Two plano convex lenses of focal lengths 20 cm and 30 cm in contact.
b) Two convex lens of focal lengths 20 cm and 10 cm in contact.
c) Two convex lenses of focal length 25 cm separated by 5 cm.

• A. a, b, c
• B. b, a, c
• C. a, c, b
• D. c, a, b

Answer 1

Answer: (B)

b, a, c

Equivalent focal length,$f_{eq}=\frac{f_1{f}_2}{f_1+f_2-d}$

$f_{eq}^1=\frac{20\times 30}{20+30-0}=12cm$

$f_{eq}^2=\frac{20\times 10}{20+10-0}=\frac{20}{3}cm$

$f_{eq}^3=\frac{25\times 25}{25+25-5}=\frac{25\times 25}{45}=\frac{125}{9}cm$