Question

A convex lens of focal length 40 cm and a concave lens of focal length 25 cm are placed in contact in such a way that they have the common principal axis. Find the power of the combination. What is the type of lens formed by their combination? [4 MARKS]

Answer 1

Formula:1 Mark
Power of each lens: 1 Mark each
Power of combination: 1 Mark
Type of lens: 1 Mark

Given:
Focal length of the convex lens, $f_1$ = 40 cm = + 0.4 m

Power of the convex lens,

$P_1$ = $\frac{1}{+0.40}$ = + 2.5 D

Focal length of the concave lens,

$P_2$ = $\frac{1}{-0.25}$ = - 4 D

Power of the combination,

P = $P_1$ + $P_2=2.5D-4D=-1.5D$

The power of the lens formed is negative, so a concave lens is formed by the combination of the lenses.