Question

A convex lens of focal length 50 cm, a concave lens of focal length 50 cm and another concave lens of focal lens 20 cm are placed in contact. The power of this combination in dioptres will be:

• A. $4.67$ D
• B. $-5$ D
• C. $3.21$ D
• D. $-3$ D

Answer 1

The correct option is B $-5$ D

$P_{conve{x}_1}=\frac{100}{50}=+2D$

$P_{concav{e}_1}=\frac{100}{-50}=-2D$

$P_{concav{e}_2}=\frac{100}{-20}=-5D$

$P_{eq}=-2-5+2$

$=-5D$