Question

A convex lens of 2 D power is joined with a concave mirror of 1 D power. Equivalent power of instrument will be

• A. $-3D$
• B. $+3D$
• C. $-5D$
• D. $+5D$

Answer 1

Answer: (A)

$-3D$

Power of the convex lens = $2D$

Power of the concave mirror = $1D$

The equivalent power of the combination of a convex lens and a concave mirror is given by the formula

$P_{equivalent}=P_{mirror}-2P_{lens}$

Substituting the values in the formula,

$P_{equivalent}=1-2\left(2\right)$

$P_{equivalent}=1-4$

$P_{equivalent}=-3D$

The equivalent power of the instrument will be -3D.