Reflection of Light — Question 8

Given,

u = 4 cm (negative)

v = 6 cm (positive)

Mirror formula:

$\dfrac{1}{\text u} + \dfrac{1}{\text v} = \dfrac{1}{\text f}$

Substituting the values in the formula above, we get,

$- \dfrac{1}{4} + \dfrac{1}{6} = \dfrac{1}{\text f} \\[0.5em] \dfrac{1}{\text f} = \dfrac{-3+2}{12} \\[0.5em] \dfrac{1}{\text f} = - \dfrac{1}{12} \\[0.5em] \Rightarrow \text f = - 12 \text{ cm} \\[0.5em]$

Hence, the focal length of concave mirror = 12 cm