Question

The refractive indices of the material of a lens for violet and red colours are respectively $1.56$ and $1.54$. If the focal length of the lens for violet light is $40\, cm$, then focal length for red colour is

• A. 41.48 cm
• B. 20.25 cm
• C. 15.3 cm
• D. 2.8 cm

Answer 1

Answer: (A)

The focal length for red colour is
$f_{R}=f_{V} \frac{\left(\mu_{V}-1\right)}{\left(\mu_{R}-1\right)}$
Given, $f_{V}=40\, cm,\, \mu_{V}=1.56,\, \mu_{R}=1.54$
$\therefore f_{R}=40 \times \frac{0.56}{0.54}$
$=41.48\, cm$