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

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

Tardigrade Admin · Accepted Answer

The focal length for red colour is fR=fV ((μV-1)/(μR-1)) Given, fV=40 cm, μV=1.56, μR=1.54 ∴ fR=40 × (0.56/0.54) =41.48 cm

Q. 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 $40 c m$ , then focal length for red colour is

41.48 cm

20.25 cm

15.3 cm

2.8 cm

The focal length for red colour is
$f_{R} = f_{V} \frac{( μ _{V} - 1 )}{( μ _{R} - 1 )}$
Given, $f_{V} = 40 c m, μ_{V} = 1.56, μ_{R} = 1.54$
$∴ f_{R} = 40 \times \frac{0.56}{0.54}$
$= 41.48 c m$

Q. 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 40cm, then focal length for red colour is