Question

The refractive index of a Plano-convex lens is 1.6 for violet and 1.5 for red light. The radius of curvature is 0.20 cm. The separation between violet and red foci of the lens is ____________

(a) 0.05 cm

(b) 0.06 cm

(c) 0.07 cm

(d) 0.08 cm

Answer 1

Right answer is (c) 0.07 cm

To explain I would say: Here, μv = 1.6, μr = 1.5, R1 = 0.20 m, R2 = ∞

Using lens maker’s formula: (frac{1}{F_v}=(μ_v-1)(frac{1}{R_1}+frac{1}{R_2}))

Fv = 0.33 m

Similarly solving for red color: (frac{1}{F_r}=(μ_r-1)(frac{1}{R_1}+frac{1}{R_2}))

Fr = 0.40 m

Separation between red and violet, Fr–Fv = 0.40 – 0.33

= 0.07 m.