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

The question was posed to me during an online exam.

This interesting question is from Optical Instruments topic in division Optics of Engineering Physics II

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.