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.