Question

Suppose that there is a dispersive medium whose refractive index depends on the wavelength as given by $n(λ) = n_0 +\frac{ a}{λ^2}-\frac{b}{λ^4}$ . The value of 𝜆 at which the group and phase velocities would be the same, is:

• A. $\sqrt\frac{2a}{a}$
• B. $\sqrt\frac{b}{2a}$
• C. $\sqrt\frac{3b}{a}$
• D. $\sqrt\frac{b}{3a}$

Answer 1

Answer: (A)

$\sqrt\frac{2a}{a}$

Answer 2

The correct option is (A): $\sqrt\frac{2a}{a}$