Question

Certain plane wavefronts are shown in figure. The refractive index of medium is

• A. 2
• B. 4
• C. 1.5
• D. Cannot be determined

Answer 1

Answer: (A)

2

Answer 2

The refractive index of a medium is the ratio of the speed of light in vacuum to the speed of light in the medium, $n = c/v$. Since the frequency of light remains constant when it passes from one medium to another, the ratio of speeds is equal to the ratio of wavelengths: $n = \lambda_{vacuum}/\lambda_{medium}$.

From the figure, we interpret the distance labeled 2m as the wavelength in vacuum ($\lambda_{vacuum} = 2$m) and the distance labeled 1m as the wavelength in the medium ($\lambda_{medium} = 1$m).

Therefore, the refractive index of the medium is $n = \frac{\lambda_{vacuum}}{\lambda_{medium}} = \frac{2}{1} = 2$.