Question

Electromagnetic waves emanating from a point A (in air) are incident on a rectangular block of material M and emerge from the other side as shown. The angles i and r are angles of incidence and refraction when the wave travels from air to the medium. Such paths for the rays are possible

• A. If the material has a refractive index very nearly equal to zero
• B. Only with gamma rays with a wavelength smaller than atomic nuclei of the material
• C. If the material has a refractive index less than zero
• D. Only if the wave travels in M with a speed faster than the speed of light in vacuum.

Answer 1

Answer: (C)

If the material has a refractive index less than zero

As can be seen from the given image, incident ray and refracted ray, both are on the same side of the normal. Usually, the refracted angle is defined as the angle between normal and refracted ray on opposite direction as that of incident ray. Hence here, the angle would be (-r).

So using Snell's law,

$\frac{\mu 1}{\mu 2}$ = $\frac{sinr}{sini}$

Here,

$\frac{\mu 1}{\mu 2}$ = $\frac{sin(-r)}{sini}$ = $\frac{-sinr}{sini}$

So, to make the ratio positive, of all factors, we can say only $\mu 2$ will have to be negative. Option C.

Such materials, with negative Refractive Index are known as Meta materials.