Which of the following generates a plane wavefront?

An initially parallel cylindrical beam travels in a medium of refractive index $\mu(I) = \mu_0 + \mu_2I$,where $\mu_0$ and $\mu_2$ are positive constants and $I$ is the intensity of the light beam. The intensity of the beam is decreasing with increasing radius. The initial shape of the wavefront of the beam is

Consider a point at the focal point of a convergent lens. Another convergent lens of short focal length is placed on the other side. What is the nature of the wavefronts emerging from the final image?

The wavefront of a distant source of unknown shape is approximately

Which of the following is correct for light diverging from a point source?

The figure shows plane waves refracted from air to water using Huygens's principle. $a, b, c, d, e$ are lengths on the diagram. The refractive index of water with respect to air is the ratio: