$A$ wavefront is the locus of all points where the particles of the medium vibrate with the same:

$A$ wave front is the locus of all the points where the particles of the medium vibrate with the same:

Mark the wrong statement.

The phase difference between any two particles in a given wavefront is . . . . . . rad.

The following figure represents a wave front $AB$ which passes from air to another transparent medium and produces a new wave front $CD$ after refraction. The refractive index of the medium is $:$ ($PQ$ is the boundary between air and the medium)

When a plane wave front is incident on a concave mirror as shown in the figure,what is the shape of the reflected wave front?