Which of the following generates a plane wavefront?

Explain how to obtain a new wavefront at time $\tau$ using Huygens' principle for a plane wavefront.

What is the shape of the wavefront on Earth for sunlight?

State Huygens' principle. What is the fundamental principle on which it is based?

The wave theory of light was given by

$A$ light wave is propagating with plane wave fronts of the type $x+y+z=$ constant. The angle made by the direction of wave propagation with the $x$-axis is