$A$ 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

The figure shows a surface $XY$ separating two transparent media,medium-$1$ and medium-$2$. The lines $ab$ and $cd$ represent wavefronts of a light wave traveling in medium-$1$ and incident on $XY$. The lines $ef$ and $gh$ represent wavefronts of the light wave in medium-$2$ after refraction. The phases of the light wave at $c, d, e$ and $f$ are $\phi_c, \phi_d, \phi_e$ and $\phi_f$ respectively. It is given that $\phi_c \neq \phi_f$.

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

By Huygen's wave theory of light,we cannot explain the phenomenon of

When a point source of light is placed at the focus of a convex lens,light emerges from the lens. What is the shape of the wavefront of the emerging light?

What is visible light? Write various views on it.