What is the most important point of Huygen's wave theory ?

The figure shows a surface $XY$ separating two transparent media, medium -$1$ and medium -$2$. The lines $a b$ and cd represent wavefronts of a light wave travelling in medium-$1$ and incident on $XY$. The lines ef and gh represent wavefronts of the light wave in medium-$2$ after refraction.

$Image$

$1.$  Light travels as a

$(A)$ parallel beam in each medium

$(B)$ convergent beam in each medium

$(C)$ divergent beam in each medium

$(D)$ divergent beam in one medium and convergent beam in the other medium.

$2.$  The phases of the light wave at $\mathrm{c}, \mathrm{d}, \mathrm{e}$ and $\mathrm{f}$ are $\phi_{\mathrm{c}}, \phi_{\mathrm{d}}, \phi_{\mathrm{e}}$ and $\phi_{\mathrm{f}}$ respectively. It is given that $\phi_{\mathrm{c}} \neq \phi_{\mathrm{f}}$

$(A)$ $\phi_{\mathrm{c}}$ cannot be equal to $\phi_{\mathrm{d}}$

$(B)$ $\phi_{\mathrm{a}}$ can be equal to $\phi_{\mathrm{e}}$

$(C)$ $\left(\phi_{\mathrm{d}}-\phi_t\right)$ is equal to $\left(\phi_{\mathrm{c}}-\phi_{\mathrm{e}}\right)$

$(D)$ $\left(\phi_{\mathrm{d}}-\phi_c\right)$ is not equal to $\left(\phi_{\mathrm{f}}-\phi_e\right)$

$3.$  Speed of the light is

$(A)$ the same in medium-$1$ and medium-$2$

$(B)$ larger in medium-$1$ than in medium-$2$

$(C)$ larger in medium-$2$ than in medium-$1$

$(D)$ different at $\mathrm{b}$ and $\mathrm{d}$

Give the answer question $1, 2$ and $3.$

Figure, shows wave fronts in still water moving in the direction of the arrow towards the interface $PQ$ between a shallow region and a deep(denser) region. Which of the lines shown may represent one of the wave fronts in the deep region?

In the adjacent diagram,$ CP$ represents a wavefront and $AO$ & $BP$, the corresponding two rays. Find the condition on $\theta$ for constructive interference at $P$ between the ray $BP$ and reflected ray $OP$

The idea of secondary wavelets for the propagation of a wave was first given by