Write Huygen's principle and principle basically composited ? 

When a plane wave front incident on a concave mirror as shown in figure then shape of reflected wave front

Figure shown plane waves refracted for air to water using Huygen's principle $a, b, c, d, e$ are lengths on the diagram. The refractive index of water wrt air is the ratio.

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 $\phi_c \neq \phi_f.$

You have learnt in the text how Huygens’ principle leads to the laws of reflection and refraction. Use the same principle to deduce directly that a point object placed in front of a plane mirror produces a virtual image whose distance from the mirror is equal to the object distance from the mirror.