The figure shows the parallel beam is incident on a prism at a instant and its corresponding plane wavefront $\mathrm{A}_{1} \mathrm{~B}_{1} \cdot \mathrm{A}_{1} \mathrm{~B}_{1}$ is normal to rays and emergent beam is shown by $\mathrm{A}_{2} \mathrm{~B}_{2}$.

Here the length of the path from $\mathrm{B}_{1}$ to $\mathrm{B}_{2}$ is greater then the length of the path from $\mathrm{A}_{1}$ to $\mathrm{A}_{2}$.

In fact, the path from $\mathrm{A}_{1}$ to $\mathrm{A}_{2}$ in prism is larger than the path from $\mathrm{B}_{1}^{\prime}$ to $\mathrm{B}_{2}^{\prime}$.

The velocity of light in prism is less than the velocity in air hence it takes longer time for the light to go from $\mathrm{A}_{1}$ to $\mathrm{A}_{2}$. As a result, $\mathrm{A}_{2}$ is lagging behind point $\mathrm{B}_{2}$. So the emergent wavefront is slightly tilted.