P-1 Refraction of light — Science STD 10 — Question

The ray of light incident at the boundary separating the two media bends away from the normal on refraction. Here, the angle of refraction $r$, is greater than the angle of incidence i.
Now ${ }_{ a } n _{ w }=\frac{\sin i}{\sin r}<1$. Here, ${ }_{ a } n _{ w }$ is the refractive index of $\sin r$ air with respect to water.
As ${ }_a n_w$ is constant, $r$ increases as i increases. For $r=90^{\circ}$, the ray travels along the boundary. If $i$ is increased further, as $r$ cannot be greater than $90^{\circ}$, light does not enter air. There is no refraction of light and all the light enters water on reflection. This is called total internal reflection.
For $r=90^{\circ},{ }_a n_{ w }=\frac{\sin i}{\sin 90^{\circ}}=\sin i$. This angle $i$ is $\sin 90^{\circ}$ called the critical angle.