Why does a light ray not bend when it strikes a boundary at a right angle?

When a light ray strikes a boundary between two transparent media at a right angle, it does not bend because the angle of incidence is zero.1

In optics, the behavior of light at a boundary is described by Snell’s law, which is expressed as:

$$n_1\sin {\theta }_1=n_2\sin {\theta }_2$$

In this equation, ${\theta }_1$ represents the angle of incidence and ${\theta }_2$ represents the angle of refraction, both measured relative to the normal (a line perpendicular to the surface). When a ray strikes the boundary at a right angle, it is traveling along the normal, meaning ${\theta }_1=0^{\circ }$. Since $\sin (0^{\circ })=0$, the equation simplifies to $0=n_2\sin {\theta }_2$. This requires $\sin {\theta }_2$ to be zero, which means the angle of refraction ${\theta }_2$ must also be $0^{\circ }$.12

Consequently, the light continues to travel in its original direction without changing its path.1