The critical angle of incidence ic for a ray incident from a denser to rarer medium, is that angle for which

The correct option is: the angle of refraction is 90°

Step-by-Step Explanation:

1. Understanding Light Travel: When a ray of light travels from an optically denser medium (like water or glass) to an optically rarer medium (like air), it bends away from the normal. This means the angle of refraction (represented as $r$) is greater than the angle of incidence (represented as $i$).

2. Increasing the Angle of Incidence: As we gradually increase the angle of incidence ($i$) in the denser medium, the angle of refraction ($r$) in the rarer medium also increases and bends further away from the normal.

3. Defining the Critical Angle: Eventually, at a specific angle of incidence, the refracted ray bends so much that it grazes the interface separating the two media. At this point, the angle of refraction becomes exactly 90° relative to the normal. This specific angle of incidence in the denser medium is defined as the critical angle ($i_c$).

4. According to Snell's Law:
$n_1\sin \left(i_c\right)=n_2\sin (90°)$
where $n_1$ is the refractive index of the denser medium and $n_2$ is the refractive index of the rarer medium. Since $\sin (90°)=1$, this confirms that the angle of refraction corresponding to the critical angle is precisely 90°.