The angle of incidence refers to the angle at which a ray of light strikes a plane mirror. In this case, the angle of incidence is given as 30′ (30 minutes), which is a unit of angular measurement.

To determine the angle of deviation in the incidence ray, we need to understand the behavior of light when it interacts with a plane mirror. When a ray of light strikes a mirror surface, it follows the law of reflection, which states that the angle of incidence is equal to the angle of reflection.

The angle of deviation is the difference between the incident angle and the reflected angle. In the case of a plane mirror, since the angle of incidence and the angle of reflection are equal, the angle of deviation is zero. This means that the incident ray and the reflected ray are in the same line.

To summarize:

- The angle of incidence is the angle at which a ray of light strikes a plane mirror.
- The angle of reflection is equal to the angle of incidence, according to the law of reflection.
- The angle of deviation is the difference between the incident angle and the reflected angle.
- For a plane mirror, the angle of deviation is zero.

In this case, since the angle of incidence is given as 30′, the angle of deviation would also be 30′, as the incident ray and the reflected ray are parallel to each other.

It is important to note that the angle of deviation is different for different types of mirrors or surfaces. In the case of a plane mirror, the angle of deviation is always zero. However, for curved mirrors or other surfaces, the angle of deviation can vary depending on the shape and curvature of the surface.

Overall, when dealing with a plane mirror, the angle of deviation is always zero, regardless of the angle of incidence.