Answer:
When a light ray impacts on a parallel mirror with an angle of incidence A compared with the normal to the surface of the mirror (this means a perpendicular line to the surface), the reflected light ray always is reflected with the same angle.
This means that if the light ray incident normally, the reflected ray will be reflected normally.
If the light ray insides from the bottom with an angle of 30°, the reflected ray will be reflected upwards with an angle of 30° (always measuring these angles from the normal of the surface of the mirror).
On moving the light bulb, the value of angle of incidence and angle of reflection are always equal, which also describes that incident ray and reflected rays are parallel to each other.
The given problem is based on the concept of reflected rays and incidence angle. When we change the position of light bulb or move it around, then the light ray impacts on a parallel mirror with an angle of incidence. In this case, the incidence ray will get reflected with same angle (known as angle of reflection) as compared with the normal to the surface of mirror.
This means that if the light ray incident normally, the reflected ray will be reflected normally. Hence, the reflected light ray always is reflected with the same angle.
And if the condition arises where the light ray enters from the bottom with an angle of 30°, the reflected ray will be reflected upwards with an angle of 30° (always measuring these angles from the normal of the surface of the mirror).
Thus, we can conclude that on moving the light bulb, the value of angle of incidence and angle of reflection are equal, which also describes that incident ray and reflected rays are parallel to each other.
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