Which of the following statements is/are true about the principal axis of a spherical mirror?
(i) It is normal to the mirror
(ii) Point of incidence always lies on the principal axis
(iii) Principal focus always lies on the principal axis

Correct option is B

The principal axis of a spherical mirror is an important reference line in optics. Let's evaluate each statement:

(i) It is normal to the mirror: This statement is true. The principal axis is a straight line that passes through the center of curvature and the pole of the mirror. It is perpendicular (normal) to the mirror’s surface at the pole.

(ii) Point of incidence always lies on the principal axis: This statement is not always true. While the principal axis is important for the geometry of spherical mirrors, the point of incidence can lie anywhere on the mirror's surface, not necessarily on the principal axis.

(iii) Principal focus always lies on the principal axis: This statement is true. The principal focus of a spherical mirror is always located on the principal axis. It is the point where parallel rays of light (after reflecting from the mirror) either converge or appear to diverge, depending on whether the mirror is concave or convex.

Therefore, the correct answer is (b), since statements (i) and (iii) are true.

Important Key Points:

1. The principal axis is the line passing through the center of curvature and the pole of a spherical mirror.
2. The principal axis is always normal (perpendicular) to the mirror at its pole.
3. The principal focus is located on the principal axis, and it plays a crucial role in determining the focal length of the mirror.
4. Rays parallel to the principal axis either converge (in a concave mirror) or diverge (in a convex mirror) at the principal focus.
5. The point of incidence on the mirror’s surface can be anywhere and is not constrained to the principal axis.
6. The principal axis is a key element in forming image locations and determining magnification in spherical mirrors.

• The principal axis provides symmetry to the system, allowing simpler ray diagrams for understanding how light reflects off the mirror.
• The principal focus of a concave mirror can form real images (when the object is placed outside the focal point) or virtual images (when the object is placed inside the focal point), while in a convex mirror, the image is always virtual and diminished, regardless of the object position.