The correct answer is The line passing through the centre of curvature and pole.. Explanation & Key Takeaways: • The principal axis of a spherical mirror is defined as the straight line that passes through the centre of curvature (C) and the pole (P) of the mirror. • This line acts as the reference line for all measurements and is perpendicular to the surface of the spherical mirror at the pole. • The principal axis is significant because it determines the symmetry of the mirror and is the axis along which the mirror's properties, such as focus and curvature, are defined. • In both concave and convex mirrors, the principal axis is the central line that helps in locating Explanation & Key Takeaways: like the focus (F) and the centre of curvature (C). • Hence, the correct definition of the principal axis is the line passing through the centre of curvature and pole, making Option 2 the right answer. Related Concepts: • Pole (P): • The pole is the geometric center of the mirror's reflecting surface. • It is the point where the principal axis intersects the mirror's surface. • Centre of Curvature (C): • The centre of curvature is the center of the sphere of which the spherical mirror is a part. • It lies outside the mirror for both concave and convex mirrors and is located on the principal axis. • Focus (F): • The focus is the point on the principal axis where parallel rays of light, after reflection, converge (in concave mirrors) or appear to diverge from (in convex mirrors). • The distance from the pole to the focus is called the focal length (f). • Radius of Curvature (R): • The radius of curvature is the distance between the pole and the centre of curvature. • It is equal to twice the focal length, i.e., R = 2f. • Convex vs. Concave Mirrors: • Concave mirrors are converging mirrors that reflect light inward, while convex mirrors are diverging mirrors that spread light outward. • The principal axis remains the same in both types, helping locate Explanation & Key Takeaways: like the focus and the centre of curvature. Important Notes: • Option 1: "The line passing through the focus and object" is incorrect because the principal axis does not depend on the object's position. • Option 3: "The tangent at the pole" is incorrect as a tangent is a line that touches the surface at one point and does not represent the principal axis. • Option 4: "The perpendicular line drawn from the pole" is vague and does not define the axis passing through the centre of curvature. • Always remember, the principal axis is the line that passes through the centre of curvature and the pole of the mirror.