• Tags: reflection, sign-convention, basics
• Difficulty: Foundation

The two laws of reflection state: (1) The angle of incidence equals the angle of reflection. (2) The incident ray, reflected ray, and normal all lie in the same plane.

The New Cartesian Sign Convention is essential for JEE: all distances are measured from the pole/optical centre. Distances in the direction of incident light are positive. Heights above the principal axis are positive, below are negative. For a concave mirror, f and the focal point are on the same side as the object (negative side), so f is negative. For convex mirror, f is positive.

Common mistake: forgetting that u is always negative for real objects (light comes from the left by convention).