Question

Define principal focus and focal length for a concave mirror.

Hint:

For a spherical mirror, the focal length is equal to half of the radius of curvature: \[ f = \frac{R}{2} \]

Answer 1

Concept: A concave mirror is a spherical mirror whose reflecting surface curves inward. When parallel rays of light fall on a concave mirror, they converge after reflection at a particular point on the principal axis.
Step 1:Principal Focus} The principal focus of a concave mirror is the point on the principal axis where all rays of light that are parallel to the principal axis meet after reflection from the mirror.
Step 2:Focal Length} The focal length of a concave mirror is the distance between the pole (P) of the mirror and the principal focus (F). \[ \text{Focal Length} = PF \] It is usually denoted by \(f\).