Page 3 - Lesson note reflection through spherical mirror

Principle focus and focal length of a Spherical Mirrors
 For understanding about principle focus and focus length of a spherical mirror
first consider the figure given below


From figure 3a we see that a number of rays parallel to the principal axis are
falling on a concave mirror. If we now observe the reflected rays we see that
they are all intersecting at a point F on the principal axis of the mirror. This
point is called the principal focus of the concave mirror.
 In case of convex mirror rays get reflected at the reflecting surface of the
mirror and these reflected rays appear to come from point F on the principle
axis and this point F is called principle focus of convex mirror.
 The distance between the pole and the principal focus of a spherical mirror is
called the focal length. It is represented by the letter f.
 There is a relationship between the radius of curvature R, and focal length f,
of a spherical mirror and is given by R=2f which means that that the principal
focus of a spherical mirror lies midway between the pole and centre of
curvature.

Image Formation by Spherical mirrors
 The nature, position and size of the image formed by a concave mirror
depend on the position of the object in relation to points P, F and C.
 The image formed can be real as well as virtual depending on the positions of
the object.
 The image is either magnified, reduced or has the same size, depending on
the position of the object.

Rules for obtaining images formed by spherical mirrors

(1) Rule 1

A ray of light which is parallel to the principle axis of the mirror passes through its
focus after reflection from the mirror as shown below in the figure

3