## Laws of Reflection

7/1/2015 CBSE

1. Laws of Reflection. The reflection at a plane surface
always takes place in accordance with the following two laws:

(i) The incident ray, the reflected ray and normal to surface at the point of
incidence all lie in the same plane.

(ii) The angle of incidence, i is equal to the angle of reflection r,������
i. e.,

2. Formation of Image by the Plane Mirror.

The formation of image of a point object O by ���a plane mirror is represented
in figure. The image formed I has the following characteristics:

(i) The size of image is equal to the size of object.

(ii) The object distance = Image distance i.e., OM = MI.

(iii) The image is virtual and erect.

(iv) When a mirror is rotated through a certain angle, the reflected ray is
rotated through twice this angle.

3. Reflection of Light from Spherical Mirror.

A spherical mirror is a part cut from a hollow sphere. They are generally
constructed from glass.

The reflection at spherical mirror also takes place in accordance with the laws

of reflection.

4. Sign Convention.

���Following sign conventions are the new cartesian sign convention:

(i) All distances are measured from the pole of the mirror & direction of the
incident light is taken as positive. In other words, the distances measured
toward the right of the origin are positive.

(ii) The distance measured against the direction of the incident light are
taken as negative. In other words, the distances measured towards the left of
origin are taken as negative.

(iii) The image is virtual and erect.

(iv) The distance measured in the upward direction, perpendicular to the
principal axis of the mirror, are taken as positive & the distances measured in
the downward direction are taken as negative.

Note. The focal length of a concave mirror is negative and that of a convex
mirror is positive.