Important points of light reflection

Important points of light reflection

Laws of reflection

(a) Angle of incidence is equal to the angle of reflection .

(b) The incident ray, the reflected ray and the normal all lie in the same plane.

Mirror : A smooth, highly polished reflecting surface is called a mirror. There are two types of mirrors :

(a) plane mirror

(b) curved mirrors

Curved mirrors are classified as spherical mirrors or parabolic mirrors depending upon the curvature of the mirror.

Concave mirror : A spherical mirror whose inner hollow surface is the reflecting surface is called concave mirror. A concave mirror forms a real, inverted image of an object if the object is placed at any place outside F. However, when  the object lies between F and P, the image formed is erect and virtual.

Convex mirror : A spherical mirror whose outer bulging at surface is the reflecting surface is called convex mirror. The image formed by a convex mirror is always erect, virtual and smaller than the object whatever may be the position of the object. A convex mirror is used as a side-mirror (driver’s mirror) on automobiles.

Aperture of a mirror : The effective width of a spherical mirror from which reflection can take place is called its aperture.

Pole : The centre of a curved mirror is called its pole.

Centre of curvature : The centre of the hollow sphere of which the spherical mirror is a part is called its centre of curvature. The centre of curvature of a concave mirror is in front of it, while that of a convex mirror is behind it.

Radius of curvature : Radius of the hollow sphere of which the mirror is a part is called its radius of curvature.

Principal axis : A straight line passing through the centre of curvature and pole of a spherical mirror is called its principal axis.

Focus : A point on the principal axis of a mirror at which the rays coming from infinity meet or appear to meet is called its focus. Focus is denoted by F.

Focal length : The distance between the pole of a spherical mirror and the focus is called the focal of a spherical mirror.

Real image : The image which can be obtained on a screen is called a real image. A real image is formed only when the reflected or refracted rays actually meet at a point.

Virtual image : The image which can be seen into a mirror but cannot be obtained on a screen is called a virtual image. A virtual image is formed only when the reflected or the refracted rays appear to come from a point.

Sign convention for spherical mirrors :

According to the sign convention for mirror, the focal length of a concave mirror is negative and that of a convex mirror is positive.

Mirrors formula : The relationship, \frac{1}{f}=\frac{1}{v}+\frac{1}{u}$ is called the mirror formula.

Magnification : The ratio of  the size of the image to that of the object is called magnification. For a mirror, magnification (M) is given by,

M = – \frac{v}{u}=\frac{{{{h}_{i}}}}{{{{h}_{o}}}}$

power\,(in\,diopters)=\,\frac{1}{{f(metre)}}$

Conclusions from the sign Convention

For spherical mirror 

Distance of the object, u is negative
Distance of the real image, v is negative
Distance of the virtual image v is positive
Focal length, f is negative for concave & f is + ve for convex
Radius of curvature, R is negative for concave &  R is + ve for convex
Height of the object, O is positive
Height of the inverted (real) image, I is negative
Height of the erect (virtual) image, I is positive