*Definition :*

*We know that a ray of light is composed of packets of energy known as photons. The photons have the ability of colliding with any surface.*

*Thus the photons transfer momentum & energy in the same way as the transfer of momentum & energy take place between any two particles during collision.*

*For elastic collision a particle gets rebounded with same speed where as the direction of its motion changes due to the impact (impulsive) force offered by the other colliding particle.*

*Similarly when a photon is incident on a fine plane surface, it gets rebounded. This is known as Reflection.*

*What are the laws of reflection ?*

*What are the laws of reflection ?*

*There are two laws governing the physical phenomenon of reflection.*

*They are-*

*(i) The incident ray, normal to the interface and the reflected ray lie on the same plane.*

*(ii) The angle of incidence is equal to the angle of reflection.*

*We can observe from the figure that two rays coming from a point object O are reflected at the points M & N. If the reflected rays are produced back they meet at a point P. This point P is known as image.*

*Since the image is formed at the back of the mirror, it is known as virtual image .*

*Geometrically we can see that object distance is equal to image distance from mirror surface p = q*

*(As ΔOMP & ΔONP are isosceles triangles)*

__Properties of image formed by plane m__

__Properties of image formed by plane m__*Properties of image formed by plane mirror*

*(a) the image is virtual erect and laterally reversed.*

*(b) the image size = object size*

*(c) object distance = image distance*

*(d) when the plane mirror is rotated through an angle θ,*

*the reflected ray is rotated through an angle 2θ.*

*(e) the numbers of images formed by the combination of two mirrors inclined at an angle θ,*

*Number of image formed ,*

*(i) ; If 360/θ is an even integer*

*(i) ; If 360/θ is an odd integer*

*e.g. When two plane mirrors are placed parallel to each other , then θ = 0*

*⇒ N = ∞*

*Therefore infinite number of images are formed.*

*Example : If an object moves towards a plane mirror with a speed V at an angle θ to the perpendicular to the plane of the mirror, find the relative velocity between the object & the image.*

*Solution :*

*Velocity of the object relative to the image is given as*

*Exercise : Prove that if the plane mirror rotates through an angle θ, the reflected ray for any incident ray is deviated through an angle 2θ.*

*Example : Find the minimum length of the mirror required to visualise the complete image of the body of a person of height H.*

*Solution: The person must have to obtain a reflected ray one from his head & another reflected ray from the feet. *

*Geometrically, AC = AB + BC where*

*i.e. length of the mirror is just half the height of person.*

*Illustration: Show that if a mirror is rotated by angle θ the reflected ray rotates through an angle 2θ When the mirror rotates by θ , the angle between the normals is also equal to θ.*

*Solution :*

*Incidence angle for M’ = i + θ*

*Angle of reflection for M’ = i + θ*

*i.e. ∠N _{2}OR_{2} = i + θ*

*∠N _{1}OR_{2} = ∠N_{1}ON_{2} + ∠N_{2}OR_{2}*

*= θ + i + θ = i + 2 θ*

*or ∠R _{1}OR_{2} = ∠N_{2}OR_{2} -∠N_{2}OR_{1}*

*= i + 2 θ – i = 2 θ*

*Hence the reflected ray rotates by an angle 2θ*

*Exercise : Find the number of images formed when two plane mirrors are placed at right angle to each other & locate the images by drawing ray diagram.*

*Exercise : Draw the ray diagram for the images formed by an object placed in between two plane parallel mirrors due to both the reflecting surfaces*