Verify Laws of Reflection Using a Plane Mirror

The Theory 


Change in direction of a light ray when it strikes a smooth surface or a plane mirror is called reflection of light. Any polished or shiny surface such as a stainless steel plate or a shining steel spoon can act as a mirror and change the direction of light. The surface of the water also reflects light and you see the reflection of trees and buildings in water bodies.  

Anything which gives out light (either of its own or reflected by it) is called an object. For example, a bulb, a candle, a tree, etc. When the light rays coming from an object are reflected from a mirror then an optical appearance is produced. This is called an image. For example, when we look into the mirror, we see the image of our face. Images are either real and inverted or virtual and erect. Real images can be obtained on a screen while virtual images cannot. 


Types of Reflection 

When reflection occurs at a plane or smooth surface such as a plane mirror, all the rays incident parallel to each other remain parallel to each other even after reflection. This is called regular reflection.  

When a beam of light falls on a rough or unpolished surface like paper, cardboard etc, the incident light rays get reflected in different directions. The reflected rays are not parallel to each other, and it is called irregular or diffused reflection. 


Reflection by a plane mirror 

A ray of light PO, incident on a smooth polished surface (plane mirror) gets reflected along OQ. The ray PO is called incident ray and ray OQ is called reflected ray. 

  • Point of incidence: The point O where the incident ray strikes the polished surface is called the point of incidence.  
  • Normal line: A line perpendicular to the reflecting surface at the point of incidence. It is named in the figure as ON. 
  • Angle of incidence: The angle made by incident ray with the normal (the ∠PON in the figure). It is indicated by ∠i. 
  • Angle of reflection: The angle made by the reflected ray with the normal (the ∠QON in the figure). It is indicated by ∠r. 
  • Plane of incidence: The plane containing the incident ray (PO) and normal (ON) is called the plane of incidence. 


Laws of reflection 

  • The angle of incidence is equal to the angle of reflection, and 
  • The incident ray, the normal to the mirror at the point of incidence, and the reflected ray, all lie in the same plane. 
  • These laws of reflection are deduced from experiments. It can be verified using a plane mirror of good quality with a good reflecting surface.  A thin mirror strip is used to avoid the formation of multiple images due to multiple reflections. 
  • Laws of reflection are also obeyed by spherical mirrors (convex mirror and concave mirror). Light always reflects according to the laws of reflection, regardless of whether the reflecting surface is flat or curved. 


Properties of Image formed by a plane mirror 

  • A plane mirror forms a virtual and erect image. These images cannot be obtained on a screen. 
  • The size of the image formed is equal to the size of the object. 
  • The image of the object is formed behind the mirror. 
  • Images are laterally inverted. Due to the reflection of light, there is a change of sides of an object in its mirror image. If you stand in front of a mirror and lift your right hand, then your image in the mirror will appear to lift its left hand. 


Learning Outcomes 

  • Students can explain the phenomenon of reflection. 
  • Students can plan and conduct experiments to verify the laws of reflection. 
  • Students can explain the nature of the image formed by a plane mirror. 
  • Students can differentiate between regular and diffused reflection. 
  • Students can draw labelled diagram to show laws of reflection.