ELCAAS Learning Solution

Class XII – Physics

Topic – Reflection of Light

1. Introduction

Light is a form of energy that travels in straight lines and enables us to see objects. When light strikes a surface, it may be absorbed, transmitted, or sent back. The phenomenon of sending back light rays into the same medium after striking a surface is known as Reflection of Light . Reflection plays a crucial role in vision, optical instruments, and daily life applications like mirrors, periscopes, and headlights.

2. Laws of Reflection

The reflection of light follows two fundamental laws:

1. Law of Incidence: The angle of incidence ( i ) is equal to the angle of reflection ( r ).
‎ i=ri = ri=r

2. Law of the Plane: The incident ray, reflected ray, and the normal to the reflecting surface all lie in the same plane.

These laws are applicable to all types of reflecting surfaces, whether smooth or rough.

3. Types of Reflection

Reflection can be classified into two main types:

a) Regular Reflection

Occurs when light falls on a smooth, polished surface like a plane mirror. The reflected rays are parallel, producing a clear image.
‎ Example: Reflection from a bathroom mirror.

b) Diffuse Reflection

Occurs when light falls on a rough or irregular surface. The reflected rays scatter in different directions, and no clear image is formed.
‎ Example: Reflection from a wall or paper.

4. Image Formation by Plane Mirror

 Nature of Image: Virtual, erect, and of the same size as the object.

 Position: Behind the mirror at the same distance as the object in front.

 Lateral Inversion: The left and right sides of the image are interchanged.

Uses: Dressing mirrors, optical instruments, periscopes, solar cookers.

5. Reflection by Spherical Mirrors

Spherical mirrors are parts of a sphere’s surface and are of two types:

 Concave Mirror: Inner surface is reflective. Can form real or virtual images depending on object position.

 Convex Mirror: Outer surface is reflective. Always forms virtual, erect, and diminished images.

Applications:

 Concave mirrors in shaving mirrors, headlights, and solar concentrators.

 Convex mirrors in vehicle side mirrors for a wider field of view.

6. Mirror Formula and Sign Conventions

For spherical mirrors:

1f=1u+1v\frac{1}{f} = \frac{1}{u} + \frac{1}{v}f1​=u1​+v1​

Where:

 fff = focal length

 uuu = object distance

 vvv = image distance

Sign conventions: All distances are measured from the mirror’s pole, with the direction of incident light taken as positive.

7. Multiple Reflection

When light is reflected multiple times between two mirrors, it can produce multiple images. This principle is used in kaleidoscopes, optical instruments, and infinity mirrors.

8. Conclusion

Reflection of light is a fundamental concept in physics with vast applications in daily life and technology. Understanding the laws and types of reflection allows us to design mirrors, optical devices, and instruments that rely on controlled light paths. Mastery of these principles forms the basis for more advanced topics like refraction, total internal reflection, and optical fibre communication.