Class 12 Physics Ray Optics Optical Instruments Introduction Term 2

 Introduction

• We will try to learn what is light and also will try to understand some important phenomenon related to light and applications of optics in different optical instruments like microscope, telescope etc.
• How the phenomenon of laws of reflection and refraction play important part in our day to day to life.
• Light helps us to see all the objects around us. The entire phenomenon related to this we try a have a look on them.
• Also try to understand why certain objects enhance properties of light more as compared to others.

• What is Light?

• Light is a form of energy which enables us to see things around us.
• Light travels in straight path.
• Speed of light is c =3 x 108m/s.
• Light waves are the most common form of Electromagnetic waves.
• Ray of light is the path of the light wave travelling from one point to another.
• Beam of light is bunch of rays of light.

• Reflection of light by Spherical Mirrors

  • Reflection of light is the bouncing back off a ray of light when it strikes a boundary between different media through which it cannot pass.
  • Reflection helps us to see different objects in this world.

  

  • Laws of Reflection :-
    • The angle of reflection is equal to the angle of incidence (angle between incident ray and the normal). <i = <r
    • The incident ray, reflected ray and the normal to the reflecting surface at the point of incidence all lie in the same plane.
  • Note: - These laws are valid at each point on any reflecting surface whether plane or curved.

  

  The incident ray, reflected ray and the normal to the reflecting surface lie in the same plane.

  Spherical Mirrors

• A spherical mirror is a part of a reflective spherical surface and they are in sphere in shape.
• It is made up of a large number of extremely small plane mirrors.

Spherical Mirrors are of two types:-

1. Concave Mirror: -

• They are silvered on the inside of sphere.
• It is a converging mirror.
• In a Concave mirror when the rays of light reflect back they meet or converge at a point that is why known as converging mirror.

2. Convex Mirror: -

• They are silvered on the outside of sphere.
• In Convex mirror the rays don’t meet at appoint after reflection that is why it is a diverging mirror.

 

 

Some terminologies related to Spherical Mirrors:-

  Pole:

• The centre of the reflecting surface of a spherical mirror.
• It lies on the surface of the mirror.
• The pole is usually represented by the letter P.

2. Centre of curvature:

• The reflecting surface of a spherical mirror forms a part of a sphere. The sphere’s centre is called as centre of curvature.
• It is represented by the letter C.
• The centre of curvature is not a part of the mirror. It lies outside its reflecting surface.
• The centre of curvature of a concave mirror lies in front of it.
• However, it lies behind the mirror in case of a convex mirror.

3. Radius of curvature

• The radius of the sphere of which the reflecting surface of a spherical mirror forms a part
• It is represented by the letter R.

4. Principal axis

• A straight line passing through the pole and the centre of curvature of a spherical mirror.
• Principal axis is normal to the mirror at its pole.

5. Principal Focus

• Rays parallel to the principal axis falling on a concave mirror meet/intersect at the point on the principal axis. The point is called principal focus of concave mirror.
• The reflected rays appear to come from a point on the principal axis when rays parallel to the principal axis fall on a convex mirror, that point is called principal focus of convex mirror.
• The principal focus is represented by the letter F.
• The distance between the pole and the principal focus of a spherical mirror is called the focal length. It is represented by the letter f.

6. Aperture

• The diameter of the reflecting surface of spherical mirror is called its aperture.
• Mirrors whose aperture is much smaller than its radius of curvature, we use R=2f.

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