Class 10 Science

# Reflection of Light

Reflection: The bouncing back of rays of light from a polished and shiny surface is called reflection or reflection of light. It is similar to bouncing back of a football after colliding with a wall or any hard surface.

## Laws of Reflection of light: Fig: Reflection of Light

• The angle of incidence and angle of reflection is equal.
• The incident ray, reflected ray and normal to the point of reflection lie in the same plane.

The angle of incidence is denoted by i and angle of reflection is denoted by r. The law of reflection is applicable to all types of reflecting surface.

## Mirror and Reflection of Light:

Mirror is a shiny polished object (glass) which reflects most of the rays of light falling upon it. One side of mirror is polished with suitable material to make the other side reflective.

#### Types of Image

Real Image: Image which is formed in front of the mirror and it can be obtained on a screen is called real image.

Virtual Image: Image which is formed behind the mirror and cannot be obtained on a screen is called virtual image.

## Types of Mirror:

Plain Mirror: A mirror having a flat surface is called plane mirror.

### Formation of image in plane mirror: Fig: Image formation in plain mirror

• A plane mirror always forms virtual and erect image.
• The distance of image and that of object is equal from the mirror.
• The image formed by a plane mirror is laterally inverted.

## Spherical Mirror

Mirrors having curved reflecting surface are called spherical mirrors. A spherical mirror is a part of a sphere.

#### Types of Spherical Mirror:

Concave Mirror: Spherical mirror with reflecting surface curved inwards is called concave mirror.

Convex Mirror: Spherical mirror with reflecting surface curved outwards is called convex mirror.

#### Important terms of spherical mirror: Fig: Concave Mirror

Pole: The centre of reflecting surface of a spherical mirror is known as Pole. Pole lies on the surface of spherical mirror. Pole is generally represented by ‘P’.

Centre of Curvature: The centre of sphere, of which the reflecting surface of a spherical mirror is a part, is called the centre of curvature of the spherical mirror. Centre of curvature is not a part of spherical mirror rather it lies outside the mirror. Centre of curvature is denoted by letter C.

In the case of concave mirror centre of curvature lies in front of the reflecting surface. On the other hand, centre of curvature lies behind the reflecting surface in the case of convex mirror. Fig: Convex Mirror

Radius of Curvature: The radius of sphere, of which the reflecting surface of a spherical mirror is a part, is called the Radius of Curvature of the spherical mirror. The radius of curvature of a spherical mirror is denoted by letter R.

Similar to centre of curvature, radius of curvature lies in front of concave mirror and lies behind the convex mirror and is not a part of the mirror as it lies outside the mirror.

Aperture: The diameter of reflecting surface of a spherical mirror is called aperture.

Principal Axis: Imaginary line passing through the centre of curvature and pole of a spherical mirror is called the Principal Axis.

Focus or Principal Focus: Point on principal axis at which parallel rays (coming from infinity) converge after reflection is called the Focus or Principal Focus of the spherical mirror. Focus is represented by letter F. Fig: Converging Mirror

In the case of a concave mirror, parallel rays (coming from infinity) converge after reflection in front of the mirror. Thus, the focus lies in front of a concave mirror. Fig: Diverging Mirror

In the case of a convex mirror, parallel rays (coming from infinity) appear to be diverging from behind the mirror. Thus, the focus lies behind the convex mirror.

Focal length: The distance from pole to focus is called focal length. Focal length is denoted by letter f. Focal length is equal to half of the radius of curvature.

Or, f=R/2

Or, R=2f

## Summary

Plain Mirror Reflecting surface is flat. Image is virtual, of same size as mirror, erect and laterally inverted. Reflecting surface is concave. Most of the images are real and inverted. Virtual image is formed when object is between F and P. Reflecting surface is convex. Image is virtual, erect and smaller than object.