NCERT (CBSE) Class 10 Science - Physics | Chapter-10 LIGHT - REFLECTION AND REFRACTION - Notes and Sample Questions | Lesson 1 of 3 - www.CbseNcertSolution.com

NCERT (CBSE) Science - Physics

LIGHT - REFLECTION AND REFRACTION

Class 10, Physics (Science) Notes and Study Materials

(Lesson 1 of 3)

Spherical Mirror

Mirrors having curved surfaces are known as Spherical Mirrors. There are two types of spherical mirrors – Concave Mirror and Convex Mirror

Concave Mirror

A concave mirror is a spherical mirror whose reflecting surface is curved inwards.

Convex Mirror

A convex mirror is a spherical mirror whose reflecting surface is curved outwards. In a convex mirror the reflection of light takes place from its outer surface.

Use of Concave Mirror

1. A concave mirror forms image according to the position of the object. If an object is placed very close to a concave mirror i.e. between the focus and the pole, then the image formed is virtual, erect and highly magnified. Because of this property concave mirrors are used as:

(a) As a dentist’s mirror (to see a larger image of teeth),

(b) For examining eyes, ears, nose and throat by Doctors

2. When a light emitting object is placed at the focus of a concave mirror, then all the reflected rays become parallel to the principal axis. This property of a concave mirror is used in;

(a) A torch

(b) Behind the headlights of vehicles and light posts etc.

3. Large concave mirrors are used to concentrate sunlight to produce heat in solar furnaces.

Use of Convex Mirror

A convex mirror forms virtual, erect and diminished image of objects which subsequently increases the field of view. Because f this property of convex mirrors they are used in –

(a) Rear-view mirrors of vehicles

(b) Safety mirrors in stores.

Pole of a Spherical Mirror

The geometrical centre of the central point of a mirror is called pole. It lies on the mirror and is denoted by the letter P (as shown in the adjacent figure).

Center of Curvature

It is the geometrical center of the sphere from which the given spherical mirror is obtained. It is denoted by the letter C.

Aperture

The width of the reflecting surface is called aperture (AB in the figure).

Radius of Curvature

The radius of the curvature is the radius of the sphere from which the spherical mirror is obtained. It is denoted by R which is equal to the distance between the center of curvature (C) and pole (P). .

Principal Axis

The imaginary line passing through the Pole and the Center of Curvature is called the Principal Axis (PC).

Focus

The focus (F) is the point on the principal axis of a spherical mirror where all the incident rays parallel to the principal axis meet or appear to be meeting after reflection. A concave mirror has got a real focus which lies on the same side of reflecting surface whereas a convex mirror has got a virtual focus which is obtained on the opposite side of the reflecting surface by extrapolating the rays reflected from the mirror surface. F is the distance between the focus and the pole of the mirror. Radius of curvature (R) and the focal length (F) of a spherical mirror are related as:

R = 2F

Focal Length

The distance between the focus (F) and the pole (P) is called the focal length. It is generally denoted by f.

(f = R/2).

Light Wave

Light is a form of energy which brings the sensation of sight. Light waves travel with a speed of 3 x 10⁸ ms^–1 in free space. Its speed depends on the medium. Light wave is a transverse wave and does not require any medium to propagate.

Ray and Beam

Light travels in a straight line. An arrow which represents the direction of propagation of light is called the ray of light.

A bundle of rays originating from the same source of light in a particular direction is called a beam of light.

Rectilinear Propagation of Light

The property of light of travelling in a straight line is called the Rectilinear Propagation of Light.

Reflection of Light

The scattering back of the light by any shining and smooth surface is known as reflection of light.

Real and Virtual Image

If light after reflection converges to a point to form an image of its own, it’s called a real image. If they are diverging (appear to be meeting at a point), then it forms a virtual image.

Real image can be obtained on a screen but it is not possible in case of virtual image.

Plane Mirror

Image formed by a plane mirror is - virtual, erect, size equal to that of the object, at the distance behind the mirror as the object is in front of the mirror, and laterally inverted.
When a plane mirror is turned by an angle 1^O, the reflected ray will turn by an angle of 2^O.
When the light falls normally on a plane mirror, it will retrace its path.
To see full size image of a person he needs a mirror of length equal to half of his height.
The radius of curvature of a plane mirror is infinity, so its R = f = α (infinity).
The magnification of the image formed by a plane mirror is +1.

Image Formation by a Concave Mirror

(Table followed by figures showing respective positions)

	Position of Object	Position of Image	Size of Image	Nature of Image
1	At infinity	At the focus F	Highly diminished, point sized	Real and inverted
2	Beyond C	Between C and F	Diminished	Real and inverted
3	At C	At C	Same size	Real and inverted
4	Between C and F	Beyond C	Enlarged	Real and inverted
5	At F	At infinity	Highly enlarged	Real and inverted
6	Between P and F	Behind the mirror	Enlarged	Virtual and erect

Image Formation by a Convex Mirror

(Table followed by figures showing respective positions)

	Position of Object	Position of Image	Size of Image	Nature of Image
1.	At infinity	At focus F, behind the mirror	Highly diminished, point sized	Virtual and erect
2.	Between infinity and Pole of the mirror	Between P and F, behind mirror	Diminished	Virtual and erect

Sign Convention (Spherical Mirrors)

The following table summarizes the new Cartesian Sign Convention for Spherical Mirrors:

Mirrors	Object distance (u)	Image distance (v)		Focal length (f)	Height of object (Ho)	Height of image (Hi)
Mirrors	Object distance (u)	Real image	Virtual Image	Focal length (f)	Height of object (Ho)	Real image	Virtual image
Convex	Negative	Image does not form	Positive	Positive	Positive	Image does not form	Positive
Concave	Negative	Negative	Positive	Negative	Positive	Negative	Positive