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

## 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
(c) Shaving mirror.
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).
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 1O, the reflected ray will turn by an angle of 2O.
• 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) Real image Virtual Image 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

(Continued to LESSON 2 of 3)
Further Study on NCERT Class 10, Physics Chapter 10, Light - Reflection and Refraction
• Cbse Class 10, Science (Physics) | Light - Reflection And Refraction | Ncert Solutions - Science In-Text Questions (to be published)
• Class 10, NCERT (CBSE) Science (Physics) | Light - Reflection and Refraction | CCE type Multiple Choice Questions (MCQ) [to be published]

1. 2. 