CBSE Class 10 Science Chapter 10 Light Reflection Refraction

Class Notes of Ch 10 : Light Reflection Refraction

Class 10th Science

Light Reflection Refraction

      Topics:  

• Introduction
• Spherical mirrors
• Terms used in mirror
• Image formation by mirrors
• Uses of mirrors
• Sign convention
• Mirror Formula & Magnification
• Refraction of Light
• Refractive Index
• Refraction by spherical lenses
• Terms related to lenses.
• Image formation by lenses
• Sign convention for spherical lenses
• Lens formula & Magnification
• Power of a lens

Introduction

• An object reflects light that falls on it. This reflected light, when received by our eyes, enables us to see things.
• We are able to see through a transparent medium as light is transmitted through it.
• The laws of reflection are:
1. The angle of incidence is equal to the angle of reflection
2. The incident ray, the normal to the mirror at the point of incidence and the reflected ray, all lie in the same plane.
• These laws are applicable to every surface.
• Properties of image formed by a plane mirror:
• Image formed by a plane mirror is always virtual and erect.
• The size of the image is equal to that of the object.
• The image formed is as far behind the mirror as the object is in front of it.
• The image is laterally inverted.

Spherical Mirrors

• Mirrors, whose reflecting surfaces are spherical, are called spherical mirrors.
• The reflecting surface of such mirrors can be considered to form a part of the surface of a sphere.
• A spherical mirror, whose reflecting surface is curved inwards, i.e. faces towards the centre of the sphere, is called a concave mirror.
• A spherical mirror, whose reflecting surface is curved outwards, is called a convex mirror.

Terms used in 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.

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.

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.

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.

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.

 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.

Image Formation by Mirrors

There are 4 rules which are to be followed for image formation:

1. A ray parallel to the principal axis, after reflection, will pass through the principal focus in case of a concave mirror or appear to diverge from the principal focus in case of a convex mirror. 

A ray passing through the principal focus of a concave mirror or a ray which is directed towards the principal focus of a convex mirror, after reflection, will emerge parallel to the principal axis. 

A ray passing through the centre of curvature of a concave mirror or directed in the direction of the centre of curvature of a convex mirror, after reflection, is reflected back along the same path.

A ray incident obliquely to the principal axis, towards pole of the mirror, on the concave mirror or a convex mirror is reflected obliquely.

Image formation by Concave Mirror

Position of Object	Position of Image	Size of Image	Nature of Image
at infinity	at F	highly diminished ,point sized	real & inverted
beyond C	between F and C	diminished	real & inverted
at C	at C	same size	real & inverted
between C and F	beyond C	enlarged	real & inverted
at F	at infinity	highly enlarged	real & inverted
between F and P	behind the mirror	enlarged	virtual & erect

Image formation by Convex Mirror

Position of Object	Position of Image	Size of Image	Nature of Image
at infinity	at F , behind the mirror	highly diminished, point sized	virtual & erect
between infinity and P	between P and F, behind the mirror	diminished	virtual & erect

Uses of mirrors

Concave

• Concave mirrors are commonly used in torches, search-lights and vehicles headlights to get powerful parallel beams of light.
• They are often used as shaving mirrors to see a larger image of the face. The dentists use concave mirrors to see large images of the teeth of patients.
• Large concave mirrors are used to concentrate sunlight to produce heat in solar furnaces.

Convex

• Convex mirrors are commonly used as rear-view (wing) mirrors in vehicles.
• Convex mirrors are preferred because they always give an erect, though diminished, image. Also, they have a wider field of view as they are curved. Thus, convex mirrors enable the driver to view much larger area than would be possible with a plane mirror.

Sign Convention

In this convention, the pole (P) of the mirror is taken as the origin. The principal axis of the mirror is taken as the x-axis (X’X) of the coordinate system.

(i) The object is always placed to the left of the mirror. This implies that the light from the object falls on the mirror from the left-hand side.

(ii) All distances parallel to the principal axis are measured from the pole of the mirror.

(iii) All the distances measured to the right of the origin (along + x-axis) are taken as positive while those measured to the left of the origin (along – x-axis) are taken as negative.

(iv) Distances measured perpendicular to and above the principal axis (along + y-axis) are taken as positive.

(v) Distances measured perpendicular to and below the principal axis (along –y-axis) are taken as negative.

Mirror Formula & Magnification

• Mirror Formula :

Where:-

• The distance of the object from its pole is called the object distance (u).
• The distance of the image from the pole of the mirror is called the image distance (v).
• The distance of the principal focus from the pole is called the focal length (f).
•  Magnification
• Magnification produced by a spherical mirror gives the relative extent to which the image of an object is magnified with respect to the object size.
• A negative sign in the value of the magnification indicates that the image is real.
• A positive sign in the value of the magnification indicates that the image is virtual.

Refraction of Light

• Light does not travel in the same direction in all media.
• It appears that when travelling obliquely from one medium to another, the direction of propagation of light in the second medium changes. This phenomenon is known as refraction of light.
• Refraction is due to change in the speed of light as it enters from one transparent medium to another.

 The laws of Refraction are :

1. The incident ray, the refracted ray and the normal to the interface of two transparent media at the point of incidence, all lie in the same plane.
2. The ratio of sine of angle of incidence to the sine of angle of refraction is a constant, for the light of a given color and for the given pair of media. This law is also known as Snell’s law of refraction.

Where :- 

• n1 = refractive index of first medium
• n2 = refractive index of second medium
• i = angle of incidence
• r = angle of refraction

Refractive Index

• The extent of the change in direction that takes place in a given pair of media is expressed in terms of the refractive index.
• It turns out that light propagates with different speeds in different media.
• The value of the refractive index for a given pair of media depends upon the speed of light in the two media.

Where n21 is the refractive index of medium 2 with respect to medium 1.

Similarly refractive index of medium 1 with respect to medium 1 is given by n12 .

• Absolute refractive index : Refractive index of a medium with respect to vacuum.

Note: optically denser medium does not mean that the medium possess greater mass density.

• When a ray of light travelling from a rarer medium to a denser medium slows down and bends towards the normal.
• When it travels from a denser medium to a rarer medium, it speeds up and bends away from the normal.

Refraction by Spherical Lenses

• A transparent material bound by two surfaces, of which one or both surfaces are spherical, forms a lens.

Terms Related to Lenses.

Centre of curvature :

• A lens has two spherical surfaces. Each of these surfaces forms a part of a sphere. The centres of these spheres are called centres of curvature of the lens.
• The centre of curvature of a lens is usually represented by the letter C. Since there are two centres of curvature, we may represent them as C1 and C2.

Principal axis:

• An imaginary straight line passing through the two centres of curvature of a lens is called its principal axis.

Optical centre :

• The central point of a lens is its optical centre.
• It is usually represented by the letter O.

Aperture:

• The effective diameter of the circular outline of a spherical lens is called its aperture.

Focus:

• When rays parallel to principal axis gets refracted by convex lens , they converge to a point on principal axis, this point is called Principal focus.
• If you pass parallel rays from opposite side of lens you get another principal focus on the opposite side.
• A lens has two principal focus, usually named F1 and F2.
•  When rays parallel to principal axis gets refracted by concave lens , they appear to diverge from a point on the principal axis , this point is called Principal focus.
• Concave lens also have two focii, usually named F1 and F2.

Image Formation by Lenses

There are three rules which are to be followed:

• A ray of light from the object, parallel to the principal axis, after refraction from a convex lens, passes through the principal focus on the other side of the lens, as shown in. In case of a concave lens, the ray appears to diverge from the principal focus located on the same side of the lens. 

• A ray of light passing through a principal focus, after refraction from a convex lens, will emerge parallel to the principal axis. A ray of light appearing to meet at the principal focus of a concave lens, after refraction, will emerge parallel to the principal axis.

• A ray of light passing through the optical centre of a lens will emerge without any deviation.

Image Formation by Convex Lens

position of object	position of image	size of image	nature of image
at infinity	at F2	point-sized	real & inverted
beyond 2F1	between F2 & 2F2	diminished	real & inverted
at 2F1	at 2F2	same size	real & inverted
between F1 & 2F1	beyond 2F2	enlarged	real & inverted
at F1	at infinity	highly enlarged	real & inverted
between F1 & O	on the same of object	enlarged	virtual & erect

Image Formation by Concave Lens

position of object	position of image	size of image	nature of image
at infinity	at F1	point sized	virtual & erect
between infinity and O	between F1 and O	diminished	virtual & erect

Sign Convention for Spherical Lenses

We follow sign conventions, similar to the one used for spherical mirrors. We apply the rules for signs of distances, except that all measurements are taken from the optical centre of the lens.

Lens formula & Magnification

Lens formula is given by: 

where :

• u – image distance
• v – object distance
• f – focal length

• Magnification: The ratio of the height of the image and the height of the object. It is represented by the letter m.

                  

• The positive sign shows that the image is erect and virtual.
• The negative sign shows that the image is inverted and real.

Power of a Lens

• The power of a lens is defined as the reciprocal of its focal length.
• It is represented by the letter P.

• The SI unit of power of a lens is ‘dioptre’. It is denoted by the letter D.
• Power of a convex lens is positive and that of a concave lens is negative.

 

SCIENCE Revision Notes

Chapter:01  Chemical Reaction & Equation
Chapter:02  Acid Base & Salt
Chapter:03  Metals & Non Metals
Chapter:04  Carbon & its Components
Chapter:05  Periodic Classification of Elements
Chapter:06  Life Processes
Chapter:07  Control & Coordinates
Chapter:08  How do Organisms Reproduce
Chapter:09  Heridity & Evolution
Chapter:10  Light Reflection Refraction
Chapter:11  The Human Eye & the Colourful World
Chapter:12  Electricity
Chapter:13  Magnetic Effect of Electric Current
Chapter:14  Source Of Energy
Chapter:15  Our Environment
Chapter:16  Management of Natural Resource

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