Reflection of Light

Introduction

Light is the form of energy that provides sensation of vision.

Some common phenomena associated with lights are image formation by

mirrors, the twinkling of stars, the beautiful colours of a rainbow, bending of
light by a medium and so on.

Light is a electromagnetic wave.

Light always propagate in a straight line.

Reflection

Bouncing back of light when it strikes on a polished surface like mirror.

Types of Reflection

There are two types of reflection

(i) Regular Reflection or specular reflection -smooth surface (a

highly polished surface) reflects a parallel beam of light incident upon
it in one direction. This is called regular reflection.

A fine example of a regular reflection is Reflection from plane mirror

(ii) Diffused Reflection or irregular reflection : Diffuse reflection is the

reflection of light from a surface such that an incident ray is reflected
at many angles rather than at just one angle as in the case of
specular reflection.

A example of a irregular reflection is Reflection from cinema screen

Laws of Reflection

(i) Angle of incidence is equal to the angle of reflection.

(ii) The incident ray, the reflected ray and the normal at the point of incidence,
all lie in the same plane.

Incident light: Light which falls on the surface is called incident light.

Reflected light: Light which goes back after reflection is called reflected light.

The angle of incidence: The angle between the incident ray and the normal.

An angle of reflection: The angle between the reflected ray and the normal.

Virtual and Real image

Image is a point where at least two light rays actually meet or appear to meet.

Real Image Virtual Image

Formed when light rays appear
Formed when light rays actually meet.
to meet.
Can be obtained on screen. Can’t be obtained on screen.
Inverted Erect
Example: image formed on cinema screen and Example: image formed by plane
formed by concave mirror. mirror or convex mirror.

Image Formed by Plane Mirror

Characteristics of Image formed by Plane Mirror

(i) Virtual and erect.

(ii) Size of image is equal to the size of object.

(iii) Image is formed as far behind the mirror as the object is in front of it.

(iv) Laterally inverted.

Lateral Inversion: The right side of the object appears left side of the image
and vice-versa.

Application of lateral inversion

The word AMBULANCE is written in reverse direction so that it can be read

correctly in rear view mirror of vehicles going in front of it.

Spherical Mirrors

Mirrors whose reflecting surface is curved.

There are two types of spherical mirrors:

Ki(i) Convex Mirror

(ii) Concave Mirror

Properties of Concave mirror

• Reflecting surface is curved inwards.

• Converging mirror
Properties of Convex mirror

• Reflecting surface is curved outwards.

• Diverging mirror

Common terms for Spherical mirrors

Principal axis: The line joining the pole and centre of curvature.

Pole (P): The centre of the spherical mirror.

Aperture (MN): It is the effective diameter of the spherical mirror.

Centre of Curvature (C): The centre of the hollow glass sphere of which the
mirror was a part.

Radius of Curvature (R): The distance between the pole and the centre of
curvature.

Focus (F): The point on principal axis where all the parallel light rays actually
meet or appear to meet after reflection.

Focal length (f): The distance between the pole and the focus.

Relationship between focal length and radius of curvature: f = R/2

Rules for making ray diagrams by spherical mirror
(i) 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.

(ii) 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.

(iii) 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.

(iv) A ray incident obliquely to the principal axis, towards a point P (pole of the
mirror), on the concave mirror or a convex mirror, is reflected obliquely. The
incident and reflected rays follow the laws of reflection at the point of
incidence (point P), making equal angles with the principal axis.

Ray diagrams for images formed by concave mirror

(i) When object is at infinity

Image Position − At ‘F’

Nature of image – Real, inverted

Size – Point sized or highly diminished

(ii) When object is beyond ‘C’

Image Position – Between ‘F’ and ‘C’

Nature of image – Real, inverted

Size – Diminished

(iii) When object is at ‘C’

Image Position – At ‘C’

Nature of image – Real, inverted

Size – Same size as that of object

(iv) When object is placed between ‘F’ and ‘C’

Image Position – Beyond ‘C’

Nature of image– Real, inverted

Size – Enlarged

(v) When object is placed at ‘F’

Image Position – At Infinity

Nature of image – Real, inverted

Size – Highly enlarged

(vi) When object is between ‘P’ and ‘F’

Image Position – Behind the mirror

Nature of image – Virtual, erect

Size – Enlarged
Uses of Concave Mirror

(i) Used in torches, search lights and vehicles headlights to get powerful
parallel beam of light.

(ii) Concave mirrors are used by dentists to see large image of teeth of
patients. (Teeth have to be placed between pole and focus).

(iii) Concave mirror is used as shaving mirror to see a larger image of the
face.

(iv) Large concave mirrors are used to concentrate sunlight to produce heat in
solar furnace.

Ray diagrams of images formed by convex mirror

(i) When object is placed at infinity

Image Position − At ‘F’

Nature of image – Virtual, erect

Size – Point sized

(ii) When object is placed between pole and infinity

Image Position – Between ‘P’ and ‘F’

Nature of image– Virtual, erect

Size – Diminished
A full length image of a tall building/tree can be seen in a small convex mirror.

Uses of Convex Mirror

(i) Convex mirrors are used as rear view mirrors in vehicles because

→ they always give an erect though diminished image.

→ they have a wider field of view as they are curved outwards.

(ii) Convex mirrors are used at blind turns and on points of merging
traffic to facilitate vision of both side traffic.

(iii) Used in shops as security mirror.

Linear Magnification

Ratio between image size and object size is called linear magnification
𝑖𝑚𝑎𝑔𝑒 𝑠𝑖𝑧𝑒 𝑖𝑚𝑎𝑔𝑒 𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒
𝐿𝑖𝑛𝑒𝑎𝑟 𝑚𝑎𝑔𝑛𝑖𝑓𝑖𝑎𝑡𝑖𝑜𝑛 = =
𝑜𝑏𝑗𝑒𝑐𝑡 𝑠𝑖𝑧𝑒 𝑜𝑏𝑗𝑒𝑐𝑡 𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒

Sujash sir

sujashkumarsaha@gmail.com

9432683001