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Title: Class 9 physics notes chapter light
Description: This is the important topic notes of physics class 9 of chapter light

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Reflection of Light
Reflection of Light




The return of light into the same medium after striking a surface is called reflection
...

A plane mirror reflects almost all the light falling on it
...




The incident rays and reflected rays are parallel beams
...




The incident rays are parallel beams, but the light reflected from different surfaces will reflect at
different angles obeying the laws of reflection
...

Point of incidence: The point at which the incident ray strikes the reflecting surface is called the point
of incidence
...

Normal: The perpendicular drawn to the surface at the point of incidence is the normal
...

Plane of incidence: The plane containing the incident ray and the normal is called the plane of
incidence
...

Plane of reflection: The plane containing the reflected ray and the normal is called the plane of
incidence
...

 Second law of reflection: The angle of incidence is equal to the angle of reflection
...

1
...
It is the image formed when light
rays actually intersect after reflection
...

2
...
It is the image formed when light rays do not
actually intersect after reflection but appear to diverge from the image
...
It is an erect image
...
They get reflected from the
mirror by obeying the laws of reflection
...




The image I is formed behind the mirror and is located by producing AC and BD backwards
...


Position of Image: The image I is as far behind the mirror as the object O is in front of it
...
Light rays travel from all points of the object in all
directions
...

In a similar way, all the points on object AB form virtual images on the mirror between A and B, giving
rise to the virtual image A′B′ of AB
...
The perpendicular distance of each point
behind the mirror is equal to the corresponding point of the object in front of the mirror
...

Any symmetrical letter will appear as is after lateral inversion
...

The word ‘
’ is written as ‘
’ on ambulances
...


Characteristics of the Image Formed by the Plane Mirror


The characteristics of the image formed by a plane mirror are
 The image formed by a plane mirror is always virtual and erect
...

 The image formed by the plane mirror is as far behind the mirror as the object is in front of it
...

360
If n 
is odd, then the number of images formed is n − 1 when the object is placed symmetrically

between the two mirrors
...


If n 

Images Formed by Two Mirrors Placed Parallel to Each Other


The number of images formed in this case will be infinite as n 

360
 
...
Thus, the number of images will be n − 1 = 3
...

In a kaleidoscope, three plane mirrors are inclined at an angle of 60° to each other
...

In a barber’s shop, two mirrors face each other so that a person can see the hair at the back of the
head
...


Spherical Mirrors









A reflecting surface which is a part of a sphere is called a spherical mirror
...

A spherical mirror whose reflecting surface is curved inwards and polished on the outer spherical
surface is a concave mirror
...

The reflecting surface of a spherical mirror forms a part of a sphere
...
It is represented by C
...

The radius of curvature of a spherical mirror is the radius of the sphere of which the reflecting
surface of the spherical mirror is a part and is represented by R
...
Thus, it is
the surface from which reflection occurs
...
It is represented by F
...
It is also represented by F
...

It is denoted as f
...




Construction of Image: Ray Diagram



There are certain rules to draw a ray diagram to locate the image formed by a spherical mirror
...




Second rule: A ray of light parallel to the principal axis, after reflection, passes through the principal
focus in case of a concave mirror or appears to diverge from it in case of a convex mirror
...




Fourth rule: A ray of light incident obliquely towards the pole of a mirror is reflected obliquely as per
the laws of reflection
...


Ray diagrams for formation of images: Concave mirror
CASE (i):
Position of object

Position of image

At infinity

At focus F

Size of image
Nature of image
Diminished to pointReal
sized

Position of object Position of image
At far distance
At focus F

Size of image
Highly diminished

Nature of image
Real and inverted

Position of object Position of image
Beyond C
Between F and C

Size of image
Diminished

Nature of image
Real and inverted

CASE (ii):

CASE (iii):
Position of object Position of image
At C
At C

Size of image
Same size

Nature of image
Real and inverted

Position of object Position of image
Between C and
Beyond C
F

Size of image

Nature of image

CASE (iv):
Magnified

Real and inverted

Size of image
Highly magnified

Nature of image
Real and inverted

CASE (v):
Position of object Position of image
At F
At infinity

CASE (vi):
Position of object Position of image
Between P and
Behind mirror
F

Size of image
Magnified

Nature of image
Virtual and erect

Ray Diagrams for Formation of Images: Convex Mirror
CASE (i):
Position of object
At infinity

Position of image
At focus F behind
the mirror

Size of image
Highly diminished,
point-sized

Nature of image
Virtual and erect

Case (ii)
Position of object
Between infinity and
the pole of the mirror

Position of image
Between P and F
behind the mirror

Size of image
Diminished

Nature of image
Virtual and
erect

Distinction of Mirrors without Touching


The mirror is held near the face and the image is seen
...

Case (ii): If the image is upright, magnified and increases in size by moving the mirror away from the
face, then the mirror is a concave mirror
...


Sign Convention
 The sign convention is as follows:
i
...
The rays are made incident
from the left
...
The distances measured along the principal axis in the direction of incident light, are taken as
positive, while those opposite to the incident light, are taken as negative
...
Distances measured above the principal axis are taken as positive and those measured below the
principal axis are taken as negative
...
It is represented as m
Title: Class 9 physics notes chapter light
Description: This is the important topic notes of physics class 9 of chapter light