Class -8 , Lesson Light

1. Light Class-8
Light_Class-8 _Dipak Subedi 1
Unit- 7
Dipak Subedi
science teacher at
Shree Bishow Shanti Secondary School ,
Kaski

2. Light_Class-8 _Dipak Subedi 2
Contents :-
 Light
 Mirror
 Types of mirror
 Ray diagram from concave mirror
 Ray Diagram from convex mirror
 Refraction of light

3. Light_Class-8 _Dipak Subedi 3
Light :- Light is a form of energy which enables us to see objects.
Light travels in a straight path through a medium
Ray of light :- The path of light is represented by a straight line with an
arrowhead which is called ray of light.
Beam :- The collection of parallel rays of light is called a beam of light.

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Mirror :-Mirror is an instrument which reflects light falling on it
and forms image of the object.

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Plane mirror Spherical mirror
Concave Mirror Convex Mirror

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Plane Mirror :- the mirror which has flat and smooth
reflecting surface is known as plane mirror.

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Characteristics of image formed by plane mirror
 The image formed by a plane mirror is erect, virtual
(that cannot be obtained on a screen), of the same
size as the object and formed at a distance equal
to the object distance.
 The image thus formed on a plane mirror is
laterally inverted (or left appearing right and right

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Concave mirror
 A spherical mirror which is polished from outer
curved side ,such that reflecting surface is towards
the hollow side is called concave mirror.
 Concave mirror is also known converging mirror.

10. Light_Class-8 _Dipak Subedi 10
Convex mirror
 A spherical mirror which is polished from the hollow
side ,such that reflecting surface is towards outer
side is called convex mirror.
 Convex mirror is also known diverging mirror.

11. Light_Class-8 _Dipak Subedi 11
Discussion Questions :-
What are the types of mirror ?
Ant three differences between concave mirror and
convex mirror.
Which mirror is also known as diverging mirror and
why ?
Say any two characteristics of image formed by plane
mirror.

12. Light_Class-8 _Dipak Subedi 12
Important terms related to a spherical mirror
a) Pole of mirror
b) Center of curvature
c) Radius of curvature
d) Principal axis
e) Principal focus
f) Focal length

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Focal length :- it is the distance
between the pole and the principal
focus.

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Formation of Virtual Image
Real and virtual image

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Discussion questions
What is real image ?
Say any two characteristics of virtual image.
Name the mirror which always forms virtual
image.
Define principal focus and focal length.

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Rules used to draw ray diagrams in a concave mirror

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Rule- 3
 The incident ray that passes through the center of curvature of
the concave mirror is reflected back along the own path.
 Rays incident through the centre of curvature of convex
retraces the path .

21. Light_Class-8 _Dipak Subedi 21
When an object is placed
at infinity, its image is
formed at focus. The
image is real, inverted
and highly diminished in
size.
Tutorial Video To draw ray diagram from concave mirror can be seen from this link https://youtu.be/7iK1nIJs49Y

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When an object is
placed at a point
beyond C, its image is
formed between C and
F. The formed image is
inverted, real and
diminished.

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When an object is
placed at C, its image is
formed at C itself. The
formed image is real
,inverted and magnified.

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When object is placed
between the focus and
the center of curvature ,
it forms a real, inverted
and magnified image
behind the centre of
curvature.

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When object is placed
between the pole of the
mirror and principal focus ,it
forms a
virtual ,erect and magnified
image behind the mirror.

26. Light_Class-8 _Dipak Subedi 26
Diagram formed by convex mirror
 The image of the object at
infinity forms on the
principal focus of the
mirror which lies behind
the mirror .
 The image is virtual ,erect
and highly diminished.

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When object is placed anywhere
between the pole of the mirror
and infinity ,it forms a virtual
,erect and diminished image
behind the mirror between the
pole P and focus F .

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Refraction of Light

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 The medium through which the light travels is called an optical
medium.
 Denser medium and Rarer medium :- The medium whose
density is relatively high is called denser medium and that having
relatively low density is called rarer medium.
 Air, water, kerosene, glass, plastic, etc. are called optical medium
because light can travel through them.
 Light can travel even without medium.
 The process of bending of light when it travels from one

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Incident ray :- The ray of light travelling from first medium to
another medium is called incident ray.
Refracted ray :- The ray travelling through the second medium
after passing it from first medium is called refracted ray.
Emergent ray :-the ray of light coming out of the second medium
after refraction is called emergent ray.

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Normal :- The perpendicular line drawn at the point of incidence
is called normal.
Angle of incidence:- The angle made by the incident ray with
the normal is called angle of incidence.
Angle of refraction :-The angle between refracted ray and
normal is called the angle of refraction.

36. Light_Class-8 _Dipak Subedi 36
Give reason.
a) Plane mirror is used for looking our face.
b) Convex mirror is used as looking glass in vehicles.
c) Half dipped pencil in water is seen as bent.
d) The apparent depth of a pond is less than its real depth.
e) Convex mirror is also called diverging mirror.

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Thank you