PHYSICS NOTES
CLASS : X
LIGHT-REFLECTION
Nature of light : There are two theories about the nature of light : wave theory and particle theory of
light.
According to wave theory , light consists of electromagnetic waves which do not require a material
medium for their propagation. According to particle theory light is composed of particles which travel in
a straight line at very high speed. The modern theory of light is called “ Quantum Theory of light “ –
Light has dual nature , both waves and particles ( depending on the situation )
Reflection-: The returning back of light to the same medium after striking any surface is called reflection.
LAWS OF REFLECTION:

The incident ray, the reflected ray and the normal at the point of incidence, all lie on the same plane.

The angle of incidence is equal to the angle of reflection.<i=<r
Characteristics of the images formed by a plane mirror-:

Virtual and erect.

Same size as the object.

Laterally inverted.

Distance between object and mirror=distance between image and mirror.
A plane mirror is used :





As a looking glass to see our image.
By interior designers to create an illusion of depth.
To fold light in a periscope and other instruments
To build a kaleidoscope and other optical instruments
To build a kaleidoscope.
If the mirrors are inclined at angle Ø then the number of images given by the relation :
n = 360 --- 1
Ø
Spherical mirror-important terms-:
Center of curvature (C)-: It is the centre of the sphere in which the spherical mirror for a part.
Radius of curvature(R)-: It is the radius of the sphere in which the spherical mirror forms a part.
Pole(P)-: It is the geometric centre of the spherical mirror.
Principal axis-: It is the straight line passing through the centre of curvature(C) and pole(P) of the
spherical mirror.
Principal focus of a concave mirror(F)-: Rays of light parallel and close to principal axis after
reflection from a concave mirror converges to a point on the principal axis. This point is known as
the principal focus of a concave mirror.
Ray diagram Page No.162 Fig 10.2(a)
Principal focus of a convex mirror(F)-: Rays of light parallel and close to the principal axis after
reflection from a convex mirror appears to diverge from a point on the principal axis. This point is
known as the principal focus of a convex mirror.
Ray diagram Page No.162 Fig 10.2(b)
Focal length(f)-: The distance between pole and principal focus of a spherical mirror.
Aperture-: The diameter of the reflecting surface of spherical mirror is called its aperture.
Principal focus and focal length of concave mirror and concave mirror
PRINCIPAL FOCUS OF CONCAVE & CONVEX MIRROR
Concave mirror-: Rules for image formation

A ray parallel to the principal axis after reflection will pass through the principal focus a concave
mirror.
page no.164 fig 10.3(a)
 A ray passing through the principal focus of a concave mirror after reflection will emerge
parallel to the principal axis.
page no.165 fig 10.4(a)
 A ray passing through the centre of curvature of a concave mirror after reflection is reflected
back along the same path.
page no.165 fig 10.5(a)
 A ray incident obliquely to the principal axis towards the pole of the mirror is reflected
obliquely.
page no.165 fig 10.6(a)
Distinguish between real and virtual images-:
REAL IMAGES
Images that can be obtained on a screen.
Always inverted.
Formed by actual intersection of reflected or
refracted rays.
VIRTUAL IMAGES
Images that cannot be obtained on a screen.
Always erect.
Not formed by actual intersection of reflected or
refracted rays but by geometrical construction
The following table gives the position , size and nature of the image formed in a concave mirror
corresponding to different positions of the object and use of the concave mirror.
POSITION OF THE
OBJECT
At infinity
Beyond the centre of
curvature.
POSITION OF THE
IMAGE
At the focus
Between the focus
and centre of
curvature.
At C
At C
NATURE AND SIZE OF USE
THE IMAGE
Real , inverted and
As collector of
diminished
radiation in solar
heating devices.
Real , inverted and
diminished
Real and inverted and
same size as the
object
Real , inverted and
magnified
As a reflecting mirror
behind a projector
lamp.
In flood lights
As a reflecting
mirror in car, head
lights, search lights
etc.
As a shaving mirror
or make up mirror
and dentist”s mirror
Between the focus
and centre of
curvature
At focus
Beyond the centre of
curvature
At infinity
Real, inverted and
magnified
Between the pole of
the mirror and the
focus
Appear behind the
mirror
Virtual, erect and
magnified
The following table gives the position, size and nature of the image formed in a convex mirror
corresponding to different position of the object .
Position of the object
At infinity
Between infinity and
pole of the mirror
Position of the image
At focus
Size of the image
Extremely diminished
Nature of the image
Virtual and erect
Between the focus
and pole
Diminished
Virtual and erect
USES OF CONVEX MIRRORS


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Rear view mirror in automobiles.
Vigilance mirrors in departmental stores.
Convex mirrors are used as shop security mirrors.
SIGN CONVENTION FOR MIRRORS
MIRROR FORMULA
The distance of the an object from the pole of a mirror is known as object distance (u).
The distance of image from the pole of a mirror is known as image distance (v). The distance of
focus from the pole of a mirror is known as focal length (f).
A formula which gives the relationship between image distance (v), object distance (v) and
focal length (f) of a spherical mirror is known as the mirror formula.
1
_______________
Image distance
+
1
=
___________________
object distance
1
________________
Focal length.
If the magnification has a plus sign then the image is virtual and erect , the magnification has a
minus sign, then the image is real and inverted.
Concave mirror can produce virtual images as well as real images the magnification produced
by a concave mirror can be positive or negative. A convex mirror forms virtual image , so the
magnification produced by a convex mirror is positive.
Magnification ‘m’ has value greater than 1 then the image is bigger than the object , that is
magnified. If “ m “ is exactly 1 then the image is same size as the object. And “m” is less than
one the image is smaller than the object.
Linear magnification produced by the concave mirror can be less than 1 , equal to 1, or more
than 1 . convex mirror forms images which are always smaller than the object so magnification
produced by a convex mirror always less than one . A plane mirror forms images which are
always of the same size as the object, therefore magnification produced by a plane mirror is
always 1.
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