SPHERICAL MIRRORS 9.pptx BY KAVYA SINGH CLASS 9

BY KAVYA SINGH
CLASS 9TH D
ROLL NO. 23

 A lens is an optical device that is used to
bend light in a specific way.
 A converging lens bends light so that the
light rays come together to a point.
 A diverging lens bends light so it spreads
light apart instead of coming together.
 Mirrors reflect light and allow us to see
ourselves.
 A prism is another optical device that can
cause light to change directions.
 A prism is a solid piece of glass with flat
polished surfaces.

 Spherical Mirror: A curved mirror formed by a
part of a hollow glass sphere with a reflecting
surface.
TWO TYPES OF SPHERICAL MIRRORS:
 Concave mirror: A concave mirror is a curved
mirror with the reflecting surface on the
hollow side.
 Convex mirror: A convex mirror is a curved
mirror with the reflecting surface on the
outer side.

 Centre of Curvature : The centre of
curvature of a curved mirror is defined as
the center of the hollow glass sphere of
which the curved mirror was {previously} a
part.
 Radius of curvature: The radius of the
hollow glass sphere of which the spherical
mirror was a part.
 Principal Axis: The imaginary line passing
through its pole P and center of curvature C.
 Pole: The pole is defined as the geometric
center of the curved mirror.

 Focus: The principal focus is defined as the
point on the principal axis where the light
rays travelling parallel to the principal axis
after reflection actually meet.

 The focal length is the distance between the pole
P Principal focus F of a curved mirror.
 The focal length is half the radius of curvature .
FOCAL LENGTH = RADIUS OF CURVATURE

To show that f = R/2 for a Concave Mirror
o Let a ray of light AB be incident, parallel to
the principal axis, on a concave mirror. After
reflection, the ray AB passes along BD,
through the focus F. BD is normal to the
concave mirror at B.
Angle ABC = Angle CBD-------------------- (1)
o We know that AB and PC are parallel to each
other.
Angle ABC =Angle BCP--------------------(2)

From equation (1) and (2) we get
Hence triangle BCF is isosceles BF = CF----(3)
If the aperture of the mirror is small then B
will be very close to P.
BF=PF-----(4)
From equation 3 and 4
CF=PF
PC=PF + CF => PC= 2 PF
BY definition PF= f (focal length) and PC= R
(RADIUS OF CURVATURE)
Then, R =2f
i.e. Focal length = Radius of curvature/2.

I. The object is always placed on the left of the
mirror and light from the object falls from
the left to the right.
II. All distances parallel to the principal axis are
measured from the pole.
III. All distance measured to the right of the pole
are taken as + ve .
IV. All distance measured to the left of the pole
are taken as – ve .
V. The height measured upwards perpendicular
axis is taken as + ve.
VI. The height measured downwards
perpendicular to the principal axis is taken as
–ve.

Mirror formula is the relationship between
object distance [u], image distance [v] and
focal length.
Mirror formula of the Concave Mirror is 1/f
=1/v + 1/u
Here and object AB at a distance u from the
pole. The image A’B’ is formed at a distance
v from the mirror. The position of the image
is obtained by drawing a ray diagram.

In angle ABC and angle A’B’C both are a similar
triangle
SO, AB/A’B, = BC/B’C ---------{1}
Similarly ,
Angle DEF ~ Angle A’B’C
DE/A’B’ = EF/ B’F
But, DE/ A’B’ = EF/B’F
But, DE = AB
So, AB/A’B’ = EF / B’F -----------{2}

From (1) and (2) BC/B’C = EF/ B’F
If D is very close to P then EF = PF
BC/ B’C = PF/B’F
We know , BC=PC – PB and B’C= PC B’F = PB’ – PF
PC – PB /PB’ – PC = PF / PB’ – PF
By sign convention
PC = -R, PB = -u
PF = -f and PB’ = -v

o When the object is at infinity the image is
formed at the focus , it is highly
diminished, real and inverted.

 When the object is beyond C, the image is formed
between C and F, it is diminished , real and
inverted.

 When the object is between C and F , the image
is formed beyond C , it is enlarged, real and
inverted.

 When the object is at F, the image is formed at
infinity, it is highly enlarged, real and inverted.

 When the object is between F and P, the image is
formed behind the mirror, it is enlarged, virtual
and erect.

 Concave mirrors are used in torches, search lights and
head lights of vehicles to get parallel beams of lights.
 They are used as shaving mirrors to see larger image
of the face.
 They are used by dentists to see larger images of
teeth.
 Large concave mirrors are used to concentrate
sunlight to produce heat in solar furnaces.

 When the object is at infinity, the images is
formed at F behind the mirror , it is highly
diminished, virtual and erect.

 When the object is between infinity and pole, the
image is formed behind the mirror, it is
diminished, virtual and erect.

 Convex mirror are used as rear- view mirror in
vehicles. Convex mirrors give erect diminished
images of objects. They also have a wider field of
view than plane mirrors.

SPHERICAL MIRRORS 9.pptx BY KAVYA SINGH CLASS 9

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