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Chapter 10 NCERT- Light
01 Laws of reflection
02 Spherical mirrors

Reflection of Light
When light falls on a polished surface it bounces back into the
same medium which is called reflection of light.

Incident Ray: The ray of light that falls on the
reflecting surface.
Reflected ray: The ray of light that sent back from
the reflecting surface.
Normal: The normal is a line at right angle to the
reflecting surface at the point of incidence.
Angle of incidence: It is the angle made by the
incident ray and the normal
Angle of reflection: It is the angle made by the
reflected ray and the normal.
Terms related to reflection of light

The angle of incidence is equal to the angle of
reflection.
The incident ray, the reflected ray and the normal to
the mirror at the point of incidence all lie in the
same plane
Laws reflection of light

1. Virtual and Erect
2. Size is same as the object
3. Distance of the image from the mirror is same as
the distance of the object from the mirror.
4. Image is literally inverted
Properties of image formed by a plane mirror

Concave Mirror: A spherical mirror that has its
reflecting surface curved inwards.
Convex Mirror: A spherical mirror that has its
reflecting surface curved outwards.
10.2 Spherical Mirrrors

I) Pole (P): It is the centre of the reflecting surface of the
spherical mirror. It lies on the reflecting surface.
II) Centre of curvature (C): It is the centre of the sphere of
which the mirror is a part. It is not apart of the mirror. For
Conave mirror it is infront of it and for convex behind it,
III) Radius of curvature (R): It is the radius of the sphere of
which the mirror is a part. CP in the diagaram.
IV) Principal axis: It is the straight line passing through the
centre of curvature and the pole. It is normal to the mirror at
the pole.
v) Aperture: The diameter of the reflecting surface of a
spherical mirror.
Terms related to spherical mirrors

Rays parallel to the principal axis after
reflection meet or intersect at a point on
the principal axis which is called the
focus (F) of the concave mirror
The distance between the pole and the
principal focus of a spherical mirror is
called its focal length (f).
For spherical mirrors of small aperture
R=2f
Focus and focal length of a Concave mirror

Rays parallel to the principal axis after
reflection appear to come from a point
on the principal axis which is called the
principle focus (F) of the convex mirror
The distance between the pole and the
principal focus of a spherical mirror is
called its focal length (f).
Focus and focal length of a Convex mirror

It is convenient to consider only two rays for
the construction of ray diagrams.
These rays are so chosen that it is easy to
know their direction after reflection from a
mirror.
Rule1:
A ray parallel to the principle axis, after
reflection, will pass through the principle
focus in case of a concave mirror.
A ray parallel to the principle axis, after
reflection, appears to diverge from the
principle focus in case of a convex mirror
10.2.2 Image Formation by Spherical Mirrors

Rule 2:
A ray passing through the principal focus of a
concave mirror, after reflection, will emerge
parallel to the principal axis.
A ray directed rowards the principal focus of a
convex mirror emerges parallel after
reflection.
Image Formation by Spherical Mirrors

Rule 3:
A ray passing through the center of curvature
of a concave mirror, after reflection, is
reflected back along the same path.
A ray directed towards the center of curvature
of a convex mirror, after reflection, is reflected
back along the same path.

Rule 4:
A ray incident obliquely to the principal axi
towards a point P (Pole) on the concave or
convex mirror, is reflected obliquely, making
equal angles with the principal axis.

Object Position: At Infinity
Position of Image: At the focus
Nature of Image: Real and Inverted
Size of Image: Highly Diminished
Image Formation by A Concave Mirror

Object Position: Beyond Center of
Curvature
Position of Image: Between F and C
Size of Image: Diminished

Object Position: At C
Position of Image: At C
Size of Image: Same as Object

Object Position: Between C and F
Position of Image: Beyond C
Size of Image: Enlarged

Object Position: At F
Position of Image: At infinity
Size of Image: Highly Enlarged

Object Position: Between P and F
Position of Image: Behind the mirror
Nature of Image: Virtual and Erect
Size of Image: Enlarged

Summary

Object Position: At Infinity
Position of Image: At the focus behind the
mirror
Size of Image: Highly Diminished
Image Formation by A Convex Mirror

Object Position: Between Infinity and Pole
Position of Image: Between P and F
Size of Image: Diminished

Summary

10.2.3 Sign convention for reflecton by spherical mirrors

The object is always placed to the left of the mirror. This
implies that the light from the object falls on the mirror from
the left-hand side.
All distances parallel to the principal axis are measured
from the pole of the mirror.
All the distances measured to the right of the origin (along
+ve x-axis) are taken as positive while those measured to
the left of the origin (along -ve x-axis) are taken negative.
Distance measured perpendicular to and above the
principal axis (along +ve y-axis) are taken as positive.
Distance measured perpendicular to and below the principal
axis (along -ve y-axis) are taken as negative.

10.2.4 Mirror formula and Magnification

Magnification
Magnification produced by a spherical miorror gives the
relative extent to which the image of an object is magnified
with respecft to the object size.
It is expressed as the ratio of the height of the image to the
height of the object. It is represented bt the letter 'm'.
It is also related to the object distance (u) and image distance
(v) as:
m = - v/u

A Concave mirror produces three times magnified Real
image of an object placed at 10 cm in front of it. Where is
the image located? Find its focal length.
Soln: Given m= -3(Real Image), u = -10 cm
Using m = -v/u
or - 3 = - v/ -10
or v = - 30 cm
Image is formed 30 cm in front of the mirror
Now Using Mirror formula 1/v + 1/u = 1/f
u= - 10 cm, v = - 30 cm
or -1/30 - 1/ 10 =1/f
(-1 -3) / 30 = 1/f
f= -30/4 = -7.5 cmm

NCERT In text question. Q 2 Pg 171
• Q. A Concave mirror produces three times magnified
(enlarged)real image of an object placed at 10 cm in
front of it, Where is the image located?
• Soln: Given m = -3 (Since image is real), u = -10 cm
• Let v = x cm
• Using m = -v / u
• Or -3 = -x / -10
• Or x= -30 cm
• Negative sign shows image is 30 cm in front of the
mirror.

NCERT End exercise Q 14 Pg. 186
• soln: Given h= 5 cm, u = - 20 cm, R= +30 cm
• Using f = R/2 =30/2 = +15 cm
• Using mirror formula: 1/v + 1/ u = 1/f
• Or 1/v +1/ (-20) = 1/15
• Or 1/v = 1/15 + 1/20
• Or 1/v= (4 + 3)/60
• Or v = 60/7 = 8.4 cm
• Positive sign shows image is virtual erect and 8.4 cm behind the
mirror.

• Image size:
• Using m = h'/h = -v/u
• or h'/5 = -60/7 x1/-20
• or h'= 3/7 = 0.42 cm

• Soln Given u= -10 cm, f= + 15 cm, v=?
• using mirror formula: 1/u + 1/v = 1/f
• 1/-10 + 1/ v = 1/15
• 1/ v = 1/15 +1/10
• 1/v = (2+3)/30
• v= 30/5 = 6 cm
• Since v is positive image is formed behind the mirror
and is virtual and erect.

PHYSICS
REFLECTION OF LIGHT
CBSE CLASS 10
BY: RAVNEET NAGI

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