CHAPTER 9: LIGHT – REFLECTION AND REFRACTION
1. INTRODUCTION TO LIGHT
We see the objects around us when light from the objects reaches our eyes.
During the day, sunlight falls on objects. The objects reflect some of this light, and the reflected light enters
our eyes. This enables us to see the objects.
Important points
• A luminous object produces its own light.
• An ordinary non-luminous object is visible because it reflects light falling on it.
• We cannot see ordinary objects in a completely dark room because there is no light reaching our eyes
from them.
• Light can pass through a transparent medium.
Example
When light from the Sun falls on a tree: Sun → Tree → Reflected light → Eye Therefore, we see the tree.
RAY OF LIGHT: The path of light is represented by a straight line with an arrow showing the direction of
propagation. Such a representation is called a ray of light.
Rectilinear propagation of light: The apparent straight-line travel of light is called rectilinear
propagation of light. The formation of sharp shadows is one observation that supports the straight-line
propagation of light.
2. REFLECTION OF LIGHT
When light falls on a polished or reflecting surface and bounces back into the same medium, the
phenomenon is called reflection of light. A mirror is a highly polished surface that reflects most of the light
falling on it.
Examples of reflection
• Image formation in a mirror
• Seeing our face in a mirror
• Reflection of sunlight from a polished surface
TERMS RELATED TO REFLECTION
o INCIDENT RAY: The ray of light that falls on the reflecting surface is called the incident ray.
Incident = Incoming
o REFLECTED RAY: The ray of light that bounces back from the reflecting surface is called the
reflected ray.
o POINT OF INCIDENCE: The point on the reflecting surface where the incident ray strikes the
surface is called the point of incidence.
o NORMAL: A line drawn perpendicular (90°) to the reflecting surface at the point of incidence is
called the normal. The normal is an imaginary reference line used for measuring angles.
o ANGLE OF INCIDENCE: The angle between the incident ray and the normal at the point of
incidence is called the angle of incidence. It is represented by (i). The angle of incidence is
measured from the normal, not from the mirror.
o ANGLE OF REFLECTION: The angle between the reflected ray and the normal at the point of
incidence is called the angle of reflection. It is represented by (r). The angle of reflection is also
measured from the normal, not from the mirror.
▪ LAWS OF REFLECTION
There are two laws of reflection.
(a) First Law of Reflection: The angle of incidence is equal to the angle of reflection.
i=r
(b) Second Law of Reflection: The incident ray, the normal at the point of incidence and the
reflected ray all lie in the same plane.
o PLANE MIRROR: A plane mirror has a flat reflecting surface. The image formed by a plane
mirror has the following properties:
, ➢ It is virtual (An image that cannot be obtained on a screen) & erect (An upright image.).
➢ It is the same size as the object.
➢ It is formed at the same distance behind the mirror as the object is in front of it.
➢ It is laterally inverted (The left and right sides appear interchanged.).
o CURVED MIRRORS: A reflecting surface may be curved instead of flat. A curved reflecting surface
can be considered as a part of a spherical surface. The most commonly used curved mirrors are
spherical mirrors.
o SPHERICAL MIRRORS: A spherical mirror is a mirror whose reflecting surface forms a part of
the surface of a sphere. There are two types of spherical mirrors:
a) CONCAVE MIRROR: A spherical mirror whose reflecting surface is curved inwards,
towards the centre of the sphere, is called a concave mirror.
b) CONVEX MIRROR: A spherical mirror whose reflecting surface is curved outwards is
called a convex mirror.
o POLE (P): The centre of the reflecting surface of a spherical mirror is called its pole. The pole is
represented by P. The pole lies on the surface of the mirror.
o CENTRE OF CURVATURE (C): The reflecting surface of a spherical mirror forms a part of a
sphere. The centre of this imaginary sphere is called the centre of curvature. It is represented by C.
The centre of curvature is not a part of the mirror; it lies outside the reflecting surface.
o RADIUS OF CURVATURE (R): The radius of the sphere of which the reflecting surface of a
spherical mirror is a part is called the radius of curvature. It is represented by R. The distance
between the pole (P) and centre of curvature (C) is equal to the radius of curvature.
▪ PC=R
o PRINCIPAL AXIS: The straight line passing through the pole (P) and centre of curvature (C) is
called the principal axis of the spherical mirror. The principal axis is normal to the mirror at its pole.
o C────────F────────P )
Principal axis
o PRINCIPAL FOCUS (F): When rays of light parallel to the principal axis fall on a spherical mirror,
they are reflected in a particular way.
Concave mirror
Parallel rays after reflection actually meet at a point on the principal axis.
This point is called the principal focus.
Convex mirror
Parallel rays after reflection appear to come from a point on the principal axis.
This point is called the principal focus. The principal focus is represented by F.
Important difference
Concave mirror Convex mirror
Parallel rays actually meet at (F) Reflected rays appear to come from (F)
Focus is in front Focus is behind
Converging action Diverging action
o FOCAL LENGTH (f): The distance between the pole (P) and the principal focus (F) is called the
focal length. It is represented by f.
o APERTURE: The reflecting surface of a spherical mirror has a circular outline. The diameter of the
reflecting surface of the spherical mirror is called its aperture.
, 3. RELATIONSHIP BETWEEN RADIUS OF CURVATURE AND FOCAL LENGTH
R=2f
This means that the principal focus (F) lies midway between the pole (P) and centre of curvature (C).
Example 1: The radius of curvature of a spherical mirror is (20) cm. Find its focal length.
Ans: Given, R=20
Since f= R/2 = 20/2 =10
Example 2: The distance between the pole & centre of curvature of a mirror is 40 cm. Find its focal length.
Ans: Since, PC=R & R=40
f=R/2 =40/2 =20
FOCUSING OF SUNLIGHT BY A CONCAVE MIRROR
When a concave mirror is directed towards the Sun, sunlight falls on it approximately as parallel rays.
The concave mirror reflects and converges these rays at its focus.
A bright, sharp spot is formed at the focus.
The concentrated sunlight produces considerable heat and can burn paper.
This observation can be used to obtain an approximate value of the focal length of a concave mirror.
REAL AND VIRTUAL IMAGES
Real Image: A real image is formed when the reflected/refracted light rays actually meet. A real image can
generally be obtained on a screen.
Example
The image of the Sun formed by a concave mirror can be obtained on a sheet of paper.
Virtual Image: A virtual image is formed when light rays do not actually meet, but appear to come from a
particular point. A virtual image cannot be obtained on a screen.
4. IMAGE FORMATION BY A CONCAVE MIRROR
The position, size and nature of the image formed by a concave mirror depend on the position of the object
with respect to: P, F and C. There are six important positions of the object.
1. INTRODUCTION TO LIGHT
We see the objects around us when light from the objects reaches our eyes.
During the day, sunlight falls on objects. The objects reflect some of this light, and the reflected light enters
our eyes. This enables us to see the objects.
Important points
• A luminous object produces its own light.
• An ordinary non-luminous object is visible because it reflects light falling on it.
• We cannot see ordinary objects in a completely dark room because there is no light reaching our eyes
from them.
• Light can pass through a transparent medium.
Example
When light from the Sun falls on a tree: Sun → Tree → Reflected light → Eye Therefore, we see the tree.
RAY OF LIGHT: The path of light is represented by a straight line with an arrow showing the direction of
propagation. Such a representation is called a ray of light.
Rectilinear propagation of light: The apparent straight-line travel of light is called rectilinear
propagation of light. The formation of sharp shadows is one observation that supports the straight-line
propagation of light.
2. REFLECTION OF LIGHT
When light falls on a polished or reflecting surface and bounces back into the same medium, the
phenomenon is called reflection of light. A mirror is a highly polished surface that reflects most of the light
falling on it.
Examples of reflection
• Image formation in a mirror
• Seeing our face in a mirror
• Reflection of sunlight from a polished surface
TERMS RELATED TO REFLECTION
o INCIDENT RAY: The ray of light that falls on the reflecting surface is called the incident ray.
Incident = Incoming
o REFLECTED RAY: The ray of light that bounces back from the reflecting surface is called the
reflected ray.
o POINT OF INCIDENCE: The point on the reflecting surface where the incident ray strikes the
surface is called the point of incidence.
o NORMAL: A line drawn perpendicular (90°) to the reflecting surface at the point of incidence is
called the normal. The normal is an imaginary reference line used for measuring angles.
o ANGLE OF INCIDENCE: The angle between the incident ray and the normal at the point of
incidence is called the angle of incidence. It is represented by (i). The angle of incidence is
measured from the normal, not from the mirror.
o ANGLE OF REFLECTION: The angle between the reflected ray and the normal at the point of
incidence is called the angle of reflection. It is represented by (r). The angle of reflection is also
measured from the normal, not from the mirror.
▪ LAWS OF REFLECTION
There are two laws of reflection.
(a) First Law of Reflection: The angle of incidence is equal to the angle of reflection.
i=r
(b) Second Law of Reflection: The incident ray, the normal at the point of incidence and the
reflected ray all lie in the same plane.
o PLANE MIRROR: A plane mirror has a flat reflecting surface. The image formed by a plane
mirror has the following properties:
, ➢ It is virtual (An image that cannot be obtained on a screen) & erect (An upright image.).
➢ It is the same size as the object.
➢ It is formed at the same distance behind the mirror as the object is in front of it.
➢ It is laterally inverted (The left and right sides appear interchanged.).
o CURVED MIRRORS: A reflecting surface may be curved instead of flat. A curved reflecting surface
can be considered as a part of a spherical surface. The most commonly used curved mirrors are
spherical mirrors.
o SPHERICAL MIRRORS: A spherical mirror is a mirror whose reflecting surface forms a part of
the surface of a sphere. There are two types of spherical mirrors:
a) CONCAVE MIRROR: A spherical mirror whose reflecting surface is curved inwards,
towards the centre of the sphere, is called a concave mirror.
b) CONVEX MIRROR: A spherical mirror whose reflecting surface is curved outwards is
called a convex mirror.
o POLE (P): The centre of the reflecting surface of a spherical mirror is called its pole. The pole is
represented by P. The pole lies on the surface of the mirror.
o CENTRE OF CURVATURE (C): The reflecting surface of a spherical mirror forms a part of a
sphere. The centre of this imaginary sphere is called the centre of curvature. It is represented by C.
The centre of curvature is not a part of the mirror; it lies outside the reflecting surface.
o RADIUS OF CURVATURE (R): The radius of the sphere of which the reflecting surface of a
spherical mirror is a part is called the radius of curvature. It is represented by R. The distance
between the pole (P) and centre of curvature (C) is equal to the radius of curvature.
▪ PC=R
o PRINCIPAL AXIS: The straight line passing through the pole (P) and centre of curvature (C) is
called the principal axis of the spherical mirror. The principal axis is normal to the mirror at its pole.
o C────────F────────P )
Principal axis
o PRINCIPAL FOCUS (F): When rays of light parallel to the principal axis fall on a spherical mirror,
they are reflected in a particular way.
Concave mirror
Parallel rays after reflection actually meet at a point on the principal axis.
This point is called the principal focus.
Convex mirror
Parallel rays after reflection appear to come from a point on the principal axis.
This point is called the principal focus. The principal focus is represented by F.
Important difference
Concave mirror Convex mirror
Parallel rays actually meet at (F) Reflected rays appear to come from (F)
Focus is in front Focus is behind
Converging action Diverging action
o FOCAL LENGTH (f): The distance between the pole (P) and the principal focus (F) is called the
focal length. It is represented by f.
o APERTURE: The reflecting surface of a spherical mirror has a circular outline. The diameter of the
reflecting surface of the spherical mirror is called its aperture.
, 3. RELATIONSHIP BETWEEN RADIUS OF CURVATURE AND FOCAL LENGTH
R=2f
This means that the principal focus (F) lies midway between the pole (P) and centre of curvature (C).
Example 1: The radius of curvature of a spherical mirror is (20) cm. Find its focal length.
Ans: Given, R=20
Since f= R/2 = 20/2 =10
Example 2: The distance between the pole & centre of curvature of a mirror is 40 cm. Find its focal length.
Ans: Since, PC=R & R=40
f=R/2 =40/2 =20
FOCUSING OF SUNLIGHT BY A CONCAVE MIRROR
When a concave mirror is directed towards the Sun, sunlight falls on it approximately as parallel rays.
The concave mirror reflects and converges these rays at its focus.
A bright, sharp spot is formed at the focus.
The concentrated sunlight produces considerable heat and can burn paper.
This observation can be used to obtain an approximate value of the focal length of a concave mirror.
REAL AND VIRTUAL IMAGES
Real Image: A real image is formed when the reflected/refracted light rays actually meet. A real image can
generally be obtained on a screen.
Example
The image of the Sun formed by a concave mirror can be obtained on a sheet of paper.
Virtual Image: A virtual image is formed when light rays do not actually meet, but appear to come from a
particular point. A virtual image cannot be obtained on a screen.
4. IMAGE FORMATION BY A CONCAVE MIRROR
The position, size and nature of the image formed by a concave mirror depend on the position of the object
with respect to: P, F and C. There are six important positions of the object.
