CHAPTER - 10
LIGHT – REFLECTION AND REFRACTION
LIGHT
An object reflects light that falls on it. This reflected light when received by our eyes,
enables us to see things.
Reflection of light
Reflection of light is the phenomenon of bouncing back of light in the same medium
on striking the surface of any object.
There are two types of reflection:
1. Regular reflection or Specular Reflection
2. Irregular reflection or Diffuse Reflection
Regular Reflection: When the reflecting surface is smooth and well polished, the
parallel rays falling on it are reflected parallel to one another, the reflected light goes in
one particular direction. This is Regular reflection or Specular reflection see below
figure.
Irregular reflection: When the reflecting surface is rough, the parallel rays falling on it
reflected in different direction, as shown in below fig. Such a reflection is known as
diffuse reflection or irregular reflection.
LAWS OF REFLECTION OF LIGHT
According to the laws of Reflection of light,
(i) The angle of incidence is equal to the angle of reflection, and
(ii) The incident ray, the normal to the mirror at the point of incidence and the
reflected ray, all lie in the same plane.
These laws of reflection are applicable to all types of reflecting surfaces including
spherical surfaces.
OBJECTS
Anything which gives out light rays either its own or reflected by it is called an
object.
LUMINOUS OBJECTS: The objects like the sun, other stars, electric bulb, tube-
light etc. which emit their own light are called luminous objects.
NON – LUMINOUS OBJECTS: The objects which do not emit light themselves
but only reflect or scatter the light which falls on them, are called non-luminous
objects. A flower, chair table, book, trees, etc are all non-luminous objects.
IMAGES
Image is an optical appearance produced when light rays coming from an object
are reflected from a mirror (or refracted through lens).
REAL IMAGE
The image which can be obtained on a screen is called a real image. In a cinema hall,
we see the images of actors and actress on the screen. So, the images formed on a
cinema screen is an example of real images.
VIRTUAL IMAGE
The image which cannot be obtained on a screen is called a virtual image. A virtual
image can be seen only by looking into a mirror. The image of our face in a plane
mirror is an example of virtual image.
LATERAL INVERSION
When an object is placed in front of a plane mirror, then the right side of object
appears to become the left side of image; and the left side of object appears to become
the right side of image. This change of sides of an object and its mirror image is called
lateral inversion.
The phenomenon of lateral inversion is due to the reflection of light.
CHARACTERISTICS OF IMAGES FORMED BY PLANE MIRRORS
The characteristics of images formed by plane mirrors are:
1. The image of real object is always virtual. Such image cannot be taken on a
screen.
2. The image formed in a plane mirror is always erect.
3. The size of the image in a plane mirror is always the same as the size of the
object.
4. The image formed in a plane mirror is as far behind the mirror, as the object is in
front of the mirror.
5. The image formed in a plane mirror is laterally inverted i.e. the left side of the
objects becomes the right side of the image and vice-versa.
SPHERICAL MIRROR
A spherical mirror is that mirror whose reflecting surface is the part of a hollow
sphere of glass. The spherical mirrors are of two types: Concave mirror and Convex
mirror.
CONCAVE MIRROR: A concave mirror is that spherical mirror in which the
reflection of light takes place at the concave surface (or bent-in surface).
CONVEX MIRROR: A convex mirror is that spherical mirror in which the
reflection of light takes place at the convex surface (or bulging –out surface).
TERMS RELATED TO SPHERICAL MIRRORS
Centre of Curvature(C): The centre of curvature of a spherical mirror is the
centre of the hollow sphere of glass of which the spherical mirror is a part. It is
represented by letter ‘C’.
Pole(P): The pole of a spherical mirror is the centre of the mirror. It is represented
by letter ‘P’.
Radius of Curvature(R): The radius of curvature of a spherical mirror is the
radius of the hollow sphere of glass of which the spherical is a part. It is represented
by the letter ‘R’.
Principal axis: The principal axis of a spherical mirror is the straight line passing
through the centre of curvature C and pole P of the spherical mirror, produced on
both sides.
Aperture: The aperture of a spherical mirror is the diameter of the reflecting
surface of the mirror.
PRINCIPAL FOCUS OF A SPHERICAL MIRROR
The principal focus of a concave mirror is a point on its principal axis to which all the
light rays which are parallel and close to the axis, converge after reflection from the
concave mirror. A concave mirror has a real focus. The focus of a concave mirror is in
front of the mirror. Since a concave mirror converges a parallel beams of light rays, it is
also called converging mirror.
The principal focus of a convex mirror is a point on its principal axis from which a beam
of light rays, initially parallel to the axis, appears to diverge after being reflected from the
convex mirror. A convex mirror has a virtual focus. The focus of a convex mirror is
situated behind the mirror. Since a convex mirror diverges a parallel beams of light rays,
it is also called diverging mirror.
Focal Length: The focal length of a spherical mirror is the distance between its pole
and principal focus. It is denoted by the letter ‘f’.
Relation between Radius of curvature and focal length of a spherical mirror
The focal length of a spherical mirror is equal to half of its radius of curvature.
R
f
2
In other words, for spherical mirrors of small apertures, the radius of curvature is
found to be equal to twice the focal length.
R = 2f
RULES FOR OBTAINING IMAGES FORMED BY SHPERICAL MIRRORS
The intersection of at least two reflected rays give the position of image of the point
object. Any two of the following rays can be considered for locating the image.
1. A ray parallel to the principal axis, after reflection, will pass through the principal
focus in case of a concave mirror or appear to diverge from the principal focus in
case of a convex mirror.
2. A ray passing through the principal focus of a concave mirror or a ray which is
directed towards the principal focus of a convex mirror, after reflection, will emerge
parallel to the principal axis.
3. A ray passing through the centre of curvature of a concave mirror or directed in the
direction of the centre of curvature of a convex mirror, after reflection, is reflected
back along the same path. The light rays come back along the same path because the
incident rays fall on the mirror along the normal to the reflecting surface.
4. A ray incident obliquely to the principal axis, towards a point P (pole of the mirror),
on the concave mirror or a convex mirror , is reflected obliquely. The incident and
reflected rays follow the laws of reflection at the point of incidence (point P), making
equal angles with the principal axis.
FORMATION OF DIFFERENT TYPES OF IMAGES BY A CONCAVE MIRROR
The type of image formed by a concave mirror depends on the position of object in
front of the mirror. There are six positions of the object:
Case–1: Object is in between P and F
When an object is placed between the pole(P) and focus(F) of a concave mirror, the
image formed is
(i) behind the mirror
(ii) virtual and erect and
(iii) larger than the object (or magnified)
Case–2: Object is at the focus(F).
When an object is placed at the focus of a concave mirror, the image formed is
(i) at infinity
(ii) real and inverted, and
(iii) highly magnified (or highly enlarged)
Case–3: Object is in between focus(F) and centre of curvature(C)
When an object is placed between the focus(F) and centre of curvature(C) of a
concave mirror, the image formed is
(i) beyond the centre of curvature
(ii) real and inverted, and
(iii) larger than the object (or magnified)
Case–4: Object is at the centre of curvature(C)
When an object is placed at the centre of curvature of a concave mirror, the image
formed is
(i) at the centre of curvature
(ii) real and inverted, and
(iii) same size as the object
Case–5: Object is beyond the centre of curvature(C)
When an object is placed beyond the centre of curvature of a concave mirror, the
image formed is
(i) between the focus and centre of curvature
(ii) real and inverted, and
(iii) smaller than the object (or diminished)
Case–6: Object is at infinity.
When an object is placed at infinity of a concave mirror, the image formed is
(i) between the focus and centre of curvature
(ii) real and inverted, and
(iii) much smaller than the object (or highly diminished)
USES OF CONCAVE MIRRORS
1. Concave mirrors are commonly used in torches, search-lights and vehicles
headlights to get powerful parallel beams of light.
2. Concave mirrors are used as shaving mirrors to see a larger image of the face.
3. The dentists use concave mirrors to see large images of the teeth of patients.
4. Concave mirrors are used as doctor’s head mirrors to focus light coming from a
lamp on to the body parts of a patient to be examined by the doctor.
5. Concave dishes are used in TV dish antennas to receive TV signals from the
distant communications satellite.
6. Large concave mirrors are used to concentrate sunlight to produce heat in solar
furnaces.
FORMATION OF DIFFERENT TYPES OF IMAGES BY A CONVEX MIRROR
The type of image formed by a convex mirror depends on the position of object in
front of the mirror. There are six positions of the object:
Case–1: Object is placed between P and infinity
When an object is placed between pole and infinity in front of a convex mirror, the
image formed is
(i) between the pole and focus
(ii) virtual and erect, and
(iii) smaller than the object (or diminished)
Case–2: Object is at infinity.
When an object is placed at infinity of a convex mirror, the image formed is
(i) behind the mirror at focus
(ii) virtual and erect, and
(iii) much smaller than the object (or highly diminished)
USES OF CONVEX MIRRORS
Convex mirrors are commonly used as rear-view (wing) mirrors in vehicles. These
mirrors are fitted on the sides of the vehicle, enabling the driver to see traffic behind
him/her to facilitate safe driving. Convex mirrors are preferred because they always give
an erect, though diminished, image. Also, they have a wider field of view as they are
curved outwards. Thus, convex mirrors enable the driver to view much larger area than
would be possible with a plane mirror.
INTEXT QUESTIONS PAGE NO. 168
1. Define the principal focus of a concave mirror.
Ans. Light rays that are parallel to the principal axis of a concave mirror converge at a
specific point on its principal axis after reflecting from the mirror. This point is known
as the principal focus of the concave mirror.
2. The radius of curvature of a spherical mirror is 20 cm. What is its focal length?
Ans. Here R = 20 cm
R 20
We know that f f 10cm
2 2
3. Name a mirror that can give an erect and enlarged image of an object.
Ans. When an object is placed between the pole and the principal focus of a concave
mirror, the image formed is virtual, erect, and enlarged.
4. Why do we prefer a convex mirror as a rear-view mirror in vehicles?
Ans. Convex mirrors give a virtual, erect, and diminished image of the objects placed in
front of them. They are preferred as a rear-view mirror in vehicles because they give a
wider field of view, which allows the driver to see most of the traffic behind him.
MIRROR FORMULA
In a spherical mirror, the distance of the object from its pole is called the object distance
(u). The distance of the image from the pole of the mirror is called the image distance
(v). The distance of the principal focus from the pole is called the focal length (f). There
is a relationship between these three quantities given by the mirror formula which is
expressed as
1 1 1
f v u
MAGNIFICATION
Magnification produced by a spherical mirror gives the relative extent to which the
image of an object is magnified with respect to the object size. It is expressed as the
ratio of the height of the image to the height of the object. It is usually represented by
the letter m. If h1 is the height of the object and h2 is the height of the image, then the
magnification m produced by a spherical mirror is given by
height of the image h2
m m
height of the object h1
The magnification m is also related to the object distance (u) and image distance (v). It
can be expressed as:
h v
m 2
h1 u
Points to be remembered:
The height of the object is taken to be positive as the object is usually placed
above the principal axis.
The height of the image should be taken as positive for virtual images. However,
it is to be taken as negative for real images.
When the image is real, it is inverted so h2 is negative which results m is –ve. A
negative sign in the value of the magnification indicates that the image is real.
When the image is virtual, it is erect so h2 is positive which results m is +ve. A
positive sign in the value of the magnification indicates that the image is virtual.
SIGN CONVENTION FOR SPHERICAL MIRRORS
The following sign convention is used for measuring various distances in the ray
diagrams of spherical mirrors:
1. Object is always placed to the left of mirror
2. All distances are measured from the pole of the mirror.
3. Distances measured in the direction of the incident ray are positive and the distances
measured in the direction opposite to that of the incident rays are negative.
4. Distances measured above the principal axis are positive and that measured below the
principal axis are negative.
INTEXT QUESTIONS PAGE NO. 171
1. Find the focal length of a convex mirror whose radius of curvature is 32 cm.
Ans.
Ans. Here R = 32 cm
R f 16cm
We know that f
2
Hence, the focal length of the given convex mirror is 16 cm.
2. 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?
Ans. Here, magnification, m = –3,
object distance, u = –10 cm and
image distance, v = ?
Putting these values in the magnification formula for a mirror, we get
m=-v/u
-3 = -v/10
v = - 30 C
