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REFLECTION OF LIGHT
LIGHT – It is a form of energy which gives us sensation of vision. It is itself invisible but makes
other things visible in its presence.
• It travels with a speed of 3 ´ 108 m/s in the air or vacuum. It is an electromagnetic wave
which does not require medium for propagation.
• According to particle theory : Light is composed of particles which travel in straight line at
very high speed. ‘Photon’ is the elementary particle or unit of light energy.
• According to ‘ Quantum nature of Light ‘ light exhbits the properties of both waves and par-
ticles.
REFLECTION OF LIGHT :– The process of returning of light ray on the same side after stricking a
surface is called reflection of light.
(i) Regular reflection :– When the parallel beam of light
ray after reflection remain parallel then it is called
regular reflection. it takes place from smooth, polished
surface like plane mirror.
(ii) Irregular reflection :– When the parallel beam of light
rays do not remain parallel after reflection, then it is
called irregular reflection or diffused reflection. It
takes place from rough, irregular surface.
– The ray of light which falls on the mirror surface is called
the incident ray.
– The ray of light which is sent back by the mirror is called
reflected ray.
– The angle made by the incident ray with the normal at the
point of incidence is called angle of incidence.
– The angle made by reflected ray with the normal at the point
of incidence is called angle of reflection.
– The angle made by incident ray with the surface is called glans angle of incidence ( 900 – i).
LAWS OF REFLECTION –
(1) The incident ray, the reflected ray and the normal at the point of incidence, all lie in the
same plane.
(2) The angle of reflection is always equal to the angle of incidence ( Ð i = Ð r).
• A ray of light incident normally on a mirror reflects back along the same path ( Ð i = 00 ).
IMAGE – Iamge is an optical appearance produced when light rays coming from an object are
reflected from a mirror or refracted through a lens.
• An image is formed when the light rays coming from an object meet or appear to meet at a
point, afte reflection or refraction.
(i) Real image :– The image which is formed by actual itersection of reflected or refracted rays is
called real image.
– It can be obtained on screen.
– It is always inverted.
– It is formed by concave mirror and convex lens.
(ii) Virtual image – The image which is formed when the reflected or refracted rays appear to meet
when produced backwards is called virtual image.
– It can not be obtained on a screen.
Reflection of light [1]
– It is aways erect or upright.
– It is formed by plane mirror, convex mirror,concave mirror, convex lens and concave lens.
FORMATION OF IMAGE BY PLANE MIRROR :–
(i) (ii)
Characteristics of image formed by plane mirror :–
(i) It is virtual and erect.
(ii) The size of image is equal to that of the object.
(iii) The distance of image from the mirror is equal to the dist. of object.
(iv) The image is laterally inverted.
Lateral Inversion :– The interchange of left and right side of an object in the image formed by plane
mirror is called lateral inversion.
eg :– The word AMBULANCE on the hospital vans is written in the form of its mirror image in
laterally inverted form. This is because the driver of the vehicle ahead of it sees the hospital van
coming from behind in the rear veiw mirror, and due to lateral inversion read it in correct
sequence as AMBULANCE give pass to it.
IMAGES FORMED BY TWO INCLINED MIRROR :–
The object and the images formed by two inclined mirrors lie on the circumference of a circle
whose centre lies at the point of intersection of the two mirrors and the radius equal to the
distance of object from the point of intersection.
• The number of images formed depends on the angle q 0 between two mirrors. Following two
cases are possible :–
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CASE (1) : If angle q 0 between the mirrors is such that n = is odd,
q0
(i) the number of images is n, when the object is placed asymmetrically between the mirrors.
(ii) the number of images is n – 1, when the object is placed symmetrically ( i.e. on the bisector
of the angle ) between the mirrors.
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Example :– If the angle between two mirrors is 720, then n = = 5. So images formed will be
72 0
n = 5 for the object placed asymmetrically between the mirrors and images will be ( n – 1) = 4,
if the object is placed symmetrically ( on the bisector ) between the mirrors because two im-
ages will overlap.
3600
CASE (2) : If n = is even, the number of images is always n – 1 for all positions of object in
q0
between mirrors.
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Example :– If the angle between two mirrors is 600, then n = = 6, the number of images is
60 0
(n–1) = 5, i.e. five images will be formed for all positions of the object between two mirrors.
IMAGES FORMED BY TWO MIRRORS :–
(i) When two mirrors are perpendicular to each other : –
3600 360 0
q = 0 900, then n = 0 = =4.
q 90 0
For an object placed in between the two perpendicular mirrors, the number of images formed
will be n – 1= 3.
(i) When two mirrors are parallel to each other : –
3600 360 0
q 0 = 00, then n = = = ¥ ( infinite).
q0 00
USE :– In a showroom and in barber’s shop mirrors are arranged in this manner.
• A thick plane mirror also forms a multiple number of images due to multiple reflections
within the glass from front surface and back reflecting surface.
Uses of Plane Mirrors :–
(i) Plane mirrors are used in making periscopes.
(ii) Plane mirrors are used to see ourselves.
(iii) Plane mirrors are fitted at blind turns of some busy roads so that drivers can see the ve-
hicles coming from the other side and prevent accidents.
(iv) Plane mirrors are fixed on the inside walls of certains shops to make them look bigger.
SPHERICAL MIRRORS :–
A spherical mirror is a part of hollow glass sphere whose one
surface is polished. it is of two types – concave mirror and
convex mirror.
(i) Concave mirror – The spherical mirror whose inner sur-
face is reflecting and outer surface is polished.
• It is a converging mirror because the light rays converge
at a point after reflection from it.
(ii) Convex mirror – The spherical mirror whose outer sur-
face is reflecting and inner surface is polished.
• It is a diverging mirror because the light rays appear to
diverge from a point after reflection from it.
TERMS :–
(i) Pole – The geometric centre of spherical mirror is called its pole.
(ii) Centre of curvature – The centre of the hollow glass sphere of which the mirror is a part is
called the centre of curvature. The radius of the hollow glass sphere is called radius of curva-
ture (R).
(iii) Principal axis – The straight line passing through the centre of curvature and pole of a spheri-
cal mirror is called its principal axis.
(iv) Aperture – That portion of mirror from which the reflection of light actually takes place is
called the aperture of the mirror.
(v) Focus – Focus of a concave mirror is a point on its principal axis to which all the light rays
parallel to principal axis converge after reflection. The distance of focus from pole is called
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focal length (f).
• The focus of convex mirror is the point on principal axis from which incident rays parallel to
principal axis appear to diverge after reflection from it.
Relationship betweeen focal length and radius of curvature :– ((
The focal length of a spherical mirror is equal to half of its radius of curvature.
R
f= .
2
CONCAVE MIRROR – Rules for image formation :–
1. A ray of light parallel to the principal axis passes through its focus after reflection.
2. A ray of light passing through the centre of curvature is reflected back along the same path.
3. A ray of light passing through the focus of a concave mirror becomes parallel to the principal
axis after reflection.
4. A ray of light which is incident at the pole of a concave mirror is reflected back making the
same angle with the principal axis.
FORMATION OF IMAGE :–
(a) When object is at infinity :–
Image – (i) Formed at focus,
(ii) Real, inverted
(iii) highly diminished.
(b) When object is beyond C but not at infinity :–
Image – (i) Formed between C and F
(ii) Real, inverted
(iii) diminished.
(c) When object is at C :–
Image – (i) Formed at C
(ii) real, Inverted
(iii) same size as the object
(d) When object is between F and C :–
Image – (i) Formed beyond C
(ii) real , Inverted
(iii) magnified.
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(e) When object is at principal focus (F) :–
Image – (i) Formed at infinity
(ii) Real, Inverted
(iii) highly magnified.
(f) When object is between pole and principal focus (F) :–
Image – (i) Formed behind the mirror
(ii) Virtual, erect
(iii) magnified.
CONVEX MIRROR :– Rules for image formation :–
1. A ray of light travelling parallel to the principal axis of a convex mirror after reflection
appear to diverge from the principal focus.
2. A ray of light going towards the principal focus of a convex mirror after reflection
travell parallel to the principal axis.
3. A ray of light going towards centre of curvature of a convex mirror is reflected back
along the same path.
4. A ray of light which is incident at the pole of a convex mirror is reflected back making the
same angle with the principal axis.
FORMATION OF IMAGE :–
(a) When object is at infinity :–
Image – (i) Formed behind the mirror,
(ii) virtual, erect
(iii) diminished to a point.
(b) When object is any where between the pole and infinity :–
Image – (i) Formed between pole and focus,
(ii) virtual, erect
(iii) diminished.
USAGES :–
CONCAVE MIRROR –
1. It is used as shaving mirror to see a large image of the face is held between focus and
the pole. Virtual, erect and magnified image is fromed behind the mirror. Hence, even
asmall peg of beard can be easilyshaved.
2. It is used as reflector in torches, vehicle head-lights and search lights to get powerful
beams of light. When the bulb is placed at the focus of concave reflector, then it pro-
duces powerful beam of parallel light rays.
3. It is used by E.N.T. doctors as head mirror to focus light rays coming from lamp on
body parts of a patient to be examined.
4. It is used by dentists to see large images of the teeth of patients.
5. Large concave mirrors are used to focus sun’s rays for heating solar furnaces.
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CONVEX MIRROR –
1. It is used as rear-view mirrors in vehicles to see the traffic at
the rear side. It always produces a virtual, erect and highly
diminished image of the object, due to which it provides
wide field of view. The convex mirror enables a driver to
view much larger area of the traffic behind him.
2. It is used as reflector in street light to diverge light rays from
the bulbs placed at its focus over large area of the road.
SIGN CONVENTION FOR SPHERICAL MIRRORS :–
1. All the distances are measured starting from pole of the mirror.
2. Distances measured in the same direction as that of incident light are taken as posi-
tive.
3. Distances measured against the direction of incident light are taken negative.
4. Distance measured upward and perpendicular to the principal axis are taken positive.
5. Distance measured downwardand perpendicular to the principal axis are taken as
negative.
Concave mirror Convex mirror
Focal length – – ve + ve
Object distance – – ve – ve
Image distance – – ve (real), +ve (virtual) + ve
Height of object – + ve + ve
Height of image – – ve (real), + ve (virtual) + ve
MIRROR FORMULA :–
1 1 1
+ = Where, v = distance of image from mirror
v u f
u = distance of object from mirror
f = focal length of mirror.
LINEAR MAGNIFICATION PRODUCED BY MIRROR :– The ratio of the height of image to
the height object is known as linear magnification.
Length or height of image (I)
Magnification (m) =
Length or height of object (O )
Distance of image (v)
=
Distance of object (u)
I v
m = =
O u
– If the magnification has a plus sign, then the image is virtual and erect.
– If the magnification has a minus sign, then the image is real and inverted.
Distinction Between Plane mirror, Concave mirror and Convex mirror without touching
them :– We bring our face close to each mirror,turn by turn.
1. A plane mirror will produce virtual, erect image of the same size as our face and we
look our normal self.
2. A concave mirror will produce a virtual, erect and magnified image and our face will
look much bigger.
3. A convex mirror will produce a virtual, erect and diminished image and our face will
look much smaller.
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