PHYSICS
UNIT - 3
Chapter 6 : FIBRE OPTICS
Total Internal Reflection (TIR)
When light travels from a denser medium of refractive index to a rarer medium of refractive
index , it bends away from the normal (as shown in fig. a), such that the angle of incidence
and the angle of refraction obey Snell’s Law:
As the angle of incidence is increased, the angle of refraction also increases, till at a certain
angle of incidence called as critical angle the angle of refraction becomes 90°, and the ray
just grazes along the boundary of the two media (fig. b). Applying Snell’s Law to this situation we
get:
( )
If the angle of incidence is further increased beyond the critical angle , then the ray of light gets
fully reflected back into the denser medium (fig. c). This is called Total Internal Reflection (TIR).
Conditions for Total Internal Reflection
i) Light must travel from denser medium to rarer medium
ii) Angle of incidence in the denser medium must be greater than the critical angle.
Structure of Optical Fibre
A practical optical fibre has
three co-axial regions. The
innermost region is the one
which carries the light and is
called the core. It is
surrounded by a co-axial
middle region known as the
cladding. The cladding has a
refractive index ( ) lower
than that of the core ( ) and
helps to keep the light within
Fibre Optics Prof. Harison Cota, Don Bosco College of Engineering, Fatorda Page 1
, the core through the phenomenon of TIR.
The outermost region is called the sheath and it protects the core and cladding from scratches,
contamination and moisture. Optical fibres are made of either glass or plastic.
Propagation of light in an optical fibre
Propagation of light in an optical fibre is
based on the principle of TIR. A ray of
light incident on one end of the optical
fibre is made to enter the core region
of the fibre. It travels through the core
in a straight line till it strikes the core-
cladding boundary. Since the ray is
travelling from denser core to rarer
cladding it undergoes TIR (provided the
angle of incidence is greater than the
critical angle) and is reflected back into
the core. It then again travels in a
straight line till it strikes another core-
cladding boundary. Here again it
undergoes TIR and is reflected back
into the core. In this way, through multiple TIRs, the ray of light propagates through the fibre and
emerges from the other end.
Structure of Optical Fibre Cable (OFC)
An optical fibre cable contains six optical
fibres uniformly distributed around a
central insulated steel cable which provides
strength to the OFC. Six insulated copper
wires are placed in the space between the
optical fibres. These are used for electrical
signal transmission, if required. The optical
fibres, copper wires and steel cable are
tightly held together using mylar binding
tape. Aluminum foil is then wrapped
around it to provide shielding from outside
interferences. A thick plastic jacket is put
on top to provide protection from dust, water and damage.
Expression for Acceptance Angle ( )
Consider an optical fibre having
core of refractive index and
cladding of refractive index .
Let the surrounding have
refractive index . (
> ). Let a ray a light be incident
from the surrounding on one
end of the fibre at an angle of
incidence . The ray refracts at
an angle and strikes the core-
cladding boundary at an angle .
If is greater than the critical
Fibre Optics Prof. Harison Cota, Don Bosco College of Engineering, Fatorda Page 2
UNIT - 3
Chapter 6 : FIBRE OPTICS
Total Internal Reflection (TIR)
When light travels from a denser medium of refractive index to a rarer medium of refractive
index , it bends away from the normal (as shown in fig. a), such that the angle of incidence
and the angle of refraction obey Snell’s Law:
As the angle of incidence is increased, the angle of refraction also increases, till at a certain
angle of incidence called as critical angle the angle of refraction becomes 90°, and the ray
just grazes along the boundary of the two media (fig. b). Applying Snell’s Law to this situation we
get:
( )
If the angle of incidence is further increased beyond the critical angle , then the ray of light gets
fully reflected back into the denser medium (fig. c). This is called Total Internal Reflection (TIR).
Conditions for Total Internal Reflection
i) Light must travel from denser medium to rarer medium
ii) Angle of incidence in the denser medium must be greater than the critical angle.
Structure of Optical Fibre
A practical optical fibre has
three co-axial regions. The
innermost region is the one
which carries the light and is
called the core. It is
surrounded by a co-axial
middle region known as the
cladding. The cladding has a
refractive index ( ) lower
than that of the core ( ) and
helps to keep the light within
Fibre Optics Prof. Harison Cota, Don Bosco College of Engineering, Fatorda Page 1
, the core through the phenomenon of TIR.
The outermost region is called the sheath and it protects the core and cladding from scratches,
contamination and moisture. Optical fibres are made of either glass or plastic.
Propagation of light in an optical fibre
Propagation of light in an optical fibre is
based on the principle of TIR. A ray of
light incident on one end of the optical
fibre is made to enter the core region
of the fibre. It travels through the core
in a straight line till it strikes the core-
cladding boundary. Since the ray is
travelling from denser core to rarer
cladding it undergoes TIR (provided the
angle of incidence is greater than the
critical angle) and is reflected back into
the core. It then again travels in a
straight line till it strikes another core-
cladding boundary. Here again it
undergoes TIR and is reflected back
into the core. In this way, through multiple TIRs, the ray of light propagates through the fibre and
emerges from the other end.
Structure of Optical Fibre Cable (OFC)
An optical fibre cable contains six optical
fibres uniformly distributed around a
central insulated steel cable which provides
strength to the OFC. Six insulated copper
wires are placed in the space between the
optical fibres. These are used for electrical
signal transmission, if required. The optical
fibres, copper wires and steel cable are
tightly held together using mylar binding
tape. Aluminum foil is then wrapped
around it to provide shielding from outside
interferences. A thick plastic jacket is put
on top to provide protection from dust, water and damage.
Expression for Acceptance Angle ( )
Consider an optical fibre having
core of refractive index and
cladding of refractive index .
Let the surrounding have
refractive index . (
> ). Let a ray a light be incident
from the surrounding on one
end of the fibre at an angle of
incidence . The ray refracts at
an angle and strikes the core-
cladding boundary at an angle .
If is greater than the critical
Fibre Optics Prof. Harison Cota, Don Bosco College of Engineering, Fatorda Page 2
