Download Total Internal Reflection and Critical Angle File

Total Internal reflection
• Ifa ray of light is passing from a medium of
higher refractive index to a medium of
lower refractive index under certain
conditions all of the light can be reflected
internally.
• This effect is traditionally investigated
using a semicircular glass block.
THE SEMICIRCULAR GLASS BLOCK
If a light ray is incident on the
semi-circular glass block along
its radius there is no deviation
of the ray because the light
strikes normal to the glass
As the the angle of
incidence (θ1) is increased
the refracted ray moves
further from the normal until
the angle of refraction (θ2) =
900
If the angle of incidence on the
inner face of the block is less
than a critical value. Refraction
occurs as expected.
θ1
θ1
θ2
θ is the critical angle. For
the glass air boundary
the value of the critical
angle is around 420.
When the angle θ1 is greater
than the critical angle all of the
light is reflected. The light now
obeys the laws of reflection
rather than those of refraction
θ1
Deriving the value of the critical
angle at the glass air boundary
Using
n1 sin 1  n2 sin  2
The critical angle θc occurs when the angle of refraction is 900.
n2 = 1 and sin θ2=1
n1 sin  c  1
θ1
1
sin  c 
n1
900
For glass with n= 1.5
sin  c  0.67
  420
Flash
Investigate refraction and TIR.
Click anywhere on the image. If your computer does not load the simulation here,
it can be found at
http://www.upscale.utoronto.ca/PVB/Harrison/Flash/Optics/Refraction/Refraction.html
More generally
denser medium
less dense medium
n1
n2
θ
n1 sin  c  n2
sin  c 
n2
n1
Where n1 is the refractive index of the more dense material
Optical Fibre
Optical fibres are replacing electrical cables
for the transmission of information.
Some Advantages:
• They can carry much more data
•Light signals are switched rapidly using
pulsed lasers to encode information rather
than fluctuating electric currents.
•Unlike currents the signals do not interfere
with each other
•Less data loss occurs
•They are immune from e.m. interference
Optical Fibre
A simple (uncladded) optical fibre consists of a fibre of
glass of uniform refractive index.
Light which strikes the fibre walls at an angle greater than
the critical angle is totally internally reflected in this way
light can be guided through the fibre by multiple reflections.
The glass is extremely pure and has a refractive index of
about 1.45.
Calculate the critical angle.
Optical Fibre
With a simple fibre like this the critical angle is fairly small
Angle greater than about 420
This means that quite a large range of possible angles of reflection can
be involved in the transmission of a pulse of light.
Optical Fibre
A way to improve on the performance of the fibre is to use a cladded fibre,
the cladding being of a less dense material with a slightly lower refractive index
than the core fibre.
This is referred to as a STEP-INDEX fibre
Calculate the critical angle if the core fibre has a refractive index of
1.45 and the surrounding fibre has a refractive index of 1.38
Some Advantages
1. The inner fibre is protected.
2. When the fibres are bundled adjacent central fibres cannot touch to leak
signal across the bundle
3. Only those rays close to the central axis of the fibre are transmitted (fewer
spurious signals)
4. A reduction in the range of possible angles for TIR mean that the light rays
in a pulse travel about the same distance and arrive at the same time. This
is known as a reduction in multipath dispersal
5. The number of reflections is reduced. This leads to
a) Less data loss
b) The time for transmission is shorter.