Transcript 13.3 Total internal reflection

When light travels from an optically denser medium to a less dense medium, rays are bent away
from the normal. The incident substance has a larger refractive index than the other substance
For a larger angle of incidence,
more light is reflected as well as
the refracted ray being bent
further from the normal
When the critical angle, c, is
reached it produces an angle of
refraction of 900
c
A little light is also reflected.
For angles of incidence greater
than the critical angle,
TOTAL INTERNAL REFLECTION
occurs.
θ
θ
N.B. The angle of incidence
= The angle of REFLECTION
TOTAL INTERNAL REFLECTION
weak reflected
ray
As the angle of incidence
increases towards the critical
angle ( glass = 420 ) the
refracted ray gets weaker
and the reflected ray gets
stronger.
Critical angle depends upon the
refractive indices of the media
2
1
C
n1 sin 1  n2 sin 2
HENCE
n1 sin C  n2 sin 90
If medium 2 is air, n2 = 1, and so
1
sin C 
n1
For Water
2
AIR
WATER
1
1
sin C 
1.33
C
C = 48.80
For Crown Glass
1
sin C 
1.50
C = 41.80
Finding the Critical Angle…
1) Ray gets refracted
3) Ray still gets refracted (just!)
THE CRITICAL
ANGLE
2) Ray still gets refracted
4) Ray gets
internally reflected
Why do diamond sparkle?
* high refractive index 2.417
sin C 
1
n1
Critical angle =?
* Colours are spread out more and
TIR occurs many times inside the diamond before emerging
Controls
End probe containing
coherent bundle,
incoherent bundle, lens
and surgical instruments
Eyepiece
ENDOSCOPE
Light injected here
This is an endoscope image of
the inside of the throat. The
arrows point to the vocal chords
The endoscope is inserted
into a body cavity,
which is then illuminated
through an in coherent
bundle of fibres
A lens over the end
of the other bundle
is used to form an
image of the body
A coherent bundle means that the fibre ends at each end
must be in the same relative positions
A COHERENT BUNDLE: A bundle of
optical fibres in which the relative
spatial coordinates of each fibres are
the same at the two ends of the
bundle. Such a bundle are used for
the transmission of images.
A NON-COHERENT FIBRE bundle, as
you would expect, does not have this
precise matrix alignment since they
need only transmit light for
illumination purposes. They are
cheaper to produce.
FIBRE OPTIC COMMUNICATIONS
PROBLEM: Input light rays cannot be precisely parallel.
cladding
core
Rays taking different paths will take different times to travel along the fibre,
resulting in the jumbling of the signal.
Solution : Monomode fibre only 5μm in diameter
Optical fibres
in communications
Purpose?
Optical fibres
in communications
Optical fibres
in communications
Pulse broadening with spectral dispersion
Pulse in - white light
Pulse out with spectral dispersion
Pulse broadening with spectral dispersion
Pulse in - white light
Pulse out with spectral dispersion
Pulse broadening with spectral dispersion
Pulse in - white light
Pulse out with spectral dispersion
Other uses of total internal reflection
1) Endoscopes (a medical device used to see inside the body):
2) Binoculars and periscopes (using “reflecting prisms”)
THE MIRAGE
Water
for my hump!
air layers
Hot Desert Sand
Air layers closer to the sand are hotter and less dense.
Light from the sky is successively bent until a critical angle
is reached and then total internal reflection occurs.
A mirage "water" illusion is seen because the mind initially
interprets the light rays reaching our eyes as having come
along a straight path originating from the ground. Thus, the
image of that patch of sky we see "on the ground" is
interpreted as a surface "pool of water."