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

A little light is also reflected.

For a larger angle of incidence, more light is reflected as well as the refracted ray being bent further from the normal

c

For angles of incidence greater than the critical angle,

TOTAL INTERNAL REFLECTION occurs.

When the critical angle, c, is reached it produces an angle of refraction of 900

θ θ N.B. The angle of incidence

= The angle of REFLECTION

TOTAL INTERNAL REFLECTION

As the angle of incidenceincreases towards the criticalangle ( glass = 420 ) therefracted ray gets weaker and the reflected ray getsstronger.

weak reflectedray

Critical angle depends upon the refractive indices of the media

2211 sinsin nn

1

1sin

nC

HENCE90sinsin 21 nCn

1

2

C

If medium 2 is air, n2 = 1, and so

1

2

C

AIR

WATER

For Water

33.1

1sin C

C = 48.80

For Crown Glass 50.1

1sin C C = 41.80

Finding the Critical Angle…1) Ray gets refracted

4) Ray gets internally reflected3) Ray still gets refracted (just!)

2) Ray still gets refracted

THE CRITICAL ANGLE

Why do diamond sparkle?

* high refractive index 2.417 Critical angle =? 1

1sin

nC

* Colours are spread out more and TIR occurs many times inside the diamond before emerging

End probe containing coherent bundle, incoherent bundle, lens and surgical instruments

Controls

Eyepiece

Light injected here

ENDOSCOPE

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

core

cladding

PROBLEM: Input light rays cannot be precisely parallel.

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 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

Pulse broadening with spectral dispersion

Other uses of total internal reflection1) Endoscopes (a medical device used to see inside the body):

2) Binoculars and periscopes (using “reflecting prisms”)

THE MIRAGE

Hot Desert Sand

air layers

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."

Water for my hump!