Transcript Optical Fibres

Noadswood Science, 2013
Optics

Saturday, April 2, 2016
To be able to calculate the refractive index using the critical
angle of a substance
Optical Fibres

Optical fibres can carry information coded in light or infrared
signals – they can carry more information than an ordinary
cable of the same diameter

Information such as computer data and telephone calls can
be converted into electrical signals – this information can
also be converted into either visible light signals or infrared
signals, and transmitted by optical fibres

Optical fibres can carry more information than an ordinary
cable of the same thickness – the signals in optical fibres do
not weaken as much over long distances as the signals in
ordinary cables
Optical Fibres
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An optical fibre is a thin rod of high-quality glass (very little
light is absorbed by the glass)

Light getting in at one end undergoes repeated total internal
reflection, even when the fibre is bent, and emerges at the
other end
Optical Fibres
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Find the critical angle for the glass block for total internal
reflection to occur where the light getting in at one end
undergoes repeated total internal reflection, even when the
fibre is bent, emerging at the other end...
Set up a ray box so a light ray
goes through the glass block –
rotate the glass block until the
light ray comes out from the side
Optical Fibres

Light getting in at one end undergoes repeated total internal
reflection, even when the fibre is bent, and emerges at the
other end
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The critical angle for glass is about 42°

Diamond’s have a critical angle of 24° – this is why they
sparkle so much, due to many internal reflections
Total Internal Reflection
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Total internal reflection occurs when light is coming out of
something dense, such as glass, water or perspex

If the angle is shallow enough the ray will not come out at all,
but it reflects back into the material – this is total internal
reflection
Total Internal Reflection
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Angle of incidence is less than critical angle

Most of the light passes through the air, but a little bit of it is
internally reflected
Total Internal Reflection
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Angle of incidence = critical angle

The emerging ray comes out along the surface, with quite a
lot of internal reflection
Total Internal Reflection

Angle of incidence is greater than the critical angle

No light comes out as it is all internally reflected (total
internal reflection)
Optical Fibres - Uses

Optical communications have several advantages over
electrical signals in wires:  The signal does not need boosting as often
 A cable of the same diameter can carry much more
information
 The signals cannot be tapped into, or suffer interference
from electrical sources

Normally no light would be lost at each reflection, however
some light is lost due to imperfections in the surface, so
boosters are needed every few km
Critical Angle
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The value of the critical angle (c) depends on the refractive
index of the material

A dense material with a high refractive index has a low
critical angle

This is why diamonds sparkle so much (much more light is
internally reflected as it has a small critical angle)
Refractive index = 1 ÷ sin c
Endoscopes
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Endoscopes are used to look inside people – they are a
narrow bunch of optical fibres with a lens system at each end
(with another set of optical fibres to carry light down inside)
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The image is displayed on a monitor, meaning operations
can be undertaken without the need to cut large holes in the
patient
Optics
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Complete the optics worksheet
Questions
1.
2.
It is totally internally reflected
Questions
3.
The first bundle is used to shine light into the stomach. The
second is used to see the inside of the stomach; a tiny lens
over the bundle forms an image on the ends of the fibres,
and the image can then be seen directly.
4.
Laser light can be used to cut or burn away and destroy
diseased tissue. It can also seal off (cauterise) leaking blood
vessels