Total Internal Reflection And Optical Fibres (T3) Flashcards
What is total internal reflection?
Waves going from a dense medium to a less dense medium speed up at the boundary.
Beyond a certain angle, called the critical angle, all the waves reflect back into the glass.
We say they are totally internally reflected.
What is the critical angle for most glass?
42 degrees
What is an optical fibre and how do they work?
An optical fibre is a very thin rod of high quality glass.
Light passes along it without any ever escaping from the sides.
Light getting in at one end undergoes repeated total internal reflection, even when the fibre is bent, and emerges at the other end.
The glass cannot be bent around a corner so tight it allows light that entered at the end to be refracted out of the side, as light will always strike the side at an angle greater than the critical angle.
When are optical fibres used in the medical world and how do they work?
- in endoscopes that allow surgeons to see inside their patients. Light travels down one bundle of fibres and illuminates the object to be viewed. Light reflected by the object travels up a second bundle of fibres. An image of the object is created by the eyepiece.
- in keyhole surgery.
Optical fibres carry enormous amounts of info as …..
Pulses of light
How do you calculate the critical angle?
sin c = 1 / n
Where…
c = the critical angle in degrees n = the refractive index
What is ‘total internal reflection’ and what is the name of the point at which is occurs?
- total internal reflection is the phenomenon of light being reflected back into the glass
- the angle of incidence at which is occurs is called the critical angle
- for most materials like glass, water and perspex the critical angle is around 40-50 degrees
- for gems such as diamond, it is much lower
What is the simplest form of optical fibre?
- an unclad fibre
- simply a thin glass tube that light refracts through
When would a clad fibre be used and how does it work?
- in practise, if two or more fibres run alongside each other and their sides touch, the light could pass from one to the next
- in such situations, a clad fibres are used
- they have outer cladding of optically less dense glass with a low refractive index and an inner core of optically dense glass with a high refractive index
- the difference in the refractive indexes means total internal reflection occurs at the internal boundary between the two materials instead of the external surface
Describe how analogue signals can be passed along optical fibres…
- analogue signals would mean that the brightness of the light would have to vary to represent the signal picked up by a microphone
- high frequency sounds would cause the brightness to change more times per second than low frequency sounds
- louder noises would be represented by more extreme variations in the brightness
- several signal could be passed at the same time using different colours of light, but this technique is rarely used because digital techniques allow far more signals to be passed down a single fibre at the same time at the same or better quality
Describe how digital signals are passed down optical fibres…
- sound signals must be converted into binary using an analogue to digital converter, then the ones and zeroes are sent in rapid succession, up to billions of times per second
- at the receiving end a digital to analogue converter gives a signal which can then be amplified and fed to loudspeakers
- thousands of phone signals can be sent down the same optical fibre at the same time by sending a short burst of each one in sequence, but the process must be controlled by a computer at each end
- fewer signals can be sent at the same time if higher quality signals like TV are used, but in either case optical fibres perform better, can deliver more signals and are less costly than old fashioned copper cable
