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What is the critical angle?
Beyond a certain angle, called the critical angle, c, all the waves reflect back into the glass.
The critical angle for most glass is about 42 °.
What is total internal reflection?
We say that they are totally internally reflected.
Which equation can be used to calculate the critical angle?
sin c = 1 ÷ refactive index
where c is the critical angle
What is an optical fibre?
An optical fibre is a thin rod of high-quality glass, high refractive index in the centre surrounded by glass with a lower refractive index.
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.
Name two uses of optical fibres.
Optical fibres are used in endoscopes that allow surgeons to see inside their patients.
Optical fibres can also carry enormous amounts of information as pulses of light.
How does an endoscope work?
Endoscopes use optical fibres. A doctor can insert a bundle of optical fibres into the body.
Some carry light into the body, and some carry light reflected off internal body surfaces back out.
This allows the doctor to see the inside of the body clearly – and help them diagnose diseases like cancer, or see what they are doing during keyhole surgery.
How is information carried over optical cables?
Information such as computer data and telephone calls can be converted into electrical signals. These are 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.
What is a laser?
A laser - 'Light Amplification by Stimulated Emission of Radiation' - produces an intense narrow beam of light. The light from a laser is monochromatic (a single colour).
Some lasers are capable of heating materials. This makes them useful in surgery and for cutting metals and other industrial materials.
Any two from: eye surgery, cauterising wounds/blood vessels, writing CDs/DVDs.
Lasers are used to scan bar codes in shops.
How would you expect a surgeon’s laser to be different from the laser in a supermarket scanner?
The surgeon’s laser would be more powerful.
The lasers in supermarkets must be safe in case a hand gets in the way or if someone looks into the laser.
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