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Fibre Optic Cables (see separate page on this - it is a very important use with many applications)

Telecommunications

Two types of optical fibre are used in telecommunications:

Step-index fibre - The core is made from on type of glass while the outer cladding has a lower refractive index.

Graded-index fibre - The refractive index of the material gradually decreases outwards from the centre of the fibre.

Before optical fibres had been developed, telecommunications used copper cables. Because of copper's resistance (changing some of the energy into heat), signals were reduced and had to be re-amplified every few kilometres. Compared to copper cables, optical fibres are far more efficient, less bulky and heavey and much cheaper (they are made from sand!).

Local Area Networks

Cable TV

CCTV

Astronomy

If light from several stars or galaxies needs to be studied simultaneously (for example to analyse red-shift or spectra). Optical fibres are then bundled together and placed at the focus of a telescope in a block. Each optical fibre receives light from parts of the image of the sky, fibres then lead the light to an instrument where it can be studied by translating the ligt into an electrical signal and feeding the collected data into a computer.

Optical Fibre Sensors

The endoscope or fibroscope

An endoscope is any instrument used to look inside the body. Thousands of optical fibres are bundled together in an endoscope which is inserted into a human body so that the doctor can 'see' inside. Light can be directed down the fibres even if they are bent, allowing the surgeon to illuminate the area under observation (an incoherent bundle is used to do this!). S/he can then view this from a television camera linked to a monitor by coherent fibres.
Usually consisting of a fiber-optic tube attached to a viewing device, endoscopes can be used to explore and biopsy such areas as the colon and the bronchi of the lungs. By employing miniature television cameras and tiny surgical implements thy allow not only exploration but also endoscopic surgery. Through small incisions; such surgery is much less traumatic to the patient than traditional open surgery. Recovery times are shorter, and less anaesthetic is required (sometimes none!).

Examples of surgical uses:

- Laparoscopic surgery, in which the endoscope is inserted through a small incision in the abdomen or chest, is used to correct abnormalities of the ovaries and as an alternative to traditional gall bladder and chest surgery.

- Arthroscopic surgery is endoscopic surgery performed on joints such as the knee or shoulder.

Endoscopes are widely used both in medical and vetenary practices. The physics principle on which they are based is total internal reflection within a fibre optic bundle of fibres.

Prismatic Optical Instruments

Some optical instruments, such as periscopes and binoculars use prisms instead of mirrors to reflect light around corners. This is because mirrors do not reflect light as totally as prisms do (mirrors only reflect about 95% of that reflected by prisms under TIR conditions). Also refraction distortion can result in using a glass fronted mirror. Therefore the image is crisper and brighter. In prismatic binoculars, total internal reflection in prisms is used to extend the path length between objective and eyepiece, effectively `folding' the optical path. This makes them compact and easy to carry.

Bicycle Reflector

If you position two mirrors at a right angle to each other, all of the incoming light will bounce twice and then retrace approximately the same path on its way out. (Try it out, and you'll discover thatt the image that you see in the mirror-pair is NOT laterally inverted as it would be in a single mirror!). If you put THREE mirrors together and look into the corner of them, you'll see an upside-down, unreversed image of your face. And no matter how you twist the mirrors or move your head, the image of your face will stay in the same spot. This device is called a CORNER-CUBE REFLECTOR. It returns incoming light back to its source.

Bicycle reflectors are composed of hundreds of tiny Corner-Cube Reflectors formed into the plastic - a 'Corner-Cube Array'.

When you look deeply into a white bicycle reflector close up you will notice that it looks black. The black colour is actually the upside-down image of your eye's dark pupil! If the reflector facets were lots bigger, you'd see an image of your eye within each one. Gaze at the reflector while slowly moving the edge of a white piece of paper across your eye, and just before it blocks your vision, you'll see small white bits appear in the facets of the bicycle reflector.

If you take one apart, you'll find that the faceted back of the plastic is NOT a metal-coated mirror. In fact, if it was metallized, it would only reflect about 80% of the light; same as normal mirrors. Without the metal, it reflects 100% of the light (if it is made of transparent plastic!) by using Total Internal Reflection instead. The reflectors, however, are manufactured in different colours of plastic material so that they only reflect a portion of the light in the desired colour (e.g. 'red' on the back of a bike). The coloured plastic works like a filter - (if you cycle see here).