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The reason why light moves down the fibre and does not come out of it depends on the properties of the glass or plastic. Light travelling down the fibre strikes the edge of the fibre with
a glancing angle and so
is continually reflected back
inside the fibre. The light never approaches the edge of the fibre
at a sharp enough angle to get
out (see: Critical angle). The light moves down the fibre by bouncing, or ricocheting, off the fibre walls.
All fibres are coated in a protective glass or plastic. A glass- coated fibre is made by placing a rod inside a glass tube and then heating and drawing out the rod and the tube. They form very fine fibres.
Fluorescence
Taking in invisible waves and changing them so that they become visible. The coating on a fluorescent tube does this, as do the tiny spots of material that cover the inside of a television tube.
The materials that do this are called phosphors. (See also: Luminescence; Phosphorescence.)
f-number (f-stop)
Camera lenses often have f-numbers around the rim. The numbers are related to the amount of light that goes through the lens. The ‘f’ in this case stands for ‘fraction’. Each f-number is the fraction of the focal length of the lens. Typical numbers are f/2.8,
f/4, f/5.6, f/8, f/11 and f/16. In
this sequence each higher number indicates a smaller lens opening that allows only half as much light to pass than the number immediately smaller than it. A camera set at f8 only allows in half as much light
as a camera set at f5.6. (See also: Aperture.)
 Fibre optics – A glass fibre keeps light inside because the light always strikes the side of the glass at more than the critical angle and is always reflected back inside the glass.
Focus, focal length, focal plane
The point at which rays of light converge, having passed through a lens or been reflected from a mirror.
The focal length is the distance from the lens to the point at which rays of light from a long way
away come together (converge) to a point. The focal length depends on the degree of curving of the mirror and the type of glass or plastic used to make the lens.
Lenses of longer focal length gather light from a smaller angle than those with a small focal
length. Such lenses, when used
on cameras, are called telephoto lenses. Lenses with a small focal length are called wide-angle lenses.
The focal plane is the upright plane running through the focal point. An image of an object
on it will be clear but inverted (upside down and back to front). Light from close-up objects comes together at a point behind the focal plane and will not be in focus. That is the reason why the position of the lens has to be changed (i.e. focused) in a camera depending on the distance away of the object that needs to be in focus.
 Focus – The focus of a lens is the place where parallel rays of light, from a source a long distance away, come together. The focal plane runs through this point.
Focus, focal length, focal plane
Focus
Focal length
Focal plane
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