The common properties of optical instruments

Optical instruments come in all shapes and sizes, from fly-on-the-wall surveillance cameras to 10 metre segmented astronomical reflecting telescopes, and in shapes from microscopes to sextants to periscopes to spectrographs to cine-projectors.

Whatever their purpose, they all have two things in common.

1. (1) They are image-forming devices, intended to make a picture, to form an image of a luminous source. The image may be on a cinema screen, on a photographic emulsion, on a CCD surface or on the retina of an eye.

2. (2) They are, with one important exception, centred systems. That is to say they comprise a series of curved surfaces of transparent materials or reflecting materials or both. The centres of curvature of the various elements all lie on a straight line called the optic axis. Light passes from the object, through successive elements until it emerges to form an image.

This is a slight over-simplification of course. There are occasional plane reflectors along the path, as in a periscope for example, but these are for convenience rather than for any peculiar optical properties they possess.

Optical elements

There are four basic optical elements: the lens, the mirror, the diffraction grating and the prism. What follows now concerns the first two of these. The others have chapters of their own.