REAL-IS-POSITIVE CONVENTION All distances are measured from the optical centre. Distances of real objects and real images are positive. Distances of virtual objects and images are negative. The focal length of a converging lens is +ve (real focus). The focal length of a diverging lens is -ve (virtual focus).
NEW CARTESIAN CONVENTION All distances are measured from the optical centre, being positive if in the same direction as the incident light and negative if against it. This convention is more easily applied if the object is always placed to the left of the lens, in which case all distances to the left of the lens are […]
Sign conventions When using the lens formula mentioned on page 267 correct signs have to be applied to object and image distances and focaiiengths, according to the convention chosen.
Optical fibres in telecommunications One application of optical fibres IS their increasing importance 1D telecommunications. Research is already far advanced and production of the necessary equipment has begun. It is expected that, by the end of the century, if not before, optical fibres will have largely replaced copper cables, not only for telephone conversations but also for transmitting pictures, […]
Optical fibres Total internal reflection (page 255) can be used to convey light through a thin glass fibre (Fig. 24.24), with very little loss of energy, provided the angle of incidence is greater than the critical angle. A bundle of such fibres forms a flexible light guide. The glass cladding of slightly lower refractive index than that […]
The astronomical telescope The function of a telescope is to increase the angle which a distant object appears to subtend at the eye, and therefore produces the same effect as if the object were either larger or else closer to the eye. Fig. 24.23 shows how this is done by an astronomical telescope. For a very distant object, e.g., […]
Angular magnitude and apparent size It is well known that street lamp-posts appear to be shorter the further they are away, although, in fact, all are of the same height. The angular magnitude of an object is the angle it subtends at the eye. This angle determines the size of the image formed on the retina […]
The compound microscope This instrument employs two lenses of short focal length arranged as shown in Fig. 24.21. The first of these, called the objective, produces an enlarged, real, inverted image 11 of a small object O. The image II then acts as an object for the eye lens, which gives a virtual, image 12 which is still […]
Defects of vision and their correction The so-called normal eye can accommodate for clear vision of objects from infinity (the far point) down to about 25 em (the near point). We shall now consider the case of a person with long sight who can see objects at infinity but whose near point is somewhat further than 25 […]
The eye Fig. 24.18 is a simplified diagram of the human eye. In many respects it is similar to the camera. It has a tough, white wall called the sclerotic of which the front portion, the cornea, is transparent. Situated in the aqueous humour in front of the eye lens is the iris orcoloured part of the eye […]