Text. Lenses

Lenses are bodies made of transparent material and bound­ed by faces having a cylindrical or spherical form. Although lenses differ much in form they may be divided into two classes according to the way in which they act on a parallel beam of light. Consider the lens in Fig. 5, a on which parallel rays are incident. Each ray is bent toward the

Fig. 5. Principal foci of simple lenses.

normal to the surface on entering the lens and away from the normal on emerg­ing from the lens. In this way, the rays above the axis PO are bent downward and those below it are bent upward. After leaving the lens, the rays converge to a point F, called the principal focus. Such a lens is a converging lens. If the incident rays are parallel to each other, the incident wave front is a plane perpendicular to the incident rays. When this wave front emerges from the lens, it has been changed to a concave wave front that converges to the focus. When the bounding surfaces of the lens are very convex, the lens converges the rays rapidly. This gives the lens a short focal length. When the bounding surfaces are slightly convex, the lens has a long focal length. In order that all the rays may come to a point after leaving the lens,

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the beam of light must be restricted to a narrow bundle near the principal axis of the lens. For an extended beam the outer rays will not pass through the same point as the rays near the axis. When the surfaces of the lens are concave instead of convex, the lens makes the rays that pass through it more divergent, and for this reason it is known as a diverging lens. In Fig. 5, b parallel rays are incident on a concave lens. On entering the lens, the rays are bent toward the normal "as before, and on leaving they are bent away from the normal. In this case, however, the emerging rays are bent away from the principal axis. They appear on leaving the lens to come from a point F behind the lens. When the incident rays are parallel to each other and to the principal axis, this point from which the rays appear to come on leaving the lens is the principal focus. This is only an apparent focus because the light does not really come from it, but the effect on the left-hand side of the lens is the same as if the light actually came from this point behind the lens. This kind of focus from which the light appears to come is a virtual focus. It is to be carefully distinguished from a real focus through which the light actually goes.

If the lens is convex, and the incident rays come from a point source, the incident wave front is spherical, and the emerging wave front is the surface of a sphere with its centre at the image. If the lens is concave, the curvature of the wave front is increased in passing through the lens, and the image is behind the lens.

Listen and read words and word combinations to be remembered:

transparent [traens'pearant] concave [ konkeiv] вогнутый

прозрачный convex ['konveks] выпуклый

emerge [i'me:dj] выходить restrict [ris'trikt] ограничивать

converge [kan've:dj] cxo диться extend [iks'tend] расширяться

bundle ['bAndl] пучок diverge [dai've:dj] pacxoДИТЬСЯ -

apparent [a'paerant] мнимый virtual ['vetjual] действительный

image ['imidj] изображение normal ['no:mel] перпендикуляр

bound [baund] граничить curvature ['ke:vatja] изгиб

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principal focus главный фокус

converging lens собирательная линза

diverging lens рассеивающая линза

bounding surface граничная поверхность

focal length фокусное расстояние

principal axis оптическая ось

emerging wave front выходящий фронт волны

incident wave front падающий фронт волны

real focus действительный фокус

point source точечный источник

Give Russian equivalents to the following words without using a dictionary:

lenses ['lenziz] parallel ['paralel]

material [ma’tieriel] perpendicular [ ,pe:pen'dikju la]

cylindrical [si'lindrikal] outer ['auta]

spherical ['sferikal] effect [i'fekt]

form [fo:m] pass [pa:s]

front [frAnt] focus [foukas]

Models