This section is from the book "Complete Self-Instructing Library Of Practical Photography", by J. B. Schriever. Also available from Amazon: Complete Self-Instructing Library Of Practical Photography.
By S. Lawrence (Of the Bausch & Lomb Optical Co., Rochester, N. Y.)
Function Of The Lens. From an object in front of the lens light is reflected in all directions. Some of this light is gathered in by the lens and transmitted to the plate, there to form an image of that object. The rays of light which travel from the object toward the lens are bearers of the image. The function of the lens is to assume control of these image bearing rays and convey them, steer them so to say, to the plate, in such a manner that the image there formed by them will be a true representation of the object. From each and every point of that part of the object facing it a set of rays reaches the lens and these are transmitted to the plate, where they form an image of that point - each image point thus being a representation of its corresponding object point. In order to understand how this is accomplished, it is necessary to have some knowledge of light and optics - the composition and transmission of light.
574. It is not my purpose to dwell on the theory of light but to explain in a simple manner and with the least possible use of scientific terms and mathematical formulae, the elementary principles of optics involved in lens construction, and the properties of photographic lenses as based on those principles.
575. One of the first and simplest optical truths is that light travels in a straight line as long as it continues in the same medium, or in passing from one medium into another of the same density. When it passes from one medium into another of different density, if it enters the new medium at right angles to its surface, it continues in a straight line; if it enters at an oblique angle, it is bent out of its original course and proceeds in a different direction.
576. Thus a ray of light Fig. 1 (a) entering from air into another medium, say glass, (A) at right angles to its surface - perpendicular, or normal, to the surface of the new medium - will proceed straight through, without deviation. If the two surfaces of the new medium are parallel, the direction of the ray will be perpendicular, or normal, to the second surface also, and again, in passing out, it will suffer no deviation, thus passing through and out in a straight line (a c).
Refraction. Another ray (b), entering at an oblique angle, will be bent out of its original course (b d). Glass having greater density than air, the ray, on entering, will be bent towards the normal (a c) (a line perpendicular to the surface of the new medium at the point of entry) and it will proceed in its new direction till it reaches the opposite surface (E). Passing here from a denser medium, glass, into a rarer medium (one of less density), air, at an oblique angle, it is again bent (E f), this time away from the normal
(E E). This bending is termed refraction. It takes place whenever a ray of light passes at an oblique angle from one medium into another of different density. The surface at which it takes place is called a refractive surface. The angle which the entering (incident) ray forms with the normal at the point of entry (incidence) is termed the angle of incidence; the angle formed by the refracted ray at the same point is termed the angle of refraction.
578. The degree of refraction depends upon two factors: I. - the angle at which the incident ray meets the refractive surface: II. - the relative density and consequent relative refractive power of the two media.
579. This refractive power of glass is of value in the construction of photographic lenses, for it makes it possible to control the image bearing rays, to make each set of rays emanating from a point (a point in the object) converge to a point (the corresponding point in the image) and form a correct image.