Fig. 20.
When light falls on a transparent body, part is reflected and part is refracted. The angle which the ray makes with the normal, or perpendicular, to the surface at the point of contact is known as the angle of incidence, and the angles which the reflected and refracted rays make with the same normal are known respectively as the angle of reflection and refraction. The reflected ray makes the same angle with the normal as the incident ray, while the refracted ray, when passing from a rarer to a denser medium, is bent toward the normal, and vice versa; the denser the medium into which the ray passes the greater is the deviation. This law allows us at once to understand the action of a lens, which may be defined as a transparent medium that from the curvature of its surface causes the rays of light traversing it to either converge or diverge. The ordinary lenses have either spherical surfaces or a combination of spherical and plane surfaces. This combination will give rise to six classes ([fig. 20]): (a) Double convex; (b) plano convex; (c) double concave; (d) plano concave; (e) converging, and (f) diverging meniscus. Those lenses which are thicker at the center than at the edges are converging or concentrating lenses, and those which are thicker at the edges than the center are diverging.
FOCUS—OPTICAL CENTER.
The focus of a lens is the point where the refracted rays or their prolongation meet; if the rays themselves intersect after refraction the focus is real, and if their prolongations meet the focus is virtual. The line passing through the centers of curvature of the two surfaces of a lens is called the principal axis and contains a point known as the optical center, which has the property by virtue of which, if a ray passes through it, the ray will not be deviated. The optical center can always be found by drawing two radii parallel to each other, one from each center of the curvature of the surface until the radii intersect their respective surfaces, then draw a line joining these two points. The intersection of this last line with the principal axis will give the optical center.
IMAGE—CONJUGATE FOCI.
Let AB be the section of a double convex lens and C and D ([fig. 21]) be the centers of curvature of the two surfaces. Draw the lines CD′ and DE from C and D parallel to each other, then join D′ and E by a straight line. The point O will be the optical center of the lens. Let us take a point R, on the principal axis as a source of light; the ray RD passes through the optical center and is not deviated. The ray RK on striking will be refracted in the direction KG toward the perpendicular to the surface KD in accordance with the law of refraction, as glass is denser than air. On emerging at G it is refracted away from the perpendicular to the surface CG, since it passes from a denser to a rarer medium, and will intersect the ray RD at the point R′. In a like way the ray RK′ will be found to intersect the ray RD at the same point, R′, which is the focus for all rays coming from R. The point R′ is said to be the image of the object R, and when the two points are considered together they are called conjugate foci. If the incident beam is composed of parallel homogeneous light, the rays will all be brought to a focus at a point on the principal axis, called the principal focus of the lens, and the distance of this point from the optical center is the principal focal length, which is always a fixed quantity for any given lens.
Fig. 21.