Fig. 377.—Forms of Lenses. 1. double convex; 2. plano convex; 3. concavo convex; 4. double concave; 5. plano concave; 6. convexo concave.
Fig. 378.—The action of a burning glass.
384. Effect of Lenses upon Light.—The most important characteristic of a lens is its effect upon a beam of light. Most persons have seen a "burning glass," a double convex lens, used to bring to a point, or focus, a beam of sunlight. To show the action of a burning glass send a beam of light into a darkened room, and place in its path a double convex lens. (See Fig. 378.) If two blackboard erasers are struck together near the lens, the chalk particles in the path of the light are strongly illuminated, showing that the light after passing through the lens it brought to a focus and that it spreads out beyond this point. This point to which the cone of light rays converges after passing through the convex lens is called the principal focus of the lens. The distance from the principal focus to the center of the lens is the focal length or principal focal distance of the lens. The focal length of double convex lenses of crown glass is about the same as the radius of curvature of either surface. The action of a convex or converging lens upon light may be better understood by studying Fig. 379 in which light is passing from S to F. The successive positions and shape of the advancing light waves are indicated by lines drawn across the beam. The light being retarded more in the thicker part of the lens, the light waves on leaving the lens have a concave front. Since light waves tend to move at right angles to the front of the wave, the light is brought to a focus. After passing the focus the waves have a convex front, forming a diverging cone.
Fig. 379.—Wave diagram of light passing through a convex lens.
385. Concave Lenses.—When sunlight passes through a concave lens a diverging cone of light is formed. (See Fig. 380.) This is caused by the edges of the wave being retarded more than the center, producing a convex wave front. This diverging cone of light acts as if it had proceeded from a luminous point at F.
This point is called a virtual focus and is nearly at the center of the curvature of the nearer surface.
