=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. [Illustration: 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. [Illustration: FIG. 380.--Wave diagram of light passing through a concave lens.] =386. The Formation of Images by Lenses.=--If a beam composed of _parallel_ rays of light, as sunlight, is sent in turn through three convex lenses of the same diameter but of different thickness, it is found that the _thicker the lens the greater is its converging power, or the shorter is its focal_ length. (See Fig. 381.) Now if a luminous body, such as a lighted candle, be placed near the convex lens but _beyond its focal length_, the light will be brought to a focus upon the other side of the lens and an image of the candle may be clearly seen upon the screen placed at this point. (See Fig. 382.) _The two points so situated on opposite sides of a lens that an object at one will form an image at the other are called conjugate foci._ [Illustration: FIG. 381.--The thicker the lens, the shorter is its focal length.]