When a ray of light passes through a piece of plane glass, such as a window pane (Fig. 67), it is refracted at the point B toward the perpendicular, and continues its course through the glass in the new direction BC.  On emerging from the glass, the light is refracted away from the perpendicular and takes the direction CD, which is clearly parallel to its original direction.  Hence, when we view objects through the window, we see them slightly displaced in position, but otherwise unchanged.  The deviation or displacement caused by glass as thin as window panes is too slight to be noticed, and we are not conscious that objects are out of position.

[Illustration:  FIG. 67.—­Objects looked at through a window pane seem to be in their natural place.]

111.  Chandelier Crystals and Prisms.  When a ray of light passes through plane glass, like a window pane, it is shifted somewhat, but its direction does not change; that is, the emergent ray is parallel to the incident ray.  But when a beam of light passes through a triangular glass prism, such as a chandelier crystal, its direction is greatly changed, and an object viewed through a prism is seen quite out of its true position.

Whenever light passes through a prism, it is bent toward the base of the prism, or toward the thick portion of the prism, and emerges from the prism in quite a different direction from that in which it entered (Fig. 68).  Hence, when an object is looked at through a prism, it is seen quite out of place.  In Figure 68, the candle seems to be at S, while in reality it is at A.

[Illustration:  FIG. 68.—­When looked at through the prism, A seems to be at S.]

112.  Lenses.  If two prisms are arranged as in Figure 69, and two parallel rays of light fall upon the prisms, the beam A will be bent downward toward the thickened portion of the prism, and the beam B will be bent upward toward the thick portion of the prism, and after passing through the prism the two rays will intersect at some point F, called a focus.

[Illustration:  FIG. 69.—­Rays of light are converged and focused at F.]

If two prisms are arranged as in Figure 70, the ray A will be refracted upward toward the thick end, and the ray B will be refracted downward toward the thick end; the two rays, on emerging, will therefore be widely separated and will not intersect.

[Illustration:  FIG. 70.—­Rays of light are diverged and do not come to any real focus.]

Lenses are very similar to prisms; indeed, two prisms placed as in Figure 69, and rounded off, would make a very good convex lens.  A lens is any transparent material, but usually glass, with one or both sides curved.  The various types of lenses are shown in Figure 71.

[Illustration:  FIG. 71.—­The different types of lenses.]

The first three types focus parallel rays at some common point F, as in Figure 69.  Such lenses are called convex or converging lenses.  The last three types, called concave lenses, scatter parallel rays so that they do not come to a focus, but diverge widely after passage through the lens.