Place a spoon in a glass of water, as shown in [Fig. 142] and it will look as if the spoon is broken in two at the point where it touches the water. The bending of the beam of light will be more clearly understood from the drawing in [Fig. 143].
If you will look at a star squarely through a thick piece of glass and the star seems to change its position a little you will know that the sides of the glass are not quite parallel.
A prism is a three-sided piece of glass, if we except the ends, as shown in [Fig. 144]. When a beam of light passes through a prism the prism affects the light in two ways: first, it bends the beam, and second, it separates the ether waves, or light waves, as they are called, according to their lengths, and as color depends on the length of the waves in the ether a prism will show the different colors on a screen and this is called the spectrum. It is shown in [Fig. 145].
Fig. 143.—How Light is Refracted.
Fig. 144.—Prism.
Lenses are pieces of glass having curved surfaces. When a beam of light passes through a lens it is also bent out of its original direction, or refracted.
A convex lens, [see Fig. 146], is a lens which is thicker in the middle than it is at the edges. A convex lens is used for magnifying an object; or for forming an image so that it can be magnified by another lens as in a telescope, for forming an image on the screen of a camera, and for bringing the heat waves of the Sun to a focus, as with a burning glass.
The point where the rays of light are brought together is called the focus. You can easily find the focus of a lens by holding a sheet of paper, or the hand under the lens and letting the sunlight pass through it; where the spot of light is smallest and brightest there is the focus. The distance of this point from the lens is called the focal length.