The Shape and Material of a Lens
The main or principal focus of a lens, that is, the point at which rays parallel to the base line AB
meet, depends upon the shape of the lens. For example, a thick lens, such as A
, focuses the rays very near to the lens; B
, which is not so thick, focuses the rays at a greater distance from the lens; and C
, which is a very thin lens, focuses the rays at a considerable distance from the lens. The distance of the principal focus from the lens is called the focal length of the lens, and from the diagrams we see that the more convex the lens, the shorter the focal length.
The position of the principal focus depends not only on the shape of the lens, but also on the refractive power of the material composing the lens. A lens made of ice would not deviate the rays of light so much as a lens of similar shape composed of glass. The greater the refractive power of the lens, the greater the bending, and the nearer the principal focus to the lens.
There are many different kinds of glass, and each kind of glass refracts the light differently. Flint glass contains lead; the lead makes the glass dense, and gives it great refractive power, enabling it to bend and separate light in all directions. Cut glass and toilet articles are made of flint glass because of the brilliant effects caused by its great refractive power, and imitation gems are commonly nothing more than polished flint glass.
FIG. - The different types of lenses.
FIG. - The more curved the lens, the shorter the focal length, and the nearer the focus is to the lens.