As seen in Fig. 15.3, lenses can be used to converge or diverge light, even in combination so that they cancel each other out.
FIG. 15.3 Lenses can be used in combination Convex lenses
Rules of refraction in a convex lens Once again, it is important to know the terms and rules in drawing ray diagrams for lenses. Since light passes through both types of lenses, they have two focal points, one on each side of the lens. There is no centre of curvature, but instead two points referred to as 2F (twice the focal length). They also have an optic centre (OC), which is the midpoint of the principle axis within the lens.
1. Any ray that comes into the lens parallel to the principal axis will emerge through the focal point on the other side (Fig. 15.4).
2F FIG. 15.4
F
OC
F
2F
2. Conversely, any ray coming into the lens from the focal point will emerge parallel to the principal axis (Fig. 15.5).
2F FIG. 15.5
3. Any ray that passes straight through the optic centre of the lens will continue on the same path out the other side of the lens (Fig. 15.6).
