20.4. Thin Lenses http://www.ck12.org
20.4 Thin Lenses
Objectives
The student will:
- Understand how to draw ray diagrams for convex lenses.
- Understand how to solve problems involving convex lenses.
- Understand how to draw ray diagrams for concave lenses.
- Understand how to solve problems involving concave lenses.
Vocabulary
- concave lens:A lens that is thinner at the center than its edges. Diverging lenses make rays parallel to the
principal axis(P)diverge as shown below. - convex lens:A lens that is thicker in the center than its edges. Converging lenses make rays parallel to the
principal axis(P)converge at the focal point as shown below. - focus/foci:The point(s) at which rays parallel to an axis converge.
- optical center:The center of the lens.
- principal axis:A straight line passing through the center of the lens and perpendicular to its surfaces.
Introduction
A lens is a piece of transparent material, cut into a particular shape. Its purpose is to refract light in useful ways.
Lenses come in many shapes, as shown inFigure20.17. We will be interested inconvex(converging)lensesand
concave(diverging)lenses. There are several kinds of these lenses:
A lens can be considered thin” if it has a much greater diameter than its maximum thickness.
You probably see convex lenses and concave lenses every day. People who use reading glasses use convex lenses.
Convex lenses are also used as magnifying glasses. People who use glasses to see distant objects use concave glasses.
When a lens refracts light, it can alter the point in space where an image forms. The manner in which a lens refracts
light determines which lens to use in which pair of glasses.
All ray diagrams for lenses can, in principle, be constructed using Snell’s Law. But as is the case with mirrors and
the Law of Reflection, there is a much simpler way to approach ray construction than measuring angles and using
tedious constructions.