Online+lesson+--+Lenses

Online lesson - Lenses
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The Anatomy of a Lens
If a piece of glass or other transparent material takes on the appropriate shape, it is possible that parallel incident rays would either converge to a point or appear to be diverging from a point. A piece of glass which has such a shape is referred to as a lens. A **lens** is merely a carefully ground or molded piece of transparent material which refracts light rays in such as way as to form an image. Lenses can be thought of as a series of tiny refracting prisms, each of which refracts light to produce their own image. When these prisms act together, they produce a bright image focused at a point. There are a variety of types of lenses. A **converging lens** is a lens which converges rays of light which are traveling parallel to its principal axis. Converging lenses can be identified by their shape; they are relatively thick across their middle and thin at their upper and lower edges. A **diverging lens** is a lens which diverges rays of light which are traveling parallel to its principal axis. Diverging lenses can also be identified by their shape; they are relatively thin across their middle and thick at their upper and lower edges. 

As we begin to discuss the refraction of light rays and the formation of images by these two types of lenses, we will need to use a variety of terms. Many of these terms should be familiar to you because they have already been discussed during [|Unit 13]. If you are uncertain of the meaning of the terms, spend some time reviewing them so that their meaning is firmly internalized in your mind. They will be essential as we proceed through Lesson 5. These terms describe the various parts of a lens and include such words as If a symmetrical lens is thought of as being a slice of a sphere, then there would be a line passing through the center of the sphere and attaching to the mirror in the exact center of the lens. This imaginary line is known as the **principal axis**. A lens also has an imaginary **vertical axis** which bisects the symmetrical lens into halves. As mentioned above, light rays incident towards either face of the lens and traveling parallel to the principal axis will either converge or diverge. If the light rays converge (as in a converging lens), then they will converge to a point. This point is known as the **focal point** of the converging lens. If the light rays diverge (as in a diverging lens), then the diverging rays can be traced backwards until they intersect at a point. This intersection point is known as the **focal point** of a diverging lens. The focal point is denoted by the letter **F** on the diagrams below. Note that each lens has two focal points - one on each side of the lens. Unlike mirrors, lenses can allow light to pass through either face, depending on where the incident rays are coming from. Subsequently, every lens has two possible focal points. The distance from the mirror to the focal point is known as the **focal length** (abbreviated by **f** ). Technically, a lens does not have a center of curvature (at least not one which has any importance to our discussion). However a lens does have an imaginary point which we refer to as the **2F point**. This is the point on the principal axis which is twice as far from the vertical axis as the focal point is. As we discuss the characteristics of images produced by converging and diverging lenses, these vocabulary terms will become increasingly important. Remember that this page is here and refer to it as often as needed.
 * Principal axis || Vertical Plane ||
 * Focal Point || Focal Length ||