JumpStart - Physical Science
IIntroduction To Lens*

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Introduction

A simple lens is a piece of glass or plastic having two polished surfaces that each form part of a sphere or ball. One of the surfaces must be curved; the other surface may be curved or flat. An example of a simple lens would be obtained if a piece of a glass ball were sliced off as shown in the illustration on the left. The piece of the ball sliced off would be a lens with a spherical side and a flat side. Lenses can be made in a variety of shapes for various applications. Some examples of lens shapes are illustrated below.

A lens thicker in the center than at the edge is called a converging or positive lens. A lens thinner at the center than at the edge is called a diverging or negative lens. In the illustration shown, lenses 1, 2, and 3 are converging or positive lenses. Lenses 4 and 5 are diverging or negative lenses.

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The Image Formed by a Converging Lens

When using a thin lens, that is, the thickness at the center of the lens is not too great, a thin lens mathematical approximation can be used. This approximation assumes the bending of light occurs in one plane inside the lens. A ray of light coming from a very distant object, such that the ray is parallel to the optical axis, will be bent by refraction at the two surfaces of the lens and will cross the optical axis at the focal point (f) of the lens, as seen in the illustration below. A ray passing through the center of the lens will pass through the lens undeviated.

The size and location of an image formed by a lens can be found by using the information from these two rays which is shown in the illustration below. The following illustration depicts two rays, which are defined in the following text. A ray (1) parallel to the optical axis passes through the focal point (f). A ray (2) passing through the center of the lens is undeviated. The image is real, smaller than the object, and upside down. If a piece of paper is placed at the image location, a real image can be seen on the paper. An example of this is taking a picture with a camera, where the photographic film is located at the image position.

A Simple Magnifier

When the object lies between the lens and the focal point, a virtual, upright, and enlarged image is obtained, as seen in the illustration below. Three rays are included in the illustration. Following are descriptions of these rays. A ray (1) leaving the object parallel to the optical axis will bend at the lens and go through the focal point (f). A ray (2) leaving the object going through the center of the lens will be undeviated. A ray (3) leaving the object as if it came from the front focal point of the lens will bend at the lens and travel in a line parallel to the optical axis. After passing through the lens, the three rays described above will appear to come from an enlarged and upright image. Any other ray leaving the tip of the object will appear to come from the tip of the image after passing through the lens. The three rays used in the illustration below were chosen because their paths are always known. Two rays are actually enough to locate the image, while the third ray is used for an additional check of the location of the image.

URL: http://spacelink.nasa.gov/products/Optics/