Optics, branch of physical science dealing with the propagation and behavior of light. In a general sense, light is that part of the electromagnetic spectrum that extends from X rays to microwaves and includes the radiant energy that produces the sensation of vision. The study of optics is divided into geometrical optics and physical optics, and these branches are discussed below.

      

Lenses made with surfaces of small radii have the shorter focal lengths. A lens with two convex surfaces will always refract rays parallel to the optic axis so that they converge to a focus on the side of the lens opposite to the object. A concave lens surface will deviate incident rays parallel to the axis away from the axis, so that even if the second surface of the lens is convex, the rays diverge and only appear to come to a focus on the same side of the lens as the object. Concave lenses form only virtual, erect, and diminished images. If the object distance is greater than the focal length, a converging lens forms a real and inverted image. If the object is sufficiently far away, the image is smaller than the object. If the object distance is smaller than the focal length of this lens, the image is virtual, erect, and larger than the object. The observer is then using the lens as a magnifier or simple microscope. The angle subtended at the eye by this virtual enlarged image is greater than would be the angle subtended by the object if it were at the normal viewing distance. The ratio of these two angles is the magnifying power of the lens. A lens with a shorter focal length would cause the angle subtended by the virtual image to increase and thus cause the magnifying power to increase. The magnifying power of an instrument is a measure of its ability to bring the object apparently closer to the eye. This is distinct from the lateral magnification of a camera or telescope, for example, where the ratio of the actual dimensions of a real image to those of the object increases as the focal length increases.

      

Convex Lens A convex lens curves outward; it has a thick center and thinner edges. Light passing through a convex lens is bent inward, or made to converge. This causes an image of the object to form on a screen on the opposite side of the lens. The image is in focus if the screen is placed at a particular distance from the lens that depends upon the distance of the object and the focal point of the lens. The lens in the human eye is convex, but unlike a glass lens, it is elastic so that it can change shape to focus on objects at varying distances. The lens becomes short and fat when viewing close objects and elongated and thin when viewing distant objects. Sometimes eye muscles are unable to focus light on the retina, the screen at the back of the eyeball. If the image forms behind the retina for nearby objects, a condition called farsightedness (hyperopia) results. Convex lenses are prescribed for hyperopics to assist the eye in making light converge on the retina for nearby objects.

      

Concave Lens A concave lens is curved inward; it is shaped like two dishes placed back-to-back. Light passing through a concave lens bends outward, or diverges. Unlike convex lenses, which produce real images, concave lenses produce only virtual images. A virtual image is one perceived by the visual portion of the brain and appears as a smaller image just in front of the actual object (in this case a shamrock). Concave lenses are generally prescribed for myopic, or near-sighted, people. Concave lenses help the eyes to produce a focused image on the retina instead of in front of it.

      

Magnifying Glass A magnifying glass is a large convex lens commonly used to examine small objects. The lens bends incoming light so that an enlarged, virtual image of the object (in this case a mushroom) appears beyond it. The image is called virtual because it is only perceived by the viewer’s brain, and cannot be produced on a screen.

      

Polarized Light Polarized light consists of individual photons whose electric field vectors are all aligned in the same direction. Ordinary light is unpolarized because the photons are emitted in a random manner, while laser light is polarized because the photons are emitted coherently. When light passes through a polarizing filter, the electric field interacts more strongly with molecules having certain orientations. This causes the incident beam to separate into two beams, whose electric vectors are perpendicular to each other. A horizontal filter absorbs photons whose electric vectors are vertical (above). The remaining photons are absorbed by a second filter turned 90° to the first. At other angles the intensity of transmitted light is proportional to the square of the cosine of the angle between the two filters. In the language of quantum mechanics, polarization is called state selection. Because photons have only two states, light passing through the filter separates into only two beams.

      

Camera Lenses The lens is as important a part of a camera as the body. Lenses are referred to in generic terms as wide-angle, normal, and telephoto. The three terms refer to the focal length of the lens, which is customarily measured in millimeters. Focal length is defined as the distance from the center of the lens to the image it forms when the lens is set at infinity. In practice, focal length affects the field of view, magnification, and depth of field of a lens. Cameras used by professional photographers and serious amateurs are designed to accept all three lens types interchangeably. In 35-mm photography, lenses with focal lengths from 20 to 35 mm are considered wide-angle lenses. They provide greater depth of field and encompass a larger field (or angle) of view but provide relatively low magnification. Extreme wide-angle, or fisheye, lenses provide fields of view of 180 degree or more. A 6-mm fisheye lens made by Nikon has a 220-degree field of view that produces a circular image on film, rather than the normal rectangular or square image. Lenses with focal lengths of from 45 to 55 mm are referred to as normal lenses because they produce an image that approximates the field of view of the human eye. Lenses with longer focal lengths, called telephoto lenses, constrict the field of view and decrease the depth of field while greatly magnifying the image. For a 35-mm camera, lenses with focal lengths of 85 mm or more are considered telephoto. A fourth generic lens type, the zoom lens, is designed to have a variable focal length, which can be adjusted continuously between two fixed limits. Zoom lenses are especially useful in conjunction with single-lens reflex cameras, for which they allow continuous control of image scale. Microsoft Illustrations & Literature Courtesy of "Optics," Microsoft (R) Encarta. Copyright (c) 1994 Microsoft Corporation. Copyright (c) 1994 Funk & Wagnall's Corporation.