Scan-reading Optical Instruments

An optical instrument uses mirrors, lenses, prisms or gratings, singly or in combination, to reflect, refract or otherwise modify light rays. Optical instruments, especially microscopes and telescopes, have probably broadened man’s intellectual horizons more than any other devices he has made.

Perhaps the best way to understand the operation of optical instruments is by geometrical optics- a method that deals with light as rays instead of waves or particles. These rays follow the laws of reflection and refraction as well as the laws of geometry.

Images formed by mirrors and lenses may be either real or virtual and of a predictable size and location. A real image, as formed by a camera or projector, is an actual converging of light rays and can be caught on a screen; virtual images cannot. The rays from object points do not pass through corresponding points of a virtual image. Images seen in binoculars are virtual.

Optical prisms are transparent solids of glass or other material whose opposite faces are plane but not necessarily parallel. They are used to bend light rays by refraction or internal reflection. The amount of bending depends on the refractive index of the prism, the angle between its faces, and the angle of incidence of the light. Since the refractive index depends also on the wavelength, prisms are often used to disperse a light beam into its spectrum.

Lenses form an image by refracting the light rays from an object. Curved glass lenses were first used as simple magnifiers in the 13th century, but it was not till nearly 1600 that the microscope was devised, followed by the telescope a decade or so later. Mirrors, which form an image by reflecting light rays, had already been known for several centuries and were easier to understand. A lens, however, has an advantage over a mirror in that it permits the observer to be on the opposite side from the incoming light.

Microscopes, projectors and enlargers are similar in principle, but they differ in purpose and design. In each, a positive lens forms a real image of a brightly illuminated object. With projectors, the image is caught on a screen; with microscopes, it is viewed through an eyepiece; and with photographic enlargers, the image is projected on light sensitive paper, where it ^- is recorded in semi-permanent form.

But description of light as traveling along rays is only approximately true; it gave us the simplest way of explaining making an image.

Light and color are so much a part of our lives that we often overlook their fundamental importance to many businesses such as astronomy, optics photography, television and many others.

Telescopes enlarge the image of far-off objects. Two types of telescopes in common use are refracting telescopes and reflecting telescopes. Refracting telescopes are often used as terrestrial (land-use) viewers. They consist of an objective lens, a long tube, and an eyepiece lens. Light rays from an object are refracted through a convex objective lens and form a real image in the tube of the telescope. However, the real image is less than one focal length of the convex eyepiece lens. As a result, the eye of the viewer sees the image of the object as a virtual image, inverted and enlarged. The magnification of a refracting telescope is found by dividing the focal length of the objective lens by the focal length of the eyepiece lens.

A reflecting telescope works in much the same way, but it uses mirrors instead of objective lenses to collect the light rays from an object. The incident light rays enter the telescope's tube and strike a concave mirror at the base of the tube. As the rays are reflected off the base mirror, they strike a mirror in the tube. The newly reflected light rays then converge at a focus in front of the eyepiece and the viewer sees an enlarged image.

Reflecting telescopes can be more powerful than refractors because large mirrors can collect more light than lenses can.