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 enlargersaresimilar 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.

Lenses

No doubt the most widely used optical device is the lens, and that notwithstanding the fact that we see the world through a pair of them. Lenses date back to the burning glasses of antiquity, and indeed who can say when people first peered through the liquid lens formed by a droplet of water.

Lenses are made in wide range of forms; for example, there are acoustic and microwave lenses; some of the latter are made of glass or wax in easily recognizable shapes, whereas others are far more subtle in appearance. In the traditional sense, a lens is an optical system consisting of two or more refracting interfaces, at least one of which is curved. Generally the nonplanar surfaces are centered on a common axis. These surfaces are most frequently spherical segments and are coated with thin dielectric filmes to control their transmission properties. A lens that consists of one element is a simple lens. The presence of more than one element makes it a compound lens. A lens is also classified as to whether it is thin or thick, that is, whether its thickness is effectively negligible or not. We will limit ourselves, for the most part, to centered systems (for which all surfaces are rotationally symmetric about a соmmon axis) of spherical surfaces. Under these restrictions, the simple lens can take the diverse forms. Lenses that are variously known as convex, converging, or positive are thicker at the center and so tend to decrease the radius of curvature of the wavefronts. In other words, the wave converges more as it traverses the lens, assuming, of course, that the index of the lens is greater than that of the media in which it .is immersed. Concave, diverging, or negative lenses, on the other hand, are thinner at the center and tend to advance that portion of wavefront, causing it diverge more than it did upon entry.

In the broadest sense, a lens is a refracting device that is used to reshape wavefronts in a controlled manner. Although this is usually done by passing the wave through at least one specially shaped interface separating two different homogeneous media it is not the only approach available. For example, it is also possible to reconfigure a wavefront by passing it through an inhomogeneous medium. A gradient-index, or GRIN, lens is one where the desired effect is accomplished by using medium in which the index of refraction varies in a prescribed fashion. Different portions of the wave propagate at different speeds, and the front changes shape as it progresses. In the commercial GRIN material the index varies radially, decreasing parabolically out from the central axis.

Today GRIN lenses are still fabricated in quantity only in the form of small-diameter, parallel, flat-faced rods. Usually grouped together in large arrays, they have been used extensively in such equipment as facsimile machines and compact copiers. There are other unconventional lenses, including the holographic lens and even the gravitational lens (where, for example, the gravity of galaxy bends light passing in its vicinity, thereby forming multiple images of a distant celestial object, such as quasars).

No lens is a thin lens, in the strict of having thickness that approaches zero. Yet many simple lenses, for all practical purposes, function in a fashion eqivalent to that of a thin lens. Almost all spectacle lenses, which, by the way, have been used at least since the thirteenth century, are in this category. When the radii of curvature are large and the lens diameter is small, the thickness will usually be small as well. A lens of this sort would generally have a large focal length, compared with which the thickness would be quite small; many early telescope objectives fit that description perfectly.