Optical Instruments

A ray of light entering the eye passes through the transparent cornea, the aqueous humor, the lens, and the vitreous humor. All help focus the light before it strikes the rods and cones, which are photoreceptors located on the retina. Here is where the actual process of seeing begins. The greatest bending of light rays occurs at the first surface of the cornea.

A group of ligaments and muscles automatically control the shape of the lens to bring objects at different distances into focus on the retina. This process is called accommodation. As one gets older, the lens gradually loses it flexibility, and the ability to accommodate decreases. [p. 84]

The human eye is the most versatile of al radiation detectors. Within the retina of the eye a chemical response to radiation is translated into electrical pulses. These very weak electrical messages travel almost instantaneously to the brain along the optic nerve fibers. The sensation of sight occurs in the brain. Because of psychological factors, the quality of visual sensations cannot be translated into physical data and there is no way to compare the visual sensations of different people with accuracy. What you see is for the most part subjective--wholly within the mind, and therefore usually not measurable. [p. 90]

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 convening 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. [p. 58]

Mirrors are the oldest and most widely used optical instruments. The plane mirror is the simplest image-forming device. Plane mirrors are found in every home. Spherical or parabolic mirrors are often used in optical instruments instead of lenses..... A plane or flat mirror produces a virtual image since the light rays do not come directly from the image. Rays from an object are redirected by the mirror so that they appear to come from an image located as far behind the mirror as the object is in front of the mirror. Object and image are the same size, but the image is reversed from left to right. [p. 58]

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. [p. 60]

A Lens forms 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 from an image by reflecting light rays, had already been known for several centuries and were easier to understand [back to the B.C.'s]. 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. [p. 64]

Simple Positive Lenses (also known as converging lenses) are single pieces of glass that are thicker at their centers than at their edges. Each surface is a section of a sphere, and a line though the two centers of curvature is the optic axis. Light passing through a lens is bent toward the thicker part of the glass. Light rays parallel to the optic axis are bent by the lens so as to converge at the focal point of the lens. Similarly, light coming from the opposite direction converges at a second focal point an equal distance on the opposite side of the lens. The distance from the center of the lens to the focal point is the focal length of the lens. All positive lenses are thicker at their centers than at their edges. They range from very thin lenses with surface of little curvature to thick lenses that are nearly spherical in shape..... Best known and widely used as a simple magnifier is the double convex lens whose surfaces usually, but not always, have the same curvature.... [pg. 64]

Simple Negative Lenses (also called diverging lenses) are thicker at the edges than at the center. A negative lens alone cannot form a real image as a positive lens does. Light passing through a negative lens parallel to the optic axis is bent away from the axis. The focal point of the negative lens is located by extending these diverging rays backward until they cross the axis. The image formed by a diverging lens is always virtual, uptight, and smaller and closer to the lens than the object.

Negative lenses are used to reduce images, to correct nearsightedness and to construct compound lenses. [pg. 66]

Magnification is the ratio of length of image to length of object. It equals the distance of the image divided by the distance of the object from the lens. Hence, an image will be larger than the object only if it is farther from the lens. The shorter the focal length of a lens, or the greater its convex curvature, the greater its magnifying power. This power, expressed in diopters, is the focal length of a lens in meters divided into 1. A lens with a focal length of 25 cm. (1/4 m.) has a magnifying power of 4 diopters. [pg. 65]

Astigmatism of the eye is due to a fault in the curvature of the cornea or lens. If either curved surface is not symmetrical, rays in different planes will not be focused at the same distance behind the lens. Thus part of the image will be out of focus. This defect can usually be corrected by wearing spectacles with cylindrical lenses instead of spherical ones.... Almost two out of three persons have at least a mild form of astigmatism. [p. 70]

The telescope was invented by a Dutch optician, Hans Lippershey, in 1608, some 300 years after the invention of spectacles. Galileo received news of this invention in 1609, and without seeing the original, he constructed a telescope consisting of one positive and one negative lens mounted in a discarded organ pipe. The same optical arrangement is used more efficiently in opera glasses, which are usually binocular. The positive lens is called the objective, and the negative lens is the ocular, or eyepiece. [p. 72]

Binoculars are used, like a small telescope, to view distant objects. They employ an optical system of lenses and prisms to produce an enlarged erect image. The ocular and the objective lenses provide the magnification and illumination. Between them is a pair of 45-90-45 degree prisms so arranged that the light passing through the binoculars is internally reflected four times, making the image erect..... Three factors are involved in the usefulness of binoculars--magnification, field of view, and light-gathering power. Magnification must be suited to the purpose. Any movement by either the observer or the observed is magnified at high power..... Higher magnifications require a tripod or other support. Field of view is largely determined by the ocular lenses. The diameter of the objective lens determines the light-gathering power--the larger the better if binoculars are used at night or in shady woods. Binoculars have central or individual focus (central preferred). They range in magnification from about 2 to 20. Each binocular has an identification mark such as 8 x 30 or 7 x 50. The first number is the magnification, the second the objective diameter in millimeters. An 8 x 30 glass has slightly greater magnification but distinctly less light-gathering power than a 7 x 50. [p. 76]

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.

Microscopes need intense illumination of the object because the image is much larger than the object and the same amount of light must be spread over a large area. The illumination is provided either by a tungsten lamp bulb or by an arc lamp, its light concentrated on the object by a lens or by a concave mirror used as a condenser. The microscope's objective lens has a short focal length (from 1 cm to less than 1mm ) and produces a sharp image of a very small field.

Image Sharpness depends on resolving power rather than on magnifying power, and is limited by diffraction effects that blur the image. Two points separated by a distance less than one half the wavelength of light cannot be resolved optically and will appear as one point instead of two. With the usual illumination, this separation is approx. 1/100,000 of an inch.

The Electron Microscope, by using a beam of electrons with an effective wavelength much shorter than that of light, obtains over 100 times the resolving power of optical microscopes.

The Magnification achieved with an optical microscope is approx. equal to the power of the objective multiplied by the power of the ocular or eyepiece, producing an overall magnification of up to about 1,500 diameters.

Projectors use high-intensity tungsten or arc lamps for illumination. Two condensing lenses concentrate the light rays though the object (usually a film slide). The converging rays pass on through the projection lenses, and the enlarged image is thrown on a screen. This is an inverted image, so slides are inserted upside down in a projector. With any given combination of lenses, the farther the image is projected the larger it will be, and greater lamp intensity will be required. With some projectors an opaque object can be reflected on the screen.

Photographic Enlargers are precision projectors with adjustments for focusing the image and controlling image size and brightness. Good enlargers provide uniform illumination, a good lens system, and a rigid mount. In operation, light from a lamp is concentrated by a parabaloidal reflector, passes through a diffusing glass (or a condensing lens), continues through he negative and then through the projection lens, which forms an enlarged image of the negative on the easel. Image sharpness is adjusted by moving the projection lens relative to the negative. [p. 77-78]

Camera comes from the Latin phrase camera obscura, or dark chamber, for all picture-taking instruments have a dark chamber to protect the sensitive film from light. The simplest camera is a light-proof box with a pinhole in one end and piece of film on the opposite inside wall. Light reaches the film only when the pinhole is uncovered, usually for a few seconds. Use of a lens instead of a pinhole allows much more light to pass through, and the same picture can be taken in much shorter time. If the area of the lens is 1,000 times as large as the pinhole, the picture which required ten seconds can be made in 1/1000 second, an average speed in many modern cameras. In addition to a lens and film, a camera usually has a shutter, an adjustable diaphragm, and some type of focusing adjustment. The shutter prevents light from striking the film except when a picture is being made. A mechanism opens the shutter and closes it automatically after a length of time. Camera shutters may have speeds from 10 seconds to 1/1,000 of a second. Some cameras take pictures at a millionth of a second, enabling man to see the unseeable. The diaphragm can be adjusted to admit varying amounts of light each time the shutter is open. The focusing mechanism moves the lens back and forth to achieve a clear image. [p. 79]

The Shutter of a camera allows light from the subject to enter the camera and strike the film for a time. Shutters are located either at the lens or at the film. Lens shutters are usually placed between or just behind the elements of the lens and usually have a set of leaves that snap open for the desired time and then snap shut. Focal plane shutters are next to the film and resemble a window blind with slots cut in it. A modern version has two curtains. As one moves and uncovers the film, the second follows so closely that the opening between them is a mere slot. Exposure time is varied by adjusting the width of the slot. [p. 80]

The f-Number of a lens refers to the ratio of its diameter (d) to its focal length (f). For example, f/8 refers to an aperture whose diameter is 1/8 of the focal length. A camera's lowest f-number corresponds to its wide-open diaphragm and is called the speed of the lens. Using a lower f-number shortens the required exposure time, but reduces the depth of field. The diaphragm is set to the highest f-number that can be used with a given shutter speed in order to get maximum depth of field. Many modern cameras have a built-in photoelectric cell that selects automatically the lens opening for the shutter speed used. [p. 80]

Focus is achieved when light from an object passes though a camera lens and forms a clear, accurate image on the film or viewer. Light rays from an object point diverge slightly as they enter a camera. With a wide-open lens, the rays pass through all parts of the lens and converge to the image point. To focus the camera, the distance from the lens to the film is adjusted so that the point of convergence will be at the film surface--not before or behind it. It is impossible to focus all points of a three-dimensional scene on the film at the same time, but a satisfactory sharpness can usually be achieved over a considerable depth of the scene.

Adjustment is most critical when focusing on a nearby object with wide-open lens. It is least critical for a distant object with the smallest lens opening. The accuracy of the focusing may be judged by observing the sharpness of the image on a translucent glass surface in reflect-type cameras or by superimposing two images in a rangefinder in some other cameras. [p. 81]

Depth of Field is the depth of a scene that is in focus on the camera's film. A large lens aperture allows only a limited depth to be in focus. The rest of the scene is fuzzy. With a smaller opening, the cone of light rays from lens to film converges and diverges less. Both near and distant objects are in better focus. [pg. 81]

Photometers are instruments for comparing the intensities of two light sources which have approximately the same hue.... [p. 82]

Colorimeters are instruments designed to measure color characteristics other than intensity.... [p. 83]

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