What are the different types of mirrors?

           Archaeological evidences had established that mirrors made of polished metal were in use during the Iron Age. The Egyptians had silver and bronze mirrors by 2500 B.C.

           A mirror is made of a sheet of glass on the back of which is sprayed a thin layer of silver or aluminium. It produces images by reflection. They obey the laws of reflection. A ray of light strikes the mirror and is reflected into our eyes. The kind and size of the images depend on the shape and structure of the mirror. Do you know how many types of mirrors are there?

          The mirrors can have flat or curved surfaces. A flat mirror is called a plane mirror. The image seen in a plane mirror is called a virtual image. Although we can see it, it cannot be projected onto a screen. This is because the image appears to be formed behind the mirror. The image in a plane mirror is erect but laterally inverted, i.e. the right-hand side of an object becomes the left-hand side in the image and vice versa. The image is always of the same size as the object. The plane mirrors are used in homes for seeing oneself, decorative purposes and in many optical instruments. 

          There are two different types of curved mirrors - convex and concave. The convex mirrors curve outwards and concave mirrors inwards. The image in a convex mirror is always smaller than the object. The image is erect and virtual. The convex mirrors are used as review mirrors in automobiles. They allow the driver to see a large area of the road behind him.

          The image in a concave mirror depends on how far the object is from the mirror. If the object is placed close to the mirror, its image is virtual and magnified. A magnified image is larger than the object. Since they have magnifying properties, the concave mirrors are used as shaving mirrors. If the object is beyond a certain distance from the mirror the nature of image changes. It becomes smaller than the object and upside down. The image now lies in front of the mirror and is said to be real. A real image can be projected onto a screen.

          The curved mirrors also have cylindrical, parabolic and ellipsoidal surfaces. The cylindrical mirrors focus a parallel beam of light to a line focus. A parabolic mirror may be used to focus parallel rays to a real focus. These are used in telescopes and search lights. An ellipsoidal mirror reflects light from one of its two focal points to the other.

          The mirrors are produced by coating the glass surfaces with metallic silver. This is done through a chemical process. Sometimes even copper and gold are used for the purpose. Mirrors are also being produced by vacuum deposition of silver or aluminium on glass surfaces. Most of the mirrors are coated with aluminium because as compared to silver it is less expensive.

Why do forward moving wheels sometimes appear turning backwards?

            You might have observed in a movie or on a television that the wheels of speeding automobiles sometimes appear turning backwards. Do you know what makes the forward-moving wheels appear going backwards? 

            To understand this strange optical effect, you have to understand the basic underlying principle of the motion picture. The film projected on the screen consists of a series of individual pictures. The pictures are joined together in such a manner that you can see them one after another in rapid succession. But there is a very small time, a fraction of a second, between each frame of picture when the screen is completely dark. Usually 32 pictures per second are shown on the screen, and they appear continuous because the impression of an image lasts for 1/16th of a second on the retina of our eye. 

            So in this case each individual picture shows the automobile moving along the road. However, if the wheel does not make a complete turn from one still picture to the next, it appears, as if, the wheel is moving backwards. For example, if we watch the top edge of the wheel in one picture, and if the edge does not make a complete turn in the next picture, but instead completes three-fourth of a turn, it looks as if the wheel turned one-fourth backward. Now in the third picture, with the wheel making only three-fourth of a turn, the top edge visible in the first picture is directly at the bottom. Again it appears, as if, the wheel has turned one-fourth backwards. Thus the automobile continues moving forward and wheels continue to appear to be going backwards. This is called the stroboscopic effect. 

What are the contact lenses?

            The contact lenses are worn directly on the cornea of the eyes to correct any defects of vision. All the defects that are corrected by regular eye glasses or others that cannot be corrected thus can be rectified by contact lenses.

            The first contact lenses to be used as an eye aid were made by A.E. Fick in 1887. These early lenses were first made by blowing the glasses and then by grinding and polishing bottoms of glass test tubes. These lenses were not successful and for a long time remained just a subject of academic study. However, concrete progress in this direction was made in 1938 when the plastic (methyl methacrylate) contact lenses were developed. From 1938 to 1950 many lenses were made by taking impressions of the eye and forming the lens on this mould. Such lenses had a fluid under them and covered most of the eye. 

            After 1950 smaller lenses were used that covered only the cornea which is the front surface of the eye and floated on a layer of tears. In this case it is not necessary to make an impression of the eye as the curvature of the cornea can be measured by optical instruments. Such lenses are usually 7 to 11 millimeters in diameter and 0.1 to 1 mm in thickness and can be worn all day without removing.

            To fit the contact lenses, the eyes are first tested for the vision defects just as they would be in case of spectacles. Then the radius of curvature of the eye surface is found out by using a device called keratometer. After deciding the diameter and power of the lens, the prescription goes to the manufacturer for making the lens.

            To make the contact lens the plastic rod is first sawn into sections and then turned on a lathe to make button-shaped tablets known as bonnets. Then they are given the right curvature with the help of machines for obtaining the desired power and then polished finally. The lenses are then examined to see whether they fit the eye well, or not. Finally they are worn on the cornea. With increasing acquaintance these lenses can be worn comfortably by most people for 12 hours at a stretch.

            Besides being invisible, contact lenses provide a much wider field of vision than the ordinary spectacles. They are more useful in active sports since they are not easily lost or broken and can even be tinted to protect against the sun. But contact lenses are not effective in all cases of eye trouble. They are also expensive and some people find difficulty in wearing them.

            As the research continues, even smaller and more flexible lenses are being developed in order to make them less irritating to eyes. Soft lenses of hydroxyethyle are used in modern contact lenses. 

What is a shadow?



               A shadow is formed when light is incident on an opaque object and cannot penetrate into the space immediately beyond it. In other words, a shadow is that part of an illuminated surface which is shielded from oncoming light rays by an object through which the light cannot pass. This infers that it is a dark patch or area on the ground or on any other surface created by cutting off passage of light. 




               The extent and the shape of a shadow mainly depend on the size of the source of light. If the source of light is very small the outline of the shadow will be sharp and well-defined and their shapes will that of the object producing it. But if the source is large, the shadow will be very dark in the middle, and lighter on the outside with indistinct outlines. The dark part of the shadow is called umbra, i.e. the region of complete darkness and the lighter portion is called as penumbra region.

               The shadows cast by the sun always have a penumbra and the shape of the shadows cast varies with the position of the sun in the sky and the angle of rays. An upright pole will cast a long shadow in the morning when the sun is rising but will grow shorter with approaching noon. As the sun declines in the sky, the shadow grows longer again.

               The human shadows have often had a mystical or magical significance. 

What is a Range Finder?

                    A range finder is an instrument used to measure long distances for a number of purposes, especially, by surveyors and the army. They are mainly of two types: optical range finder and laser range finder. Radar is also a form of non-optical range finder. It measures the time lapse of an electromagnetic echo and translates the time into distance. 

                    The military range finders are usually long tubes with eyepieces at the centre. The lenses and prisms are located at each end of the tube. The operator looks through the eyepiece and adjusts the prisms so that the target can be sighted through both the ends of the tube. The difference in the direction of the two lines of sight from the ends of the tube is called the parallactic angle. The angle depends upon the distance of the target. The angle is measured on a dial from which the distance of the target can be read directly.

                    There are two types of optical range finders, coincidence and stereoscopic. In the coincidence range finder, the operator looks through a single eyepiece and sees two images of the target. By turning a knob, these two images move together. When this happens, the distance to the target can be read on a dial. 

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