Category Optics

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 is light?

            For thousands of years many scientists groped in dark to understand the true nature of light. The ancient Greeks believed that light consisted of rays of matter given out by whatever object was being looked at. Plato and his followers believed that it was a mixture of different matters coming from the sun. But in the 11th century it was Alhazen, the Arabic scientist who was the first to propound the theory that light could be given out by all luminous object.

            In the 17th century, the British scientist Sir Isaac Newton put forward the corpuscular theory of light. According to this theory light travels in the form of corpuscles in straight lines through imaginary medium called ether. This theory could not explain some of the observed phenomena such as interference and diffraction. In an attempt to explain these phenomena, Christian Huggens of Holland proposed the wave theory of light. He maintained that light consists vibrations at right angles to the direction of propagation. It travelled in the form of waves which spread in straight lines. He continued that the medium in which light travels was ether which was believed to be an invisible and omnipresent substance. This classical wave theory existed for hundred years. Although it explained the phenomenon of reflection, refraction, interference and diffraction of light, it could not explain the transverse nature of light. 

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