<text><span class="style10">ptics (3 of 5)</span><span class="style7"></span><span class="style10">The lens</span><span class="style7">A lens is a piece of transparent material made in a simple geometric shape. Usually at least one surface is spherical, and often both are. In diagram 3 the features of lenses are described. Under appropriate conditions a lens will produce an image of an object by refraction of light. It does this by bending rays of light from the object.Some rays are refracted more than others, depending how they arrive at the surface of the lens. The lens affects the velocity of the rays, since light travels more slowly in a dense medium such as the lens than in a less dense medium such as air. In this way, the expanding geometric wavefront that is generated by the object is changed into a wavefront which, for a </span><span class="style26">convex</span><span class="style7"> or </span><span class="style26">converging lens</span><span class="style7">, converges to a point behind the lens. If the object is located a long way from the lens (strictly an </span><span class="style26">infinite</span><span class="style7"> distance, but a star is an excellent approximation for practical purposes) this point is known as the </span><span class="style26">rear focal point</span><span class="style7"> or </span><span class="style26">principal focus</span><span class="style7"> of the lens. Notice that a lens has two principal foci - one on each side. The distance between the optical center of the lens and the principal focus is the </span><span class="style26">focal length (f)</span><span class="style7">. If a point source of light is placed at the principal focus of the convex lens, the rays of light will be refracted to form a parallel beam.Because of the effects of dispersion, the distance from the lens at which red light and blue light from an object will be focused will be different. This can be demonstrated in the color fringes that can be seen in simple hand magnifiers (small magnifying glasses). Such fringes are unacceptable in, for example, camera lenses. A lens made from two different types of glass can be made to bring two colors to exactly the same focus with only a very small variation for other frequencies. Such a lens is called </span><span class="style26">achromatic</span><span class="style7">. Single-element lenses are, therefore, only used for simple applications. Lenses for cameras, binoculars, telescopes and microscopes are made with many elements, with different curvatures. These lenses are made from glasses with different refractive indices and dispersions. The additional elements allow the lens designer greatly to reduce the faults or </span><span class="style26">aberrations</span><span class="style7"> of the lens.</span><span class="style10">Mirrors</span><span class="style7">Mirrors are reflecting optical elements. Plane mirrors are used to deviate light beams without dispersion or to reverse or invert images. Curved mirrors, which usually have spherical or parabolic surfaces, can form images, and are often used in illumination systems such as car headlamps.Mirrors can be coated with metals such as aluminum or silver, which have high reflectance for visible light (or gold for the infrared). Alternatively, they may be coated with many thin layers of non-metallic materials for very high reflectances over a more restricted range of frequencies. A freshly coated aluminum mirror will reflect about 90% of visible light. Special mirrors, such as those used in lasers, can reflect over 99.7% of the light at one frequency.Mirrors do not introduce any chromatic aberrations into optical systems. Those with large diameters are also lighter than glass lenses of equivalent size. For these reasons they are always used as the primary reflectors of large astronomical telescopes.</span></text>
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<text><span class="style10">. Properties of lenses.</span><span class="style7"> The lens at the top is </span><span class="style26">convex</span><span class="style7"> (i.e. it is thicker in the middle), while the one below it is </span><span class="style26">concave</span><span class="style7"> (i.e. it is thinner in the middle).</span></text>
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<text>ΓÇó THE HISTORY OF ASTRONOMYΓÇó QUANTUM THEORY AND RELATIVITYΓÇó WAVE THEORYΓÇó ELECTROMAGNETISMΓÇó ATOMS AND SUBATOMIC PARTICLESΓÇó PHOTOGRAPHY AND FILMΓÇó RADIO, TELEVISION AND VIDEOΓÇó TELECOMMUNICATIONSΓÇó SEEING THE INVISIBLE</text>
