1:13 PM old.uqu.edu.sa 569 28 oF 50 LASORATOR Focal Length of Lenses OBJECTIVES

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1:13 PM old.uqu.edu.sa 569 28 oF 50 LASORATOR Focal Length of Lenses OBJECTIVES Investigate the properties of converging and diverging lenes. Determine the focal length of converging lenes both by a seal image of a dient dbjet aad by finite object and image distances. Determine the focal length of a diverging kns by using2inccelbintimwacueverp glms to foem a real image. EQUIPMENT LIST Optical bench, holders for lenses, a screen to form images, meter stick, tape ·Lamp with obed on face (illuminated object), three knse( «M +1o,-30on) THEORY When a beam of light rays parallel to the central anis of a lens is incident upon a convenging lens the rays are brought together at a point caled the focal point of the ens The distance from the center ofthe lens to the focal point is called fthe focal length of thw kms, and it potveu-y for converging lens. When a parallel beam of light rays is incident upon a divenging lens the rays diverge asthey leave the lens; however, if the paths of thw outgang tayarv traced badwaed, the tays appear to have emerged froen a point called the focal point of the lens The distance froem the center od the lens to the focal point is called the focal length fo the lens, and it is a negative quantity foe a diverging lens. Figure 41-1 two common types of lenses are pictured ln general, a k Bomverging or diverging depending upon the eurvabure ol its surlaces In Figure 41-1 the nd of urvature of the surfaces of the two lenses are denoted as Ry and Re The relationship that determines the focalength fin terms of the radu of curvature and the indes of refraction n of the glass o the ens is caled the lens makers equation. It is E4 1) For the coverging ens shown in Figure 41-3a) the radius Ry is positive and the radiR sgatie but foe the diverging lens od part (, the radius &, is negative and the adius R is pasitive. The signs of these radii are determined according to a sign convention that is described in all eementary bnbools

Explanation / Answer

We have taken lens C as a diverging (concave) lens which cannot form an image. or more precisely, which cannot form a real image as the rays which pass through the lens always diverge and form an image at the point of apparent divergence. Hence, the lens C cannot form a real image alone.

The solutin is to put a convex (convergin) lens in contact with the lens C so that it forms a real image of the virtual image obtained from the lens C.

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