question_answer 20)

(a) Determine the effective focal length of thecombination of the two lenses in the question 9.10 if they are placed 8.0 cm apart with theirprincipal axes coincident. Does the answerdepend on which side a beam of parallel light isincident ? Is the notion of effective focal lengthof this system useful at all ? (b) An object 1.5 cm in size is placed on theside of the convex lens in the abovearrangements. The distance between the objectand the convex lens is 40 cm. Determine themagnification produced by the two-lens system,and the size of the image.  

Answer:

(a) As we have i.e. This image formed by convex lens appearsas an object placed at 30 - 8 cm = 22 cmfrom the convex lens on the right hand side. Using we have, i.e. The parallel beam of light appears todiverge from (220-8) ==212 cm from theconvex lens from the left. Now let the parallel beam of light fall on theconcave lens after''which the convex lens isplaced at 8 cm. Then or the image is formed -20 cm to the left ofconcave length and .'. it is 20 + 8 = 28 cmto the left of convex lens. Again, i.e. i.e. the light appears to diverge from a pointat 420 cm to the left of convex lens. Clearly the answer depends upon the side ofincidence so the notion of effective focallength is not useful in this case. (b) Using we have i.e. Magnification produced by convex lens, Now (i.e. objectis to the right of concave lens) or magnification produced by concave lens,  Net magnification Size of image = m x size of object