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Sahay Sir > Question Answers > NCERT Exemplar Class 12 > Physics > Chapter 9 - Ray Optics and Optical Instruments > a) Determine the effective focal length of the combination of the two lenses of focal length 30cm and -20 cm. if they are placed 8.0cm apart with their principal axes coincident. Does the answer depend on which side of the combination a beam of parallel light is incident? Is the notions of effective focal length of this system useful at all? b) An object 1.5 cm in size is placed on the side of the convex lens in the arrangement a) above. The distance between the object and the convex lens is 40cm. Determine the magnification produced by the two-lens system, and the size of the image.

a) Determine the effective focal length of the combination of the two lenses of focal length 30cm and -20 cm. if they are placed 8.0cm apart with their principal axes coincident. Does the answer depend on which side of the combination a beam of parallel light is incident? Is the notions of effective focal length of this system useful at all? b) An object 1.5 cm in size is placed on the side of the convex lens in the arrangement a) above. The distance between the object and the convex lens is 40cm. Determine the magnification produced by the two-lens system, and the size of the image.

Consider the diffraction pattern for a small pinhole. As the size of the hole is increased