$\endgroup$ – user4552 Jun 23 '19 at 15:52 $\begingroup$ I don't think it's true in general that the combination of two thin lenses acts like a thin lens. Combined focal length (F): =f 1 f 2 /[f 1 + f 2] F = 10x(-20)/[10-20] = +20 cm The focal length of the combination is positive and so it acts as a convex lens. Two thin lenses separated by some distance : If two thin lenses are held a distance d apart from each other, then focal length of the combination is given by the relation, 1/F = 1/f 1 + 1/f 2 – d/f 1 f 2 Practice Problems : 1. Diagram. Procedure The focal length of combination will be (a) 1 cm (b) 20 cm (c) 100 cm (d) 100/11 cm 15. Two converging lenses of equal focal length 'f' are placed in contact. Photo.net Photography Forums. Find the focal length of the combination. Both of these have the same value of the refractive index. It may be true as an approximation when the lenses are close compared to their focal length. A focal System. If focal length of one of the lenses is 20 cm, then power of the other lens is (a) +6 ∙0 D (b) –3 ∙75 D Two thin lenses are in contact and the focal length of combination is 80 cm. Combination of Lenses I. While dealing with the combination of lenses or compound lenses, you may come across the following terms. FOCAL LENGTH AND POWER OFCOMBINATION OF THIN LENSES FOR Board, NEET and JEE. Find the net focal length of a combination of lenses whose focal length are Ask for details ; Follow Report by Anyajain6435 21.03.2019 Log in to add a comment With a convex lens, the real image of a distant object is formed at a distance equal to its focal length. The reciprocal of focal length in metre is called power of lens in dioptre (D). The combined focal length for two thin lenses separated by a distance a (Figure 2) is given by the equation: \mathrm{A}$ second converging lens, this one having a focal length of $60.0 \mathrm{cm},$ is located 300.0 $\mathrm{cm}$ … If f 1 and f 2 be the focal lengths of the two lenses and F be the focal length of the combination. Find the combined focal length. Could someone supply the formula for the resulting focal lengths when elements of convertible lenses are combined? A 1.20 -cm-tall object is 50.0 $\mathrm{cm}$ to the left of a converging lens of focal length 40.0 $\mathrm{cm} . A bi-convex lens of focal length = 10 cm and a bi-concave lens of focal length = 20 are kept in contact with each other. ... Focal Length of Convertible Lens Combinations. Discussion in 'Large Format' started by keith_pitman, Jun 1, 2001. keith_pitman.

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