Two thin lenses are in contact and the focal length of combination is 80 cm. While dealing with the combination of lenses or compound lenses, you may come across the following terms. Find the combined focal length. Combination of Lenses I. 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 Diagram. Photography Forums. Find the focal length of the combination. ... Focal Length of Convertible Lens Combinations. Discussion in 'Large Format' started by keith_pitman, Jun 1, 2001. keith_pitman. $\endgroup$ – user4552 Jun 23 '19 at 15:52 The reciprocal of focal length in metre is called power of lens in dioptre (D). FOCAL LENGTH AND POWER OFCOMBINATION OF THIN LENSES FOR Board, NEET and JEE. Could someone supply the formula for the resulting focal lengths when elements of convertible lenses are combined? Two converging lenses of equal focal length 'f' are placed in contact. The focal length of combination will be (a) 1 cm (b) 20 cm (c) 100 cm (d) 100/11 cm 15. It may be true as an approximation when the lenses are close compared to their focal length. If f 1 and f 2 be the focal lengths of the two lenses and F be the focal length of the combination. \mathrm{A}$ second converging lens, this one having a focal length of $60.0 \mathrm{cm},$ is located 300.0 $\mathrm{cm}$ … A focal System. Both of these have the same value of the refractive index. 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} . 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. Procedure The combined focal length for two thin lenses separated by a distance a (Figure 2) is given by the equation: 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. $\begingroup$ I don't think it's true in general that the combination of two thin lenses acts like a thin lens. 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. 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.