NEETPhysicsRay Optics
Match List-I with List-II. List-I describes different optical configurations involving glass lenses ( = 1.5 ) with a radius of curvature of 20 cm for their curved surfaces. List-II gives the equivalent focal length of the system. List-I List-II (A) An equiconvex lens in air (I) +10 cm (B) Two such equiconvex lenses placed in contact in air (II) -100 cm (C) Two plano-convex lenses placed with their curved surfaces fac
Options
- A(A)-(IV), (B)-(II), (C)-(I), (D)-(III)
- B(A)-(I), (B)-(IV), (C)-(II), (D)-(III)
- C(A)-(IV), (B)-(I), (C)-(II), (D)-(III)
- D(A)-(IV), (B)-(I), (C)-(III), (D)-(II)
Correct answer
C. (A)-(IV), (B)-(I), (C)-(II), (D)-(III)
Step-by-step solution
(A) For the equiconvex lens in air: 1 f_A = ( - 1) ( 1 R - 1 -R ) = (1.5 - 1) ( 2 20 ) = 1 20 cm ⁻¹ f_A = +20 cm . (B) For two such lenses in contact: 1 f_B = 1 f_A + 1 f_A = 1 20 + 1 20 = 2 20 = 1 10 cm ⁻¹ f_B = +10 cm . (C) The combination consists of two plano-convex lenses and a biconcave liquid lens in the middle. Focal length of each plano-convex lens: 1 f_p = (1.5 - 1) ( 1 20 ) = 1 40 cm ⁻¹ . Focal length of the liquid lens: 1 f_L = (1.6 - 1) ( 1 -20 - 1 20 ) = 0.6 (- 2 20 ) = - 6 100 cm ⁻¹ . Equivalent foca