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PRACTICE PAPER BOARD PREPARATION Class: XII Subject: Physics Question Paper (Theory) Topic: OPTICS Maximum Marks: 25 Marks Time Allowed: 60 minutes Sr. no 1 2 3 4 Marks Section A All questions are compulsory. In case of internal choices, attempt any one of them. Differentiate a ray and a wavefront. Which part of electromagnetic spectrum is used for LASIK eye surgery, and why? The radius of curvature of the curved surface of a plano-convex lens is 20 cm. If the refractive index of the material of the lens be 1.5, it will (a) act as a convex lens only for the objects that lie on its curved side. (b) act as a concave lens for the objects that lie on its curved side. (c) act as a convex lens irrespective of the side on which the object lies. (d) act as a concave lens irrespective of side on which the object lies. Section B Questions 10 Case Study based questions and are compulsory. Attempt any 4 sub parts from each question. Each question carries 1 mark. Newton s rings is a phenomenon in which an interference pattern is created by the reflection of light between two surfaces: a spherical surface and an adjacent Touching flat surface. It is named after Issac Newton, who investigated the effect in his 1704 treatise Optics. When viewed with monochromatic light, Newton s rings appear as a series of concentric, alternating bright and dark rings centred at the point of contact between the two surfaces. When viewed with white light, it forms a concentric ring pattern of rainbow colors, because the different wavelengths of light interfere at different thickness of the air layer between the surfaces. (i) In an interference experiment the ratio of amplitudes of coherent waves is 1 1 = 3. The ratio of the maximum and minimum intensities of fringes will be 2 (a) 4 (b) 9 (c) 2 (d) 18 (ii) Two coherent sources of intensity 10 W m 2 and 25 W m 2 interfere to form fringes. Find the ratio of maximum intensity to minimum intensity. (a) 15.54 (b) 16.78 (c) 19.72 (d) 18.39 (iii) Which of the following does not show interference? (a) Soap bubble (b) A thick film (c) Excessively thin film (d) Wedge shaped film (iv) In a Young s double-slit experiment, the slit separation is doubled. To maintain the same fringe spacing on the screen, the screen-to-slit distance D must be changed to (a) 2D (b) 4D (c) D/2 (d) D/4 1 1 1 4 5 (v) Two slits in Young s double slit experiment have widths in the ratio 81 : 1. The ratio of the amplitudes of light waves is (a) 3 : 1 (b) 3 : 2 (c) 9 : 1 (d) 6 : 1 Section C All questions are compulsory. In case of internal choices, attempt anyone. In a double-slit interference experiment, the two coherent beams, have slightly different intensities I and (I + dI) where dI << I. Show that the resultant intensity at the maxima is nearly 4I, while at the minima is nearly 6 7 8 9 10 2 | |2 4 The ratio of the intensities at minima to the maxima in the Young s double slit experiment is 9 : 25. Find the ratio of the widths of the two slits. Section -D All questions are compulsory. In case of internal choices, attempt anyone. A ray of light incident on the horizontal surface of a glass slab at 70 just grazes the adjacent vertical surface after refraction. Calculate the critical angle and refractive index of the glass. (a) A Galilean telescope obtains the final image erect (like in a terrestrial telescope) without an intermediate erecting lens. It does so by using a diverging lens for its eyepiece. Show that the angular magnification of a Galilean telescope is given by the formula: m = f0/fe (negative sign because fe is negative). (b) What is the main disadvantage of this type of telescope? An object of size 3.0 cm is placed 14 cm in front of a concave lens of focal length 21 cm. (a) What is the nature, size and position of the image formed? (b) What happens if the object is moved further away from the lens? Section E All questions are compulsory. In case of internal choices, attempt anyone. A beam of light consisting of two wavelengths, 800 nm and 600 nm is used to obtain the interference fringes in a Young s double slit experiment on a screen placed 1.4 m away. If the two slits are separated by 0.28 mm, calculate the least distance from the central bright maximum where the bright fringes of the two wavelengths coincide. 2 3 3 3 5
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