Tiêu đề 9. RAY OPTICS AND OPTICAL INSTRUMENTS.pdf ✅

Ray Optics and Optical Instruments Upright image means virtual image m = + ve (for virtual image m = – ve (for real image) Object distance is always negative. For real image, image distance is negative. For virtual image, image distance is positive. Do not use sign of convention for unknown. Answering Tip Concept Applied 1 1 v u + = 1 f 3. Which mirror does have a real focus? U [OEB] 4. Why are convex mirror preferred over plane mirrors as rear view mirrors? U [OEB] 5. Name the mirror which are used: (i) As Security mirror (ii) In Solar furnace Ap [OEB] Ans. (i) Convex mirror (ii) Concave mirror 1⁄2+1⁄2 = 1 Short Answer Type Questions-I (2 marks each) 1. Use the mirror equation to show that an object placed between f and 2f of a concave mirror forms an image beyond 2f. E [Foreign 2017; Delhi 2015] Ans. Applied concept: 1 1 v u + = 1 f (f is – ve) 1⁄2 If, u = – f Þ 1 v = 0 Þ v = ¥ 1⁄2 If, u = – 2f Þ 1 v = −1 2 f 1⁄2 Þ v = – 2f Hence, if – 2f < u < – f then, – 2f < v < ¥ 1⁄2 [Marking Scheme, 2017] 2. A concave or convex mirror is held under water. Will its focal length change? U [OEB] 3. State two positions in which a concave mirror produces a magnified image of a given object. List two differences between the two images. U [OEB] Ans. A concave mirror forms a magnified image of an object when: Case-1: Object is in between centre of curvature (C) and principal focus (F) of the mirror. Case-2: Object is in between principal focus (F) and pole (P) of the mirror. Difference: In first case the image is real and inverted while in second case, the image is virtual and erect. 1+ 1 Short Answer Type Questions-II (3 marks each) 1. An object is placed in front of a concave mirror of focal length of 12 cm. There are two possible positions of the object for which the image formed is three times the size of the object. (a) Draw the ray diagram for the each case, and (b) Find the distance between the two positions of the object. U & A [OD Set 2, 2020] Concept Applied 1 1 1 f v u = + and m = hi h0 . Ans. (a) When the object is placed between the pole and the focus, a magnified, virtual and erect image is formed by concave mirror. 1⁄2 When the object is placed between centre of curvature and focus, a magnified, real and inverted image is formed by concave mirror. 1⁄2 (b) For virtual image; Applied concept: h h v u i 0 = − = 3 \ v = – 3u Putting in mirror formula: 1 1 1 v u f + = 1⁄2 or, 1 3 1 1 − 12 + = u u − ∴ = u −8 cm 1⁄2 For real image; h h v u i 0 = − = 3 \ v = 3u 1⁄2