Page 125 - Numerical questions for class 10th // Chapter 1 ( light )
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| Formula |
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1. mirror formula is  where, u is the distance of the object v is the distance of the image f is the focal length 2. Lens formula is  where, u is the distance of the object v is the distance of the image f is the focal length 3. f = R/2 4. magnification (m) = h2/h1 where, h1 is the height of the object h2 is the height of the image 5. m = -v / u {for the mirror } 6. m = v / u {for the Lens } Note :- 1. When an imaginary image is formed in any mirror or lens, m is positive. 2. When a real image is formed in any mirror or lens, m is negative. 3. In any mirror, m = -v / u. 4. In any lens, m = v / u. |
| Numerical problems and their solutions |
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| Question 1. An object is placed at 60 cm in front of a concave mirror of focal length 20 cm, what will be the position and nature of the image of the object? Solution:- ∵ f = -20 cm , u = -60 cm , v = ? , Nature = ?  ∵ v is negative ∴ image is real Hence, the image distance is -30 cm and the nature of the image is real. Question2. A concave mirror is placed at a distance of 20 cm from the pole whose image is formed in front of the mirror at a distance of 10 cm from the pole, then find the focus of the mirror. Solution :- ∵ u = -20 cm , v = -10 cm , f = ?   Question 3. Where should an object be placed in front of a spherical mirror of 15 cm focus distance so that its 3 times magnified image will be obtained? Solution :- ∵ the image is enlarged ∴ the mirror is concave Hence , f = -15 cm , m = 土3 , u = ? When the image is imaginary, m = 3 since ,  When the image is real, m = -3 since ,  Question 4. The focal length of a concave mirror is 20 cm. By this 5 times magnified and straight image is formed, then find the position of the object is. Solution :- Question 5. A concave mirror of focal length 30 cm is placed at a distance of 90 cm from a wall. At what distance should an object be placed from the wall so that its real image is formed? Solution :- |
