In an experiment, an object was placed on the principal axis of a convex lens 25 centimeters away from the lens. A real image 4 times the size of the object was obtained. The focal length of the lens is:
Correct answer: A. 20 cm
- A. 20 cm
- B. 25 cm
- C. 33 cm
- D. 50 cm
- E. 100 cm
Explanation
The explanation is given below:u = 25 cmm = -4 [a real image so it must be inverted]v = - m * u = - * (-4) * 25 = 100 cmf = u * v / (u + v) = 25 * 100 / (25 + 100) = 2500 / 125 = 20 cmSo the focal length of the lens is 20 cm.
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