CUET · PHYSICS · PYQ PAPER 2025
A screen is placed at a distance of 90 cm from an object. The image of the object on the screen is formed by a convex lens placed at two different locations separated by 20 cm. The focal length of the lens is approximately
- A 22 cm
- B 20.8 cm
- C 21.4 cm
- D 19.2 cm
Answer & Solution
Correct Answer
(C) 21.4 cm
Step-by-step Solution
Detailed explanation
\(f = \frac{D^2 - x^2}{4D}\) \(f = \frac{(90 \text{ cm})^2 - (20 \text{ cm})^2}{4 \times 90 \text{ cm}}\) \(f = \frac{8100 \text{ cm}^2 - 400 \text{ cm}^2}{360 \text{ cm}}\) \(f = \frac{7700 \text{ cm}^2}{360 \text{ cm}} \approx 21.4 \text{ cm}\)
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