AP EAMCET · PHYSICS · Ray Optics
Two thin convex lenses are kept in contact coaxially. If the focal length of the combination of the lenses is 4 cm and sum of the focal lengths of the two lenses is 18 cm, then the focal length of the lens of low power is
- A 8 cm
- B 10 cm
- C 6 cm
- D 12 cm
Answer & Solution
Correct Answer
(D) 12 cm
Step-by-step Solution
Detailed explanation
\( \frac{1}{F} = \frac{1}{f_1} + \frac{1}{f_2} = \frac{f_1 + f_2}{f_1 f_2} \) \( \frac{1}{4} = \frac{18}{f_1 f_2} \) \( f_1 f_2 = 4 \times 18 = 72 \) Form a quadratic equation: \( x^2 - (f_1 + f_2)x + f_1 f_2 = 0 \) \( x^2 - 18x + 72 = 0 \) \( (x - 6)(x - 12) = 0 \)…
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