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Physics · PYQ NEET UG 2019(1)
हिन्दीTwo similar thin equiconvex lenses, of focal length f each, are kept coaxially in contact with each other such that the focal length of the combination is \( F_1 \). When the space between the lenses is filled with glycerin (which has the same refractive index (\( \mu = 1.5 \)) as that of glass) then the equivalent focal length is \( F_2 \). The ratio \( F_1 : F_2 \) will be:
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Correct answer · Option D
1 : 2
Explanation
For two lenses in contact, \( \frac{1}{F_1} = \frac{1}{f} + \frac{1}{f} = \frac{2}{f} \implies F_1 = \frac{f}{2} \). When glycerin of same refractive index is filled, the middle acts as a concave lens of focal length \( -f \). Thus, \( \frac{1}{F_2} = \frac{1}{f} - \frac{1}{f} + \frac{1}{f} = \frac{1}{f} \implies F_2 = f \). Ratio \( F_1 : F_2 = \frac{f/2}{f} = 1 : 2 \).
