Question

# Two 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 F1. When the space between the two lenses is filled with glycerin (which has the same refractive index (μ=1.5) as that of lens, the equivalent focal length is F2. The ratio F1 : F2 will be

A
2 : 1
B
1 : 2
C
2 : 3
D
3 : 4

Solution

## The correct option is B 1 : 21F1 = 1f+1f⇒F1 = f2 The space between convex lenses filled with glycerin will act as concave lens of focal length -f. 1F2 = 1f+1f−1f⇒ F2 = f⇒F1F2 = 12Co-Curriculars

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