First slide

Ray optics

Question

The effective focal length of the lens combination shown in figure is – 60 cm. The radii of curvature of the curved surfaces of the plano– convex lenses are 12 cm each and refractive index of the material of the lens is 1.5. The refractive index of the liquid is

Moderate

A

1.33
B

1.42
C

1.53
D

1.60

Solution

$\frac{1}{f} = \frac{1}{f_{1}} + \frac{1}{f_{2}} + \frac{1}{f_{3}}; f = - 60 cm \frac{1}{- 60} = \frac{(1.5 - 1)}{12} + \frac{(μ_{1} - 1) (- 2)}{12} + \frac{(1.5 - 1)}{12} \frac{1}{- 5} = 1 - 2 μ_{1} + 2; 2 μ_{1} = 3 + \frac{1}{5} = \frac{16}{5}; μ_{1} = \frac{8}{5} = 1.6$

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