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Two identical thin plano-convex glass lenses (refractive index 1.5) each having radius of curvature of 20 cm are placed with their convex surfaces in contact at the centre. The intervening space is filled with oil of refractive index 1.7. The focal length of the combination is

1. -25 cm
2. -50 cm
3. 50 cm
4. -20 cm

Correct answer: -50 cm

Solution

The focal length of the combination can be calculated using the lens maker's formula for each lens and the oil layer. The effective focal length is negative due to the diverging nature of the oil layer (higher refractive index than the lenses). After calculation, the focal length of the combination is -50 cm.

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