Two concave lenses L1 and L2 are kept in contact with each other. If the space between the two lenses is filled with a material of smaller refractive index, the magnitude of the focal length of the combination

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increases The focal length of the combination will increase. For finding out the combination of lens we have the formula: 1/F = 1/f_1 + 1/f_2 - d/f_1 f_2 Where, F is the focal length for the combination d is the separation between two lenses Here, d = 0 1/F = 1/f_1 + 1/f_2 F = f_1 f_2/f_1 + f_2 Hence, the focal length will increase.

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