Two concave lenses L1 and L2 are kept in contact with each other. If

1.	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 refractive index μ=1 the magnitude of the focal length of the combination(a) becomes undefined(b) remains unchanged(c) increases(d) decreases
Answer» (c) increases EXPLANATION: The focal length F of the combination is given as 1/F = 1/f₁ + 1/f₂ It is like resistors connected in parallel. The numerical value of equivalent resistance is less than the smallest resistor. Similarly, the numerical value of the equivalent focal length will decrease. Since the 1/F is the power of the lens and in the case of a diverging lens, it is negative as in this case. Hence F is also negative. If the numerical value of F decreases that means its negative will increase.

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 refractive index μ=1 the magnitude of the focal length of the combination(a) becomes undefined(b) remains unchanged(c) increases(d) decreases

Answer»

(c) increases

EXPLANATION:

The focal length F of the combination is given as 1/F = 1/f₁ + 1/f₂

It is like resistors connected in parallel. The numerical value of equivalent resistance is less than the smallest resistor. Similarly, the numerical value of the equivalent focal length will decrease. Since the 1/F is the power of the lens and in the case of a diverging lens, it is negative as in this case. Hence F is also negative. If the numerical value of F decreases that means its negative will increase.

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