Two concave lenses L_1 and L_2 are kept in contact with each other. If the space between the two lenses is filled with a material
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
1 Answers
(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.