Question

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
(a) becomes undefined
(b) remains unchanged
(c) increases
(d) decreases.

Answer 1

Answer:

Explanation:

it will decrease

in some books it is given as that the net focal length will increase but it is not true because

1/f1 = (u - 1)(-2/R) = -2(u-1)(1/R) - BY LENS MAKERS FORMULA

f1 = R/ -2(u-1) = R/ 2(1-u)            - 1 )

SIMILARLY

1/f2 = -2(u-1)(1/R)

1 /Fnet = 1/f1 + /f2

          = 2( -2(u-1)(1/R)

         = -4(u-1)/R

Fnet    = R/ -4(u-1) - reciprocal

Fnet   = R / 4 ( 1-u)                       - 2 )

now because

there is 2 in denominator in equation 1 and 4 in equation 2 therefore F net will

be smaller or decrease