Question

Two lenses of -15D and +5D are in contact with each other.
A) what is the focal length of the combination.
B) an object of size 4cm is placed at 20cm from this combination. Calculate the position and size of the image.​

Answer 1

Explanation:

i/F = 1/f1+ 1/f2

     =  -1/15 + 1/5

    = 10 / 75

F = 7.5m ( +ve means act as convex lens)

u =  -20cm

ho = 4cm

1/v - 1/u = 1/f

1/v + 1/20 = 1/7.5

v = 12cm

hi/ho   = -v/u

hi / 4   = 12 /20

    hi    = 2.4m