Question

The lens of power minus 1. 5 D and + 2. 5 + then contact the focal length of combination will be

Answer 1

power of first lense =-1.5D
power of 2nd lense =+2 5D
power of combination =2.5D-1.5D
1D
as power =100/f
f=100/1 =100cm

Answer 2

power of combination of lense
P =P1 + P2
P1 = -1.5D
P2= +2.5 D
SO P= -1.5+2.5
=1 D
P = 100/ f where f in mtr.
so , f = 100 cms.