Question

Two thin convex lenses of focal lenght 15cm and 30cm are placed in contact what will be power of the combination?

Answer 1

f1 = 15cm,  f2 = 30cm
Focal length of combination, 

1/f=1/f1+1/f2
1/f= 1/15+1/30
1/f= (15+30)/(15*30)
f = 450/45
f= 10cm

Answer 2

1m =100 cm
so, 0.15m and 0.30m
now, p=1/f
           =1/0.15
          =20/3 D ........1
now , p=1/0.30
            =10/3 D........2
adding 1 & 2
    =30/3 D
     =10 D
ANS = 10 D [D STANDS FOR DIOPTRE SI UNIT OF POWER]