Question

derive the lens formula two lenses of power 10 d and -5 d. calculate the power of new lens.

Answer 1

Power of lens=10 + (-5)=5
=>power of lens=5d
Lens Formula
1/v+1/u=1/f
where v is the image distance,u is the object distance and f is the focal length of the lens.

Answer 2

if the two lenses are combined - placed just after another at the geometric centers.

1/f = 1/f1 + 1/f2

P = P1 + P2 = 10 - 5 = 5 Dioptres

f = 1/P = 0.20  meter = 20 cm