Two lenses of power +3.5 D and -2.5 D are kept in contact. Find the focal length of the combination of lens.

$\text{Power of lens one} =\text{ P}1 = +3.5\text{D}$

$\text{Power of lens two} = \text{P2} = -2.5\text{D}$

$\text{Power of combination} = \text{P1 + P2}$

$\text{Then, Power of combination} = +3.5\text{D}+(-2.5\text{D})$

$= 3.5\text{D}-2.5\text{D}$

$= 1 D$

$\text{Power} = \frac{1}{\text{f}}$

$1 = 1/\text{f}$

$\text{f} = 1/1$

$\text{f }= 1\text{m}$

$\text{Thus,the focal length of combination} = 1 \times 100 = 100\text{cm}$