Question

Give power of lens of 20 cm focal length?

Answer 1

Answer: P = ± 5 D.

Explaination:

Power of a lens is given by,

P = 1/f = 100/f (in cm)

=> P = (100)/(± 20 cm) = ± 5 D.

More:

• Power of combination of lens = P = Total power of each lens by sign convention
• Power of combination of two lens separated by a distance d = $\rm \dfrac{f_1 f_2}{f_1 + f_2 - d}$.
• Net magnification = m(net) = m1 × m2 × m3 × ... .
• Power of lens to correct hypermetropia = $\rm \dfrac{1}{-F}$ (F should be in mts.)
• Power of lens to correct myopia = $\rm 4 - \dfrac{1}{N.P.}$ (NP = near point, should be in mts.)