Question

When two lenses of focal length +10cm and -10cm are placed in contact then find the net power?

Answer 1

When two lenses of focal length f₁ and f₂ are kept in contact, the net power is given by

$P_{net}= \frac{1}{f_1}+ \frac{1}{f_2}$

f₁ = 10 cm
f₂ = -10 cm
$P_{net} = \frac{1}{10} + \frac{1}{-10}=0$

Answer 2

Net Power P = 1/f1 + 1/f2
                  = 1/10  + [1/-10]
                  = 1/10-1/10
                  = 0
Hence the net power of 2 lenses when they placed in contact = 0