Question

Two thin lenses of focal length + 10 centimetre and minus 5 cm are kept in contact find the focal length and power of the combination of lenses

Answer 1

Answer:

If two lens are put in contact their powers will add

P = P₁ + P₂

Power of lens = 1/focal length of lens(in meters)

$\frac{1}{f} = \frac{+1}{0.1} + (\frac{-1}{0.05} ) = 10 - 20 = -10\\\\f = \frac{-1}{10} m = -10cm \\\\ = \frac{1}{f_1} + \frac{1}{f_2}$

Power of the combination = 1/f = -10 D

the combination of these two lenses give us a diverging lens of focal length 10cm