Question

plz help for school exam
two lenses of focal length 20cm and -40cm are held in contact. if an object lies at infinity image formed by lens combination will be at
(a)infinity
(b)20cm
(c)40cm
(d)60cm

Answer 1

Answer:

given  =  focal length =  20cm , and 40cm

             

to find =  if an object lies at infinity image formed by lens combination will be at?

solution =

$\frac{1}{f} = \frac{1}{f1} + \frac{1}{f2} \\\\\frac{1}{f} = \frac{1}{20} - \frac{1}{40}$

2 - 1 / 40

f = 40cm

$\frac{1}{v} - \frac{1}{u} = \frac{1}{v} - \frac{1}{infinite }$

v = f = 40cm

so ,we can conclude that the u is equal to infinite

so the option 1st is correct answer i.e  ∞