Question

Find the focal length of lens which should be placed in contact with a lens of focal length 10cm so that the power combination becomes 5d

Answer 1

Power(P2) of lens with 10 cm focal length = 10010=10D
Let Power of the lens is P1
Total power of combination = P1 +P2
​5D=​ P1 + 10D
P1 = -5D
focal length = −15=−0.2m⇒−20 cm
Power of the lens is negative so the given lens is concave lens
 

Answer 2

1/f1+ 1/f2= 5

10/ 1000 + f2= 5

0.001+ f2= 5

f2= 4.999 cm