Question

Find the focal length and power of convex lens which,when put in contact with a concave lens of focal length 25cm forms a real image 5 times the size of an object placed 20cm from the combination

Answer 1

Magnification = 5

Magnification = image distance / object distance

5 = i / 20

Image distance = 20 × 5 = 100

Lens formula :

1/F = 1/u + 1/v

1/F = > where F is the combined focal length.

1/F = 1/20 + 1/100

1/F = (5 + 1) /100

1/F = 6/100

Let the focal length of the first lens be f₁ and the lens of the second lens be f₂.

f₁ = 25 cm

1/F = 1/f₁ + 1/f₂

6/100 = 1/25 + 1/f₂

6/100 - 1/25 = 1/f₂

(6 - 4) / 100 = 1/f₂

2/100 = 1/f₂

1/50 = 1/f₂

f₂ = 50 cm

Power = 1/focal length

Power = 1/50 = 0.02

Answer :

Focal length = 50cm

Power = 0.02