Question

A convex lens forms an image three times the size of the object on a screen. If the distance between the screen and the object is 120cm. Determine;

1. The object distance.    
2. The focal length of the lens. 

Answer 1

1. ^v/_μ=3
     v=3μ
   μ+ v =120cm
   μ+3μ=120cm
       4μ=30cm   
2. f= μv   =  3μ²ⁱ =  3 × 30 × 30  =22.5cm
      μ+v      μ+v          120