Question

also please give the explaination for each answer​

Answer 1

(a) The focal length of the lens is 20cm. This is because only when the object is placed at 2F1, the image will be formed at 2F2. According to the table, when u=-40cm, v=40cm. Therefore 2F = 40cm => F=20cm.

(b) S. No. 6. This is because when we use the lens formula,

          $\frac{1}{f} = \frac{1}{v} - \frac{1}{u}\\\\\frac{1}{120} - \frac{1}{-40} = \frac{4}{120} = \frac{1}{30} \neq \frac{1}{20}$