Question

hey guys plz answer it ...
1) an object is placed at the distance of 100 cm from lens of power -4D calculate the image distance.

Answer 1

$power = \frac{1}{focal \: \: length}$
$- 4 = \frac{1}{f} \\ f = \frac{1}{ - 4} m \\ f = - 25cm$
now
$\frac{1}{v} - \frac{1}{u} = \frac{1}{f} \\ \frac{1}{v} = \frac{ - 1}{25} - \frac{1}{100} \\ = \frac{ - 5}{100} \\ v = - 20$