The image of an object which is 10cm from a lens is formed on the same side as the object. If the image is 10cm behind the object, Find the focal length of the lens.

Focal length is 6.67 cm.

Explanation:

Object distance, u, from the lens = 10 cm

Image distance, v, to the lens = 10 cm + 10 cm

                                                 =  20 cm

Apply the lens formula to determine the focal length, f, of the lens.

= +

 = +

    =

 =

Find the inverse of both sides, so that;

f =

  = 6.67

f = 6.67 cm

The focal length of the lens is 6.67 cm.