WAEC Physics 2015 Theory — Question 18

Model answer

(i) Using 1/f = 1/v − 1/u with f = −18 m (diverging lens) and u = 90 m: 1/v = 1/f + 1/u = −1/18 + 1/90 = −5/90 + 1/90 = −4/90, so v = −22.5 m (the negative sign shows the image is virtual, formed 22.5 m from the lens on the same side as the object). (ii) Magnification m = v/u = 22.5/90 = 0.25. Since image length = m × object length, object length (length of shark) = image length / m = 1.0/0.25 = 4.0 m.