Question

A lens forms a real image of an object. The distance from the object to the lens is x cm and that from the lens to the image is y cm. The graph shows the variation of y with x.
It can be deduced that the lens is

A. converging and of focal length 10cm
B. converging and of focal length 20cm
C. converging and of focal length 40cm
D. diverging and of focal length 20cm

Answer 1

Correct Answer - a.
A diverging lens is ruled out because both x and y are positive values. Both x and y equal 20cm at their smalles su, which occurs when
` x+y=40cm=4f`
`:. f=10cm`
This indicates a converging lens of focal length `= 10cm`