A student has obtained a point image of a distant object using the given convex lens. To find the focal length of the lens he should measure the distance between the :
(A) lens and the object only
(B) lens and the screen only
(C) object and the image only
(D) lens and the object and also between the object and the image
Solution
In the given case, the student has obtained the point image of a distant object using a convex lens.
According to the lens formula,
`1/f=1/v-1/u`
Here,
v = Image distance
u = Object distance
f = Focal length of the lens
Because the object is placed very far, the object distance is taken to be infinity. As u = ∞
`"Thus, "1/u=0" and "f="v"`
To find the focal length of the lens, the student should know the image distance, which is the distance between the lens and the screen.
Hence, the correct option is B.
