Question

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

Answer 1

The correct option is B

lens and the screen only

In the given case, the student has obtained the point image of a distant object using a convex lens.
According to the lens formula,
$\frac{1}{f}=\frac{1}{v}-\frac{1}{u}$
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 $u=\infty$
Thus, $\frac{1}{u}=0$ 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.