Mohan, Sohan and Rahul separately measured the focal length of the same convex lens using the same distant tree as object. All of them measured the distance between the lens and the inverted image of the tree on the screen. Mohan obtained a sharp image on the screen and labelled the measured distance as $f_1$. Sohan obtained a slightly smaller and somewhat blurred image on the screen. He labelled the measured distance as $f_2$. Rahul obtained a slightly magnified and blurred image on the screen. He labelled the measured distance as $f_3$. The three measured values of focal length are likely to be related as

According to the principles of optics described in Activity 9.2 and 9.3, light rays from a distant object (at infinity) converge at the principal focus.

1. Mohan ($f_1$): A sharp image of a distant object is only formed at the focal plane. Thus, $f_1$ represents the true focal length ($f$).
2. Sohan ($f_2$): If the screen is moved closer to the lens from the focus, the light rays have not yet converged to a point, resulting in a blurred and smaller cross-section of the light cone. Thus, $f_2 < f_1$.
3. Rahul ($f_3$): If the screen is moved further from the lens beyond the focus, the rays have already converged and are now diverging, resulting in a blurred and larger (magnified) image. Thus, $f_3 > f_1$.

Combining these, we get $f_3 > f_1 > f_2$.