List four precautions which a student should observe while determining the focal length of a given convex lens by obtaining image of a distant object on a screen.

The four precautions which a student should observe while determining the focal length of a given convex lens by obtaining image of a distant object on a screen are as follows:

1. The lens should be fixed vertically in the lens holder throughout the experiment.

2. While measuring all the distances the scale should be kept horizontal and it must be parallel to the ground.

3. Distance should be measured only when the sharp and well-defined image of the object is obtained on the screen.

4. The base of the lens and the white screen should be in a line with the measuring scale.