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.

Explanation

To determine the focal length of a given convex lens by obtaining the image of a distant object on a screen, a student should observe the following four precautions:

1. **Ensure the Object is Clearly Visible**: The distant object used for the experiment should be clearly visible. This ensures that the light rays coming from the object are well-defined and can be focused properly by the lens.

2. **Obtain a Sharp and Well-Defined Image**: The image formed on the screen should be sharp and well-defined. A blurry image indicates that the lens is not properly aligned or that the distance is not correctly adjusted, which can lead to inaccurate measurements.

3. **Keep the Scale Parallel to the Ground**: When measuring distances, the scale should be kept parallel to the ground. This ensures that the measurements taken are accurate and not distorted by any angle.

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