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.
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Precautions :
1. Fix the convex lens vertically in the lens holder.
2. The base of the lens stand and white screen should be in line with the measuring scale.
3. Record the position of the lens and screen only when a well defined sharp image is formed.
The four precautions needed to be taken for performing the given experiment are as follows:
1. It is important to ensure that the screen, as well as the convex lens, are vertical in orientation.
2. It should be ensured that the image that is formed is the clearest focus possible.
3. The positions of object and image should be marked and taken into consideration only when a proper image is formed on the screen.
4. Distances should be measured carefully from the centre of the lens and parallax error should be avoided as much as possible.