Add few lines about the experiment by which the focal length is calculated.
We can also add a diagram (like such these are copyright)
1. The lens should be help properly and vertically.
2. At least 3-4 observations should be taken .
3. The lens should be moved forward and backward to get a clear and sharp image on the screen.
4. The base of lens, screen and measuring scale should be in a straight line.
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