A visitor had left his spectacles behind. How would you know if he is long sighted or short sighted by examining the spectacles

A concave lens will indicate Myopia.
A convex lens will indicate hypermetropia.

What you should do?

Look a distant object through the lens.
If the image is smaller in size and erect, it is a concave lens.
If it is the same it is a simple glass slab, used for protection from dust.
If none of the above, look at a close by object. If the image is enlarged and erect, or blurry and inverted, it is a convex lens.

(Go through the study material and study image formation by lens if you require the reasons for the same)

Happy to help you!

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