In a certain murder investigation, it was important to discover whether the victim was long-sighted or short-sighted. How could a detective decide by examining his spectacles?

(a) If the victim's spectacles are fitted with a convex lens, the detective can conclude that the victim was long-sighted, or hypermetropia.

Explanation

Hypermetropia, also known as long-sightedness or far-sightedness, is a defect of vision in which a person can't see the nearby object clearly (appears blurred), though can see the distant objects clearly. The near point of a hypermetropic eye is more than 25 cm away.

This defect occurs either due to the low converging power of the eye lens, (because of its large focal length). Or, due to the eye-ball being too short, which causes light to focus beyond the retina, instead of directly on the retina. 

It is corrected by using spectacles containing convex lenses, which should be of such a focal length (or power) that it forms a virtual image of the object (lying at the normal near point N of 25 cm) at the near point 'N' of the hypermetropic eye.  

(b) If the victim's spectacles are fitted with a concave lens, the detective can conclude that the victim was short-sighted, or myopia.