What does the power like -2.5or.1.5 refers to in-case of eye defects?
Both the above answers are correct in that a negative sign refers to a concave (diverging) lens used to correct nearsightedness, and a positive number refers to a convex (converging) lens used to correct farsightedness (which I have). The number itself is the magnifying power (or diverging power, if negative) in units called diopters. Diopters refer to the reciprocal of the focal length of the lens in meters, for example, a 1-diopter lens has a focal length of 1 meter, a 2-diopter lens has a focal length of ½ meter or 50 cm, a 2.5 diopter lens has a focal length of 40 cm, etc. So your -2.5 diopter lens is a concave, or diverging lens, of focal length -40 cm. (concave lenses have negative focal lengths) and the 1.5 diopter lens has a focal length of 66 2/3 cm.
