Try this.
Make a list of students in your class using spectacles. Record the power of their lenses. Find out and note which type of defect of vision they suffer from. Which defect is most common among the students?

Explanation

`{:("Name of the student","Power of spectacles " , "Defect"),(,,):}`
The power of the lenses should be noted down along with its sign. This sign (positive or negative) of the power helps in determining the type of defect of vision the student is suffering from. If the power of the lens is negative, the lens used is concave i.e., the student is suffering from nearsightedness. If the power of the lens is positive the lens used is convex i.e., the student is suffering from farsightedness. The most common defect among the students ts nearsightedness or myopia.