Science (086) — CBSE Class 10 board question

The lens of power −4.0 D is for the correction of distant vision (myopia), as it is a concave lens (negative power).

Its focal length:

$$f = \frac{1}{P} = \frac{1}{-4.0} = -0.25 \text{ m}$$

The focal length of the lens for distant vision is −0.25 m.

• In bifocal lenses for presbyopia, the concave (negative power) lens corrects distant vision (myopia) and occupies the upper portion; the convex (positive power) lens corrects near vision (hypermetropia) and occupies the lower portion.
• Use $f = \frac{1}{P}$ with $P$ in dioptres to get $f$ in metres. Don't forget the negative sign — examiners check it.