Q49
General SciencePhysics
The power of a lens is $+2.0\,D$. The focal length of the lens and its nature are
- a.50 cm, concave
- b.50 cm, convex
- c.100 cm, convex
- d.100 cm, concave
Answer: (B) 50 cm, convex
Focal length $f=\frac{1}{P}=\frac{1}{2.0}\,m=0.5\,m=50\,cm$. A positive power means a converging, i.e., convex lens; a concave (diverging) lens has negative power. A lens of 100 cm focal length would have a power of only $\pm1\,D$. Convex lenses correct hypermetropia (long-sightedness) and concave lenses correct myopia; the SI unit of power is the dioptre ($1\,D=1\,m^{-1}$), and the powers of thin lenses in contact simply add.