Question

1 D equals ______.

Options

• 1 m

• 1 cm

• \(1 \mathrm{~m}^{-1}\)

• \(10 \mathrm{~m}^{-1}\)

Answer 1

1 D equals 1 m.

Explanation:

\[ {1\,D =1\, m^{-1}} \]

Here, D stands for dioptre, the SI unit of power of a lens.

The power of a lens is:

\[ P=\frac{1}{f} \]

where \(f\) is the focal length in metres.

Therefore, a lens having a focal length of \(1\,m\) has a power of:

\[ P=\frac{1}{1}=1\,D \]

Hence,

\[ {1\,D=1\,m^{-1}} \]