Question

Find the focal length of a lens of $power-2D$ . What type of lens is this?

Answer 1

Hint: We just need to use the mathematical relation between focal length and power of the lens.

Formula Used $P=\dfrac{1}{f}$
Where $P=\text{is }power\text{ of lens in }Dioptres$
And $f=\text{is focal length of the lens in }meters$

Complete step by step solution
Here $P=-2D$
Using $P=-\dfrac{1}{f}$
          $\begin{align}
  & -2=\dfrac{1}{f} \\
 & -2f=1 \\
 & f=-\dfrac{1}{2} \\
 & \text{ }=-0\cdot 5m \\
 & f=-0\cdot 5m \\
\end{align}$

Hence, the focal length of the lens is $-0\cdot 5m$ .

Since the focal length here is negative, it is inferred that the lens used here is a concave lens.

Additional Information: study the lens properties i.e. concave and convex lens and also study the sign of conventions used for the lenses to understand the positive and negative signs.

Note: Do not forget to use the signs of power and focal length of the length must be standardized in $dioptres\text{ and }meters$ respectively.