Question

Define the power of a lens. What is its unit? One student uses a lens of focal length 50 cm and another of –50 cm.

Answer 1

1. Defining power and its units.

1. The degree of convergence and divergence provided by a lens is called the power of the lens.
2. The power of a lens is given by the reciprocal of its focal length. i.e., $power=\frac{1}{focallength}$ or $P=\frac{1}{f}wherefisinmetres$.
3. The larger the focal length of the lens, the smaller the power and vice-versa.
4. The SI unit of power of the lens is diopter D.$Iff=1m,P=\frac{1}{1m}=1D$.
5. Hence the power of a lens is said to be one diopter if its focal length is one metre.

2

step 1: Given data

$$\begin{gathered} f_1=50cm=0.5mFocusof1stlens \\ {f}_2=-50cm=-0.5mFocusof2ndlens \end{gathered}$$

Step 2: Calculation

By convention, the focal length of a convex lens is taken as positive and that of a concave lens is taken as negative.

Hence the lens $L_1$ must be a convex lens and the lens $L_2$ must be concave.

$Poweroffirstlens,P_1=\frac{1}{f_1}=\frac{1}{0.5}=2dipoter$

and $powerofsecondlens,P_2=\frac{1}{f_2}=\frac{1}{-0.5}=-2diopter$.

Hence, Power would be $-2diopter,2diopter$. for 2nd and 1st Lens.