Define the power of a lens. Also mention its SI unit. A student uses a lens of focal length 30 cm and another of −10 cm. Write the nature and power of each lens.  

 

The power of the lens is defined as the reciprocal of the focal length of the lens expressed in metres.  

$P=1/focal length$

The SI unit of power is Dioptre denoted by D.  Focal length of the first lens,  

$$F_1=30 cm$$

$$=0.3 m$$

Since, the focal length is positive, it is a convex lens.  

Power of lens,  

$$P_1=1/F_1$$

$$=1/0.3$$

$$=3.33 D$$

Focal length of the second lens,

$$F_2= -10 cm$$

$$= -0.1 m$$

Since the focal length is negative, it is a concave lens.  

Power of lens,

$$P_2=1/F_2$$

$$=1/(-0.1)$$

$$= -10 D$$