A student uses a lens of focal length 40 cm and another of -20 cm. Write the nature and power of each lens.
The power (P) of a lens is given by P=1f
where fff is the focal length of the lens in meters.
Given that, the focal length of a lens = fff = +40cm+40 cm+40cm = 0.4m0.4 m0.4m.
Therefore the power of the lens given by, P=10.4m=+2.5D.
As the power of the lens is positive, it is a converging (convex) lens.
Similarly given that the focal length of a lens = −20cm-20 cm−20cm = −0.2m-0.2 m−0.2m.
Hence the power of the lens given by P=1−0.2m=−5D.