Question

Define power of lens and give si unit if a convex lens of focal length 15 cm is placed in contact coaxially with concave lens of focal length 20cm what is the power of the combination

Answer 1

$\underline{\bold{Given:-}}$

Focal length of convex lens f1 = 15 cm

Focal length of concave lens f2 = -20 cm

$\underline{\bold{Solution:-}}$

Let f be the focal length of the combination.

So,

$\frac{1}{f} = \frac{1}{ f_{1}} + \frac{1}{ f_{2}} \\ \\ \frac{1}{f} = \frac{1}{15} + \frac{1}{ - 20} \\ \\ \frac{1}{f} = \frac{4 - 3}{60} \\ \\ \frac{1}{f} = \frac{1}{60} \\ \\ f = 60 \\ \\ focal \: length \: of \: combination = 60 cm$

Now, we have to find out the power of combination.

Let P be the power of combination

We know that

$P \: = P_{1} + P_{2} \\ \\ = \frac{1}{ f_{1} } + \frac{1}{ f_{2} } \\ \\the \: condition \: is \: same \: in \: above \\ \\ = \frac{1}{60} \\ \\ Power \: = \frac{1}{60} Dioptre$