Question

If a focal length of a lens is -20 cm. Find the nature and the power of lens. If that lens is combined with another lens of power -5D. Find the combined power of these two lenses. ​

Answer 1

Your Answer:

Given:-

• focal length of a lens is -20cm

To find:-

• The nature and power of the lens
• If that lens is combined with another lens of power -5D. Then find power of combined lenses.

Solution:-

The lens has a negative focal length so no doubt it is a diverging lens. And if it is a diverging lens then it is a concave lens.

We know that

$\tt \bf \blue {D = \dfrac{1}{f}}$

where D is the power in diopters and f is the focal length in meters.

So, we have to first convert the the focal length in meters.

$\tt f =-20cm=\cancel{\dfrac{-20}{100}}m =- 0.2m$

So, putting value of f in above equation

$\tt D = \dfrac{1}{-0.2} = -5D$

Now finding combined power.

$\tt -5D-5D = -10D$