Question

a doctor advised to use combinations of lenses of power 2d & 1d what will be the focal length​

Answer 1

For finding the focal length, we can use the formula for power of lens which is reciprocal of focal length itself. The formula is as follows:

D=1/f, where 'D' is the power of the lens in metres and 'f' is its focal length.

Since we have to find the focal for combination of power lens:

D1= 1/f1,   D2=1/f2

where,

D1= 2,

D2= 1

this means,

1/f = 1/f1 + 1/f2

1/f = 2 + 1

Focal length = 3 metres

hope this helped...