Question

In the following question :

if lens 1 has focal length 15 cm and lens 2 has focal length 30 cm find which one has more power ?

the solution given says to find power we should write 1/15cm

where as in the video sir says 1D = 1/m

so please clarify if we have to convert the 15cm to meters first or not.

Answer 1

If you are going to write power in terms of Dioptre you should always need to convert cm to m since one dioptre is defined as 1 dioptre = 1 m⁻¹ .

But if you are simply comparing which one is greater,the you don't need to convert to m just for easy calculation.

But for board exam you should convert to m and express power as Dioptre.

The above question is just comparing,so no need to convert to m,but remember not to express it in terms of Dioptre.Here it is wrongly expressed in Dioptre.