Question

Two lenses A and B have power of (i) +2D and (ii) −4D respectively. What is the nature and focal length of each lens? 

Answer 1

 Power of lens A = +2D
     Power of lens B = -4D  

Power =`1/focal length` 

or  focal length= `1/power`   

Thus, focal length (f_A ) of A =`1/2=0.5m =50 cm ` 

Focal length (f_B ) of B = `1/-4=-0.25m=-25  cm `  

Therefore, lens A is a convex lens as it has a positive focal length and lens B is concave as it has a negative focal length.