Power of lens (P):
1) Power of lens is defined the reciprocal of focal length of lens
SI unit of power of lens is 1/meter (per meter) or dioptre (D).
Power of convex lens is positive, while that o concave lens is negative.
2) If ‘n’ number of lenses of different power P1, P2, P3,….. are arranged such that there optical centre align on same principal axis, then the total power of combination is sum of individual powers, given as
P = P1+P2+ P3+……….
Ex.1) Doctor has prescribed the spectacles of power +1.50 D to Sandip. Find the focal length of the lens of spectacle, hence identify the defect Sandip is having.
Solution:
Data: P=+1.25 D
Formula:
On substituting the value, we get
The focal length of the lens used in spectacles of Sandip is positive; hence it is convex lens which is used to correct Hypermetropia. Thus Sandip have defect of Hypermetropia.
Ex.1) For manufacturing of microscope in laboratory lens required are of power +1.25 D, +2.25 D and +0.75 D. Find the total power of combination of lenses used in microscope.
Solution: Data given,
P1= +1.25 D, P2=+2.25 D and P3=+0.75 D
Total power is given by,
P = 1.25 +2.25 + 0.75
= 4.25 D