The Power of a Lens measures its ability to converge (bring together) or diverge (spread out) light rays. It is a fundamental concept in Class 10 Physics (Light chapter).
Opticians use dioptres to prescribe corrective lenses. A higher dioptre number (e.g., +4.0 D vs +1.0 D) means the lens bends light more strongly and has a shorter focal length.
The power of a lens is defined as the reciprocal of its focal length.
Formula: P = 1 / f (in metres)
If the focal length is given in centimetres (cm), the formula becomes: P = 100 / f (in cm)
The SI unit of power of a lens is Dioptre (D).
Definition of 1 Dioptre: 1 Dioptre is the power of a lens whose focal length is 1 metre (1 D = 1 m⁻¹).
Example: If your spectacles prescription is -2.0 D, you are wearing concave lenses (to correct short-sightedness/myopia).
When multiple thin lenses are placed in contact, their total power is the algebraic sum of their individual powers: P_total = P₁ + P₂ + P₃... (Remember to include the + or - signs when adding).
Since it is a convex lens, f = +25 cm. P = 100 / f = 100 / 25 = **+4.0 D**.
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