Lens Power
Optical power in dioptres (f in metres).
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Example
f=0.5m ⇒ 2 D
How it works
Lens power tells you how strongly a lens bends light: the shorter the focal length, the stronger the lens. It is measured in dioptres, and the unit works out as 1/metre because power is defined as the reciprocal of focal length in metres. A positive power usually means a converging lens, while a negative power means a diverging lens.
When to use it
Use this formula when you need to convert a lens’s focal length into optical power, such as for eyeglass prescriptions, camera lenses, or simple ray-diagram problems. It is also useful when comparing how strong different lenses are.
Worked example, step by step
A converging lens has a focal length of 0.25 m. P = 1/f P = 1/0.25 P = 4 So the lens power is 4 D.
Common mistakes to avoid
- using focal length in centimetres instead of metres
- forgetting that a diverging lens has negative power
- mixing up power with magnification or assuming a larger focal length means stronger lens
Did You Know?
Optometrists measure lenses in dioptres, the reciprocal of focal length in metres, so stronger lenses have shorter focus.
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