Physics · Ch 6 — Optics
Power of a Lens
Power of a Lens
The power of a lens measures how strongly it converges or diverges light, defined as the reciprocal of its focal length, , with SI unit dioptre (D), where . Power is positive for a converging lens and negative for a diverging lens. From the lens maker's formula, , so a larger refractive index, or more sharply curved ('bulkier') surfaces with smaller radii, both give a larger power (and correspondingly shorter focal …
What this figure shows. Two lenses, (a) and (b), are shown bending an incoming ray by different amounts; lens (b) bends the ray through a noticeably sharper angle than lens (a), meaning it has the greater deviating strength (power) and correspondingly the shorter focal length, since the two are always inversely related -- the more strongly a lens bends light, the closer …
Worked out. A glass lens has focal length 150 cm, i.e. f = 1.5 m. Using P = 1/f directly gives P = 1/1.5 = 0.67 dioptre. Since the power comes out positive, this confirms the lens is converging -- a fairly weak one, since its focal length is quite long and hence its power quite small in magnitude. …