Power of a lensलेंस की क्षमताPower of a lens

NCERT Fig 9.18 · Class 12 Physics · Chapter 9 · interactive diagram

Parts and what they mean

• Convex (converging) lens (उत्तल (अभिसारी) लेंस) — Thicker in the middle; f is positive.
• Principal axis (मुख्य अक्ष) — Line through the optical centre O and the foci.
• Optical centre O (प्रकाशिक केंद्र O) — A ray through it goes straight on, undeviated. All distances are measured from O.
• Ray parallel to the axis at height h (अक्ष के समांतर किरण, ऊँचाई h पर) — Falls on the lens at distance h from the optical centre.
• Ray bent through angle δ (कोण δ से मुड़ी किरण) — A shorter focal length bends the ray more, so the lens is more powerful.
• Height h (ऊँचाई h) — Distance of the ray from the axis; take h = 1 in the definition.
• Focal length f (फोकस दूरी f) — Use it in metres to get power in dioptre: P = 1/f. f = +40 cm → P = +2.5 D; f = −25 cm → P = −4 D.
• Angle of convergence δ (अभिसरण कोण δ) — Power = tan δ for a ray at unit distance; for small δ, P = δ = 1/f.
• Focus F (फोकस F) — The bent ray crosses the axis here.

Original diagram drawn by Simple Learning Hub, based on NCERT Fig 9.18. Report a mistake