Plane angle and solid angleसमतलीय कोण और घन कोण

NCERT Fig 1.1 · interactive diagram

Parts and their functions

• Arc length ds (चाप की लंबाई ds) — The length of the arc cut off by the two radii. dθ = ds/r.
• Radius r (त्रिज्या r) — Distance of the arc from the centre O. Same unit as ds, so the ratio has no unit dimension.
• Plane angle dθ (समतलीय कोण dθ) — dθ = ds/r. Unit: radian (rad). A full circle = 2π rad.
• Intercepted area dA (अंतःखंडित क्षेत्रफल dA) — The patch of the sphere’s surface cut off by the cone.
• Cone from O (O से शंकु) — Lines from the apex O to the edge of dA. The sphere is centred at O.
• Solid angle dΩ (घन कोण dΩ) — dΩ = dA/r². Unit: steradian (sr). Whole sphere = 4π sr.
• Sphere of radius r (त्रिज्या r का गोला) — Drawn about O as centre; dA lies on its surface.

Original diagram drawn by Simple Learning Hub, based on NCERT Fig 1.1. Report a mistake