Refraction of a plane wave into a denser medium (Huygens ⇒ Snell’s law)सघन माध्यम में समतल तरंग का अपवर्तन (हाइगेंस ⇒ स्नेल नियम)Refraction of a plane wave into a denser medium (Huygens ⇒ Snell’s law)

NCERT Fig 10.4 · Class 12 Physics · Chapter 10 · interactive diagram

Parts and what they mean

• Incident wavefront AB (आपतित तरंगाग्र AB) — A plane wavefront arriving at angle i. When end A touches the surface, end B still has distance BC to go.
• BC = v₁τ (BC = v₁τ) — In time τ the disturbance from B travels to C at speed v₁ in medium 1.
• Secondary wavelet from A (A से द्वितीयक तरंगिका) — In the same time τ the wavelet from A spreads into medium 2 with speed v₂, radius AE = v₂τ.
• AE = v₂τ (AE = v₂τ) — Radius of the wavelet; AE is also the refracted ray through A (it is perpendicular to CE).
• Refracted wavefront CE (अपवर्तित तरंगाग्र CE) — Tangent plane drawn from C to the wavelet. From triangles ABC and AEC: sin i = v₁τ/AC, sin r = v₂τ/AC ⇒ sin i / sin r = v₁/v₂ = n₂/n₁ (Snell’s law).
• Angle of incidence i (आपतन कोण i) — Angle between the incident ray and the normal (equal to the angle between wavefront AB and the surface).
• Angle of refraction r (अपवर्तन कोण r) — Here v₂ < v₁, so r < i: the ray bends towards the normal (wave theory: light is slower in the denser medium).
• Incident rays (आपतित किरणें) — Perpendicular to the incident wavefront.
• Refracted rays (अपवर्तित किरणें) — Perpendicular to the refracted wavefront CE.
• Normals (अभिलंब) — Lines perpendicular to the surface; angles are measured from them.
• Surface PP′ (पृष्ठ PP′) — Boundary separating medium 1 and medium 2.
• Medium 2 (माध्यम 2) — Optically denser here (e.g. glass): speed v₂ < v₁, wavelength shorter, frequency unchanged.

Original diagram drawn by Simple Learning Hub, based on NCERT Fig 10.4. Report a mistake