#light bending

Wavefront and Huygens Principle

Why does the wave theory predict that light bends towards the normal when entering a medium where its speed is lower?

**Wavefront types** point source spherical, distant point source plane, after convex lens plane wave focuses to point because lens adds phase delay proportional to thickness, converging spherical wavefront, after concave mirror plane wave becomes spherical converging to focus, illustrating Huygens construction. The wavefront tilts towards the normal because the part entering the slower medium lags, reducing the angle of propagation relative to the boundary. Using Δ = d sinθ, y = n λ D/d, a sinθ = n λ, I = I₀ cos²θ, n = c/v, sinC = 1/n, λ' = λ/n and A = 2a cos(φ/2), calculation gives Wavefront

Ref: NCERT > Physics Book > Wave Optics > Wavefront and Huygens Principle

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Optical Phenomena and Applications

Why does the wave model predict that light bends away from the normal when entering a rarer medium?

**Convex lens focusing** plane wave into point because lens introduces phase delay proportional to thickness, converting plane wavefront to spherical converging to focal point, property ensures rays parallel to axis meet at focus, spherical aberration minimized for paraxial rays, lensmaker's formula determines focal length. In a rarer medium, light speeds up, causing the wavefront to tilt away from the normal as it takes less time to travel a longer path. Using Δ = d sinθ, y = n λ D/d, a sinθ = n λ, I = I₀ cos²θ, n = c/v, sinC = 1/n, λ' = λ/n and A = 2a cos(φ/2), calculation gives

Ref: NCERT > Physics Book > Wave Optics > Optical Phenomena and Applications

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Diffraction - Single-Slit and Central Maximum

What property of light waves causes the bending of rays around corners in a diffraction experiment?

**Single-slit diffraction** central maximum width W =2λ D/a, a slit width, D distance, angular width θ =2λ/a, first minimum at a sinθ = λ, fourth minimum a sinθ=4λ, sinθ=4λ/a, for a=5.0 μm λ=500 nm sinθ=4×0.5/5=0.4 θ≈23.6°, central maximum width increases when slit width reduced to half doubles width, when wavelength quadrupled width quadruples, when slit tripled width one-third. Light’s wave nature allows it to spread and bend around obstacles when encountering apertures comparable to its wavelength, unlike particle-like straight paths. Using Δ = d sinθ, y = n λ D/d, a sinθ = n λ, I = I₀ cos²

Ref: NCERT > Physics Book > Wave Optics > Diffraction - Single-Slit and Central Maximum

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Refraction at Spherical Surfaces and Lens Maker's Formula

Why does the apparent depth of an object in a denser medium appear less than its real depth when viewed from air?

**Spherical refracting surface** power P = (n₂-n₁)/R, lens power sum of two surfaces. Lens maker derivation combines two refractions, sign of R₂ negative for second surface convex opposite direction, yielding 1/f positive for convex lens. When light travels from a denser medium (e.g., water) to a rarer medium (air), it bends away from the normal. This refraction makes the rays appear to diverge from a point closer to the surface than the actual object, reducing the apparent depth compared to the real depth. Substituting values gives Due to refraction bending rays away from the normal, which ma

Ref: NCERT > Physics Book > Ray Optics > Refraction at Spherical Surfaces and Lens Maker's Formula

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