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#light behavior

8 public questions tagged with this topic.

Refraction, Refractive Index and Critical Angle

Why does the angle of refraction increase beyond 90° become impossible when light moves from a denser to a rarer medium?

**Critical angle** sinC = n₂/n₁, n₁ denser, n₂ rarer 1 for air, for C=36.9° n₁=1/sin36.9°=1/0.6=1.67, for C=39° n=1/sin39°=1/0.629=1.59, for n=1.85 C=arcsin(1/1.85)=arcsin0.5405=32.7°, total internal reflection occurs only when light travels from denser to rarer and incidence > C, because Snell's law would require sinθ₂>1 impossible. Beyond the critical angle, the sine of the refraction angle exceeds 1, which is mathematically impossible, leading to total internal reflection. 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), calculati

Ref: NCERT > Physics Book > Wave Optics > Refraction, Refractive Index and Critical Angle

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Polarization and Malus Law

What allows light to continue propagating as a transverse wave after passing through a polaroid?

**Polarization** transverse wave property, light after polaroid polarized along pass-axis, Malus law I = I₀ cos²θ, θ angle between pass-axes, initial unpolarized intensity I₀ after first polaroid I₁ = I₀/2, after second at 45° I₂ = I₁ cos²45°= I₀/2×0.5= I₀/4, after two perpendicular 90° I=0 because cos90°=0, for 60° I= I₀/2×cos²60°= I₀/2×0.25= I₀/8. The transverse nature of light, with electric fields oscillating perpendicular to propagation, is preserved, but restricted to the polaroid’s pass-axis direction. Using Δ = d sinθ, y = n λ D/d, a sinθ = n λ, I = I₀ cos²θ, n = c/v, sinC = 1/n, λ' =

Ref: NCERT > Physics Book > Wave Optics > Polarization and Malus Law

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Diffraction - Single-Slit and Central Maximum

Why does the wave theory predict a change in wavelength but not frequency during refraction?

**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. Frequency is source-dependent and constant, while wavelength adjusts inversely to the speed change in the medium (v = fλ), maintaining the wave equation. Using Δ = d sinθ, y = n λ D/d, a sinθ = n λ, I = I₀ cos²θ, n =

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

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Polarization and Malus Law

Why does the wave theory require light to propagate as transverse waves to explain polarization?

**Polaroid rotation** intensity varies sinusoidally with angle due to Malus law I = I₀ cos²θ, when polaroid rotated 90° from initial, intensity goes from max to zero, for unpolarized light rotating polaroid does not change intensity after first polaroid because average, but second polaroid intensity depends on relative angle, explains why intensity transmitted through two polaroids drops to zero when perpendicular. Only transverse waves have oscillations perpendicular to propagation that can be filtered in specific directions, unlike longitudinal waves, enabling polarization. Using Δ = d sinθ,

Ref: NCERT > Physics Book > Wave Optics > Polarization and Malus Law

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Wave Properties, Frequency and Energy Conservation

What happens to the wavelength of light when it enters a denser medium?

**Frequency of light** remains unchanged when refracts from air into water because frequency determined by source, energy E= h f conserved, speed decreases v= c/n, wavelength decreases λ'=v/f= λ/n, energy of wave proportional to amplitude² not speed, intensity I =½ c ε₀ E₀², energy not depend on speed directly, when speed decreases amplitude may change but energy conserved, interference redistributes energy, total energy same, bright regions gain from dark. In a denser medium, the speed decreases, and since frequency remains constant, wavelength decreases ( λ = (v/nu) ). Using Δ = d sinθ, y =

Ref: NCERT > Physics Book > Wave Optics > Wave Properties, Frequency and Energy Conservation

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Wave Properties, Frequency and Energy Conservation

What happens to the frequency of light when it refracts from air into water?

**Frequency of light** remains unchanged when refracts from air into water because frequency determined by source, energy E= h f conserved, speed decreases v= c/n, wavelength decreases λ'=v/f= λ/n, energy of wave proportional to amplitude² not speed, intensity I =½ c ε₀ E₀², energy not depend on speed directly, when speed decreases amplitude may change but energy conserved, interference redistributes energy, total energy same, bright regions gain from dark. The frequency of light depends on the source and remains constant during refraction, as only speed and wavelength change with the medium.

Ref: NCERT > Physics Book > Wave Optics > Wave Properties, Frequency and Energy Conservation

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Refraction Through Prism and Minimum Deviation

When light travels from a denser medium to a rarer medium and the angle of incidence exceeds a certain value, what pheno

**Prism formula** for small A, δ_m≈(n-1)A, for 30°, n=1.6, δ_m≈0.6×30°=18°, approximate, exact using sin formula. For 45°, n=1.6, sin[(45+δ_m)/2]=1.6×sin22.5°=1.6×0.3827=0.6123, (45+δ_m)/2=37.8°, δ_m=30.6°, showing deviation increases with A and n. When light travels from a denser to a rarer medium (e.g., glass to air) and the angle of incidence exceeds the critical angle, total internal reflection occurs. This is because the refracted ray would otherwise require a sine value greater than 1, which is physically impossible. Substituting values gives Total internal reflection, which matches expe

Ref: NCERT > Physics Book > Ray Optics > Refraction Through Prism and Minimum Deviation

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Total Internal Reflection and Critical Angle

What is the significance of the critical angle in the context of total internal reflection?

**Total internal reflection** occurs when light travels from denser to rarer medium and incidence > C, condition sinC = 1/n for air interface. For glass n=1.52 C≈41°, water n=1.33 C≈48.8°, so at 49° water-air TIR occurs, explaining why ray does not emerge. The critical angle is the angle of incidence above which total internal reflection occurs when light travels from a denser to a rarer medium. At this angle, the refracted ray grazes the boundary (angle of refraction = 90°), and beyond it, all light is reflected back, enabling applications like optical fibers. Substituting values gives It is

Ref: NCERT > Physics Book > Ray Optics > Total Internal Reflection and Critical Angle

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