Light Theory: Models, Interference, Refraction, and Applications

Particle model

Newton's model of light that describes it as particles.

Wave model

Huygens' model of light that describes it as waves.

Interference of light

A phenomenon that proves the wave theory, demonstrated by Thomas Young in 1801.

Electromagnetic wave

A unified concept of light proposed by James Clerk Maxwell in 1865.

Photon

A quantum of light energy that interacts like a particle and has frequency like a wave.

Energy of a photon

E = hƒ, where h = 6.63 × 10⁻³⁴ J·s and ƒ is frequency.

Light ray

An imaginary line that shows the direction of light travel, perpendicular to wavefronts.

Specular reflection

Reflection from a smooth surface where reflected rays remain parallel (mirror-like).

Diffuse reflection

Reflection from a rough surface, scattering light in many directions.

Law of Reflection

Angle of incidence = angle of reflection (θᵢ = θᵣ), both measured from the normal.

Refraction

The bending of light when it passes between two media with different speeds.

Snell's Law

n₁ sinθ₁ = n₂ sinθ₂, relating angles of incidence and refraction to refractive indices.

Index of refraction

n = c / v, where c = speed of light in vacuum and v = speed of light in medium (n ≥ 1).

Frequency of light

Stays constant when light passes from one medium to another; speed (v) and wavelength (λ) change.

Dispersion

Splitting of white light into component colors due to wavelength dependence of refractive index.

Violet light bending

Violet light bends more than red light in a prism because it has a higher refractive index (lower speed).

Rainbow formation

Light refracts into raindrop, reflects internally, and refracts out; different wavelengths deviate at slightly different angles (red ≈ 42°, violet ≈ 40°).

Critical angle

The angle of incidence that produces a refracted ray along the boundary (θ₂ = 90°).

Total internal reflection

Occurs when θᵢ > critical angle, all light is reflected back into the medium; no refraction occurs.

Applications of total internal reflection

Fiber optics (telecom, endoscopy), diamond sparkle.