Question 1

What is Snell's Law?

Accepted Answer

Snell's Law describes how light bends when it passes from one medium to another. It states that n₁sinθ₁ = n₂sinθ₂, where n₁ and n₂ are the refractive indices of the two media and θ₁, θ₂ are the angles of incidence and refraction measured from the normal to the interface. When light enters a denser medium (higher n) it bends toward the normal; when entering a less dense medium it bends away.

Question 2

What is total internal reflection and how is the critical angle calculated?

Accepted Answer

Total internal reflection (TIR) occurs when light travels from a denser medium (higher n₁) to a less dense medium (lower n₂) and the angle of incidence exceeds the critical angle θ_c = arcsin(n₂/n₁). At and beyond this angle, no refracted ray exists and all light is reflected. For example, glass (n=1.5) to air (n=1.0) has a critical angle of arcsin(1/1.5) ≈ 41.8°. This principle underlies optical fiber cables and the brilliance of diamonds.

Question 3

What is Fresnel reflectance?

Accepted Answer

Fresnel reflectance describes the fraction of light reflected at an interface. It depends on the angle of incidence and the refractive indices of both media, and differs for s-polarized and p-polarized light. At normal incidence the reflectance simplifies to R₀ = ((n₁−n₂)/(n₁+n₂))². At total internal reflection R = 1 (complete reflection). For unpolarized light, the average of s and p reflectances is used.

Question 4

How is Snell's Law applied in engineering and CAE?

Accepted Answer

Snell's Law is fundamental to optical design (lenses, prisms, fiber optics), anti-reflection coating design, and solar cell optimization. In non-destructive testing (NDT), the analogous law for sound waves governs ultrasonic beam refraction at material interfaces — the acoustic version uses sound speeds instead of refractive indices. CAE ray-tracing and ultrasonic simulation tools (e.g. CIVA) apply Snell's Law at every boundary.

Snell's Law & Refraction Simulator

Theory