Refraction, Reflection & Polarisation

Revise Refraction, Reflection & Polarisation for Physics 2 (IAL) WPH12 (AS Level) — revision notes and instant AI marking.

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Edexcel IAL Physics · Waves

Refraction, Reflection & Polarisation

When light crosses into a new material or bounces off a surface, its speed, direction, and even the way it wiggles can all change — and every rule in this chapter is really just describing those three things happening in different situations.

Summary — What This Chapter Covers

Intensity is power spread over an area (I = P/A), and it obeys an inverse-square law with distance for a point source.
Refraction happens when light crosses a boundary between materials — it changes speed, and (unless it hits the boundary straight on) it changes direction too.
Refractive index (n) tells you how much a material slows light down compared to a vacuum. Snell's Law (n₁sinθ₁ = n₂sinθ₂) connects refractive index to the angles either side of a boundary.
The critical angle is the special angle of incidence (inside a dense material) at which the refracted ray grazes along the boundary at exactly 90°.
Total internal reflection (TIR) happens beyond the critical angle, when going from a denser to a less dense material — 100% of the light reflects, none escapes.
The refractive index experiment uses a ray box, a perspex block, and careful angle-marking to measure n practically.
Plane polarisation restricts a transverse wave's oscillations to a single plane. Only transverse waves can be polarised — longitudinal waves never can.
Light can be polarised by filters, reflection, or refraction, and polarisation has real uses — from sunglasses to stress-testing materials to analysing sugar solutions.

1. Intensity of Radiation

Think about a wave — light, sound, whatever — as a courier carrying energy. Intensity just measures how much energy that courier delivers, per second, to each square metre of surface it hits. That's it. It's power density.

Core Formula

I = P / A

Intensity (W m⁻²) = Power (W) ÷ Area (m²)

Now here's the part that trips people up conceptually: intensity isn't just about how much energy the source is pumping out — it's about how concentrated that energy is when it lands. The same 100 W lightbulb feels blinding up close and barely noticeable from across a football field, even though it's emitting the exact same power the whole time. What's changing is the area that power is spread across.

Amplitude and Frequency

Intensity also depends on the wave's amplitude and frequency — specifically, it's proportional to the square of each:

Proportionality

I ∝ A² and I ∝ f²

Double the amplitude → intensity increases by a factor of 4. Double the frequency → same thing.

Analogy: Imagine shaking a rope. Shake it with twice the amplitude (bigger swings) and you're putting in noticeably more energy — not double, but quadruple, because energy in a wave scales with the square of how far the particles swing. Same logic applies to how fast you shake it (frequency).

Spherical Waves & the Inverse Square Law

A point source (like a bare lightbulb or a speaker) radiates energy equally in all directions — forming an expanding sphere of wavefronts. The area of that sphere at radius r is 4πr². So as the wave travels outward, the same total power P is spread across a bigger and bigger sphere:

Intensity from a Point Source

I = P / (4πr²)

This means I ∝ 1/r² — the famous inverse square law.