IB Physics: Reflection, Refraction & Diffraction FAQ

You need the laws of reflection and refraction, Snell's law with refractive index, critical angle and total internal reflection, plus diffraction through a single slit and a grating. According to the IB Physics guide (first assessed 2025), all of this sits under Topic C.3, Wave Phenomena, with extra grating and resolution content added at HL.

Reflection is a wave bouncing off a boundary at an equal angle of incidence and reflection. Refraction is a wave changing speed and direction as it crosses into a different medium. Diffraction is a wave spreading out after passing through a gap or around an obstacle — no boundary crossing involved at all.

Snell's law states $n_1\sin {\theta }_1=n_2\sin {\theta }_2$, linking the refractive indices and angles either side of a boundary. Use it any time a ray crosses from one medium into another — glass into air, water into glass — to find an unknown angle or refractive index.

Total internal reflection happens when light travelling from a denser to a less dense medium hits the boundary at an angle greater than the critical angle, so all the light reflects back with none refracting out. The critical angle is found from $\sin {\theta }_c=n_2/n_1$, where $n_1$ is the denser medium.

The narrower the slit relative to the wavelength, the more the wave spreads out after passing through — diffraction becomes most noticeable when slit width and wavelength are roughly the same size. Widen the slit relative to wavelength and the wave passes through with far less spreading, behaving more like a ray.

These fall under Topic C: Wave Behaviour, specifically subtopic C.3, Wave Phenomena, in the current IB Physics guide (first assessed 2025). It sits alongside C.1 simple harmonic motion, C.2 the wave model, C.4 standing waves and C.5 the Doppler effect, so questions often link across the whole topic.

Yes — SL students need reflection, refraction, Snell's law, critical angle, TIR and qualitative single-slit diffraction. HL adds the diffraction grating equation and resolution, including the Rayleigh criterion for resolving two close sources. HL exam questions also push further into multi-step calculations combining two or three effects.

The three I see every mock exam: mixing up which medium is denser when applying critical angle, forgetting units when converting lines-per-millimetre into slit spacing $d$, and describing diffraction as 'bending' instead of 'spreading'. All three cost easy marks that have nothing to do with understanding the physics.

Use $n=\sin {\theta }_1/\sin {\theta }_2$ from Snell's law when angles are given, or $n=c/v$ if you're given the speed of light in the medium. Always state which angle is in which medium clearly before substituting — examiners mark the setup, not just the final number.

I always tell my students to draw the ray diagram first, then attach the equation to it — not the other way round. Practise past paper questions that combine reflection, refraction and diffraction in one scenario, since that's exactly how the IB tends to test C.3 at both SL and HL.

They can — refraction through lenses or prisms and diffraction gratings are popular, measurable IA topics because they give clean numerical data for a graph and a clear independent variable. Reflection alone is less common as a standalone IA since it's harder to generate a rich enough dataset for high marks.

Not conceptually harder, but there's more of it — HL adds the diffraction grating equation and resolution using the Rayleigh criterion on top of everything SL covers. The real difficulty jump is in multi-step Paper 2 questions that chain refraction, TIR and diffraction together across several sub-parts.

Look for resources that separate SL and HL content clearly and include worked calculations, not just definitions — that's where most marks are lost on this topic. A good question bank with mark schemes lets your child self-check the working, not just the final answer, which matters most for Snell's law and grating questions.