IGCSE Physics: waves, light and sound

Key ideas, board by board

General wave properties are shared: wavefront, wavelength, frequency, amplitude, wave speed, transverse and longitudinal waves, reflection and refraction. Cambridge Core (3.1) also includes diffraction through a gap and at an edge, shown in a ripple tank; Supplement explains how wavelength and gap size affect diffraction. Cambridge models seismic S-waves as transverse and P-waves as longitudinal. Edexcel adds the Doppler effect (3.8): a source moving towards you gives a higher observed frequency and shorter wavelength, moving away gives a lower frequency and longer wavelength.

Light: both boards require the law of reflection, ray diagrams, refraction experiments with glass blocks, critical angle and total internal reflection. Cambridge Supplement holds n = sin i / sin r, n = 1 / sin c, refractive index as a ratio of speeds and optical fibres in telecommunications; Edexcel requires both equations of every student (3.18, 3.22) and lists the refractive index of a glass block as a practical (3.19). Thin lenses and dispersion by a prism are Cambridge content (3.2.3, 3.2.4); Edexcel does not include lenses.

Electromagnetic spectrum: in order of decreasing wavelength and increasing frequency, radio, microwaves, infrared, visible, ultraviolet, X-rays, gamma rays. All travel at the same speed in a vacuum. Both boards list uses and harmful effects; Cambridge Supplement adds communication systems and digital versus analogue signals. Sound: Cambridge Core includes the 20 Hz to 20 000 Hz range, the speed of sound in air (about 330 to 350 m/s), echoes and ultrasound; Supplement adds compressions and rarefactions and sonar calculations. In Edexcel, sound as a longitudinal wave is on both papers (3.23) but hearing range, the speed of sound practical, oscilloscopes, pitch and loudness are 3.24P to 3.29P, Paper 2 only.

The electromagnetic spectrum: uses and dangers

The uses in each board's list differ slightly; learn the examples printed in your own syllabus.

Worked examples

1. Wave equation. An FM radio station broadcasts at 100 MHz. The speed of radio waves is 3.0 x 10^8 m/s. Wavelength = v / f = 3.0 x 10^8 ÷ 1.0 x 10^8 = 3.0 m. Remember 1 MHz = 1 000 000 Hz.

2. Period. A wave on an oscilloscope has a period of 0.020 s. f = 1 / T = 1 ÷ 0.020 = 50 Hz.

3. Refractive index. A ray enters a glass block with angle of incidence 40 degrees and angle of refraction 25 degrees. n = sin 40 / sin 25 = 0.643 ÷ 0.423 = 1.52. Angles are measured from the normal, not from the surface.

4. Critical angle. For glass with n = 1.5, sin c = 1 / 1.5 = 0.667, so c = 41.8 degrees, about 42 degrees. A ray inside the glass hitting the surface at more than 42 degrees is totally internally reflected.

5. Sonar (Cambridge Supplement). An ultrasound pulse returns to a ship 0.40 s after it is sent. Sound travels at 1500 m/s in seawater. The pulse travels down and back, so depth = (1500 x 0.40) ÷ 2 = 300 m.

Common mistakes

• Measuring angles of incidence and refraction from the surface instead of the normal.
• Forgetting to halve the distance in echo and sonar questions.
• Saying light slows down and bends "towards the surface". Entering a denser medium it bends towards the normal.
• Writing that sound can travel through a vacuum. It needs a medium.
• Mixing up the order of the electromagnetic spectrum, or saying that higher frequency waves travel faster. All travel at the same speed in a vacuum.
• Confusing amplitude (loudness, energy) with frequency (pitch).
• Saying total internal reflection happens when light goes from air into glass. It only happens going from the denser medium towards the less dense one, beyond the critical angle.

Exam technique and mark-scheme language

Ray diagrams are marked for accuracy: use a ruler and a sharp pencil, draw the normal as a dashed line at 90 degrees to the surface, add arrows on rays, and measure angles with a protractor. For total internal reflection, examiners look for two conditions: light travelling from a more dense to a less dense medium, and angle of incidence greater than the critical angle.

In "describe an experiment" questions, name the apparatus (ray box, glass block, protractor, sheet of paper), describe what you measure, how many values, and how you process the results, for example plotting sin i against sin r, where the gradient is n. For speed of sound in air, describe two people a measured distance apart (at least 100 m) timing the delay between seeing and hearing a clap or starting pistol, repeating and averaging.

How one-to-one lessons help with this topic

Waves mixes calculation, drawing and recall, and students often neglect one of the three. A tutor uses the shared whiteboard for ray diagrams the student draws live, checks the trigonometry in refractive index questions (where many slip on calculator mode), and uses quick-fire recall rounds for the electromagnetic spectrum. For Cambridge Extended candidates the tutor also covers lens ray diagrams, which are hard to learn from a textbook alone.

Self-check

1. Define wavelength, frequency, amplitude and wavefront.
2. Give two examples each of transverse and longitudinal waves.
3. Use v = f x wavelength and f = 1 / T with prefixes such as kHz and MHz.
4. Draw ray diagrams for reflection and refraction with correct normals.
5. Calculate refractive index and critical angle.
6. Explain how optical fibres use total internal reflection.
7. List the electromagnetic spectrum in order with one use and one danger for each region.
8. Describe how to measure the speed of sound in air.