Fisica IGCSE: onde, suono e luce, guida d'esame

Most waves marks are lost before any physics happens.

The content itself is not the hard part: a wave transfers energy without transferring matter, $v = f\lambda$ does most of the arithmetic, and light behaves predictably at a boundary. What separates a grade 6 from a grade 8 on a waves question is whether the student answered the question that was actually asked.

Examiners write every question around a command word, and that word is a contract. It tells you how long the answer should be and what kind of sentence earns credit. Learn to spot it, circle it, and let it shape the answer before you write a single line.

• State or Write down: one short line, no reasoning. "State one use of ultrasound." A three sentence answer here wastes ninety seconds and earns exactly the same one mark.
• Describe: say what happens, not why. "Describe the motion of the particles in a sound wave." You need the observable behaviour (particles vibrate parallel to the direction of energy transfer, producing compressions and rarefactions), not a causal chain.
• Explain: what happens and why. This is the word that costs students marks, because they describe and stop. If the question says explain, at least one of your sentences must contain a because, a therefore, or a physical cause.
• Suggest: the examiner knows this exact context is not in your syllabus. They want standard physics applied to an unfamiliar situation, so the mark scheme is generous. Never leave these blank.
• Calculate or Determine: working is compulsory. Determine often means "get the number from a graph or a diagram first", so check whether you are supposed to read a gradient or measure a wavelength before you start substituting.
• Complete the diagram or Draw: this is a practical skill question in disguise. Ruler, sharp pencil, arrows showing direction, and the normal drawn as a dashed line.

A quick habit that pays for itself: the mark allocation is a word count. A [1] answer is one clause. A [3] answer is three genuinely separate physics statements, not one statement said three ways.

IGCSE mark schemes for waves are built from independent scoring points. The marker reads your answer looking for specific physics statements, ticks each one once, and ignores everything else. This is enormously good news, because it means you can build long answers mechanically instead of hoping inspiration arrives.

The skeleton is: what changes, what stays the same, what the consequence is. Almost every waves explanation at this level fits it.

Take a classic three mark question: "Explain why a ray of light changes direction as it passes from air into glass."

• What changes: the light slows down when it enters the glass, because glass is optically denser than air.
• What stays the same: the frequency of the light is unchanged, so the wavelength decreases in proportion to the speed.
• The consequence: because one side of the wavefront enters the glass before the other, the ray changes direction and bends towards the normal.

Three sentences, three distinct physics ideas, three marks. Notice that the weak version ("the light bends because the glass is denser") contains only one of those three ideas and is worth one mark, even though it feels like a complete answer to the student writing it.

The same skeleton handles the other high frequency explanation questions. Why does the pitch of a note rise when frequency increases? Why do sound waves not travel through a vacuum? Why does a prism produce a spectrum from white light (different colours travel at different speeds in glass, so they have different refractive indices, so they refract through different angles)? In every case, name the change, name the invariant, then state the observable result.

One more discipline that costs nothing: write about the named object in the question. If the stem says "the ray of red light", your answer should say "the red light", not "it". Ambiguous pronouns are the single most common reason a technically correct sentence fails to score, because the marker cannot tell which thing you meant. For more on this pattern across the whole paper, see our guide to IGCSE Physics calculations students get wrong.

Calculation questions in the light topic are where method marks are handed out most generously and thrown away most often. Here is a full worked example laid out the way the mark scheme wants to see it.

Question.

A ray of light travels from air into a rectangular glass block of refractive index $n = 1.50$. The angle of incidence at the surface is $42^{\circ}$. (a) Calculate the angle of refraction. [3] (b) Calculate the critical angle for this glass. [2]

Part (a), step by step.

Write the equation first, in symbols, before any numbers touch the page:

$$n = \frac{\sin i}{\sin r}$$

Rearrange in symbols, still with no numbers:

$$\sin r = \frac{\sin i}{n}$$

Substitute:

$$\sin r = \frac{\sin 42^{\circ}}{1.50} = \frac{0.6691}{1.50} = 0.4461$$

Evaluate:

$$r = \sin^{-1}(0.4461) = 26.5^{\circ}$$

That is three clean marks: one for the correct equation, one for correct substitution, one for the final answer with its unit. Crucially, a student who makes an arithmetic slip and writes $r = 62^{\circ}$ still collects two of the three marks, provided the equation and the substitution are visible on the page. A student who writes only a wrong final answer collects zero. That is the entire argument for showing working, and it is worth several grade points across a full paper.

Part (b).

The critical angle is the angle of incidence inside the denser medium for which the angle of refraction is $90^{\circ}$:

$$\sin c = \frac{1}{n} = \frac{1}{1.50} = 0.6667$$

$$c = 41.8^{\circ}$$

And then the follow up that examiners love: since the ray in part (a) strikes the second surface at an angle greater than $41.8^{\circ}$, total internal reflection occurs there. Being able to link the two parts is what turns a competent answer into a top band one.

Three habits that protect these marks.

Keep your calculator in degrees, not radians, and check it in the first thirty seconds of the exam. Carry at least four figures through the middle of a calculation and round only at the end. And measure every angle from the normal, never from the surface, both in your calculations and in your diagrams. A full refresher on the equations you must recall is in IGCSE Physics formulas explained.

The sound sub topic is small, predictable and heavily reused, which makes it the best value revision in the whole waves unit. There are essentially four things examiners ask, and once you can produce all four on demand you have covered the great majority of what appears.

One. The nature of the wave.

Sound is a longitudinal wave: the particles of the medium vibrate parallel to the direction of energy transfer, producing regions of compression (particles close together, higher pressure) and rarefaction (particles further apart, lower pressure). Sound requires a medium, so it cannot travel through a vacuum. Write that last sentence exactly like that, because "there is nothing to vibrate in space" is a common answer that often fails to score on its own.

Two. The speed of sound by the echo method.

Stand a measured distance from a large flat wall, make a sharp sound, and time the interval until the echo returns. Suppose the wall is 200 m away and the echo arrives after 1.2 s. The sound travels there and back, so the distance is $2 \times 200 = 400$ m:

$$v = \frac{d}{t} = \frac{400}{1.2} = 333 \text{ m/s}$$

The mark that students drop here is the factor of two. The second mark they drop is the improvement question at the end: repeat the measurement several times and take a mean, because human reaction time is the dominant source of error, and use a longer distance so that the time interval is large compared with that reaction time.

Three. The wave equation applied to sound.

A tuning fork of frequency 256 Hz sounds in air where the speed of sound is 340 m/s:

$$\lambda = \frac{v}{f} = \frac{340}{256} = 1.33 \text{ m}$$

Note the units: frequency in hertz, speed in metres per second, wavelength in metres. If a question gives you kilohertz or centimetres, convert first and write the conversion down, because an unconverted unit turns three marks into one.

Four. Pitch, loudness and the audible range.

Higher frequency means higher pitch. Larger amplitude means louder sound. Those two are independent, and swapping them is one of the most frequent errors on the multiple choice paper. The normal human audible range is approximately 20 Hz to 20 000 Hz, and ultrasound is any sound above that upper limit, used for cleaning, sonar depth finding and medical imaging.

Sound also appears in the practical papers as a timing and mean value exercise, which is treated in detail in our guide to the IGCSE Physics practical papers.

On the extended theory paper the arithmetic is brutally simple: the total mark count and the number of minutes are close to equal, so you have roughly one minute per mark, including the time you spend reading, drawing and checking. Check the exact figures on the front cover of your own board's paper, but plan around one mark a minute and you will not be far wrong.

That single number should govern how you behave on a waves question:

• A [1] answer gets under a minute. State questions should be answered in the time it takes to write the sentence. If you are still thinking after forty seconds, write your best guess and move on. There is no penalty for a wrong statement.
• A [3] explanation gets three minutes, and it deserves them. This is where the skeleton from the previous section earns its keep, because you are not composing, you are filling a template.
• A ray diagram gets its full allocation and no more. Students routinely spend six minutes perfecting a four mark diagram. The marks are for the ruled straight lines, the dashed normal, the arrows showing the direction of travel and the correct geometry, not for beauty.
• The ninety second rule for calculations. If you have written the equation, substituted and are still stuck after ninety seconds, stop, leave the space, mark the question number in the margin and go on. Come back at the end. The two method marks are already banked on the page, which is the whole point.

The reading minute.

Before you start writing, spend one minute turning the pages of the paper. You are looking for two things: the long structured waves question (so you know it is coming and roughly what it is about) and any question that looks unfamiliar. Knowing what is ahead stops the panic that makes students rush the easy early marks.

The last five minutes.

Do not use them to reread everything. Use them in a fixed order: first, fill in every blank you skipped with something plausible, because a blank scores zero with certainty and a guess does not. Second, check that every calculated answer has a unit. Third, check that every diagram has arrows. Those three sweeps are worth more than rereading one hard question for the fourth time.

If you want the wider version of this approach across all topics, our article on IGCSE Physics tips to get an A star covers the same discipline applied to the full syllabus.

Every year the same handful of errors appears in the same places. None of them is a gap in understanding. All of them are habits, which means all of them can be trained out in a fortnight of deliberate practice.

• Angles measured from the surface instead of the normal. This is the number one error in the light topic, in calculations and in diagrams alike. Draw the dashed normal first, before the rays, every single time.
• Missing arrows on ray diagrams. A line without an arrowhead is not a ray, and mark schemes frequently make the arrow a separate scoring point.
• Assuming frequency changes on refraction. It does not. When light enters a new medium, speed and wavelength both change, frequency is fixed by the source. This appears as a multiple choice distractor constantly.
• Confusing amplitude with frequency. Amplitude relates to loudness and brightness, frequency to pitch and colour.
• Forgetting the factor of two in echo, sonar and ultrasound scanning questions.
• Units left off, or the wrong prefix. Wavelength in nanometres, frequency in megahertz and speed in metres per second cannot be combined until you have converted. Write the conversion as its own line so it can earn credit.
• Answering Explain with a description. Covered above, and worth checking one last time before you turn the page.
• Excessive rounding mid calculation. Rounding $\sin 42^{\circ}$ to 0.7 early can move your final angle by more than a degree and take you outside the mark scheme tolerance.
• Describing total internal reflection without both conditions. You need the light to be travelling from a denser to a less dense medium and the angle of incidence to exceed the critical angle. One condition alone is half an answer.
• Leaving Suggest questions blank because the context is unfamiliar. The context is meant to be unfamiliar. Apply the standard physics anyway.

A practical way to use this list: after every past paper, do not just total your score. Write down which item on this list cost you each lost mark. Within three or four papers a pattern emerges, and almost always two or three items account for most of the damage. Fix those two, and the grade moves without you learning any new physics at all.