Feedback: Topic 3 Waves Challenge - Tara's IGCSE Study Hub

🌟 What you're excellent at

Your light and optics work was perfect — 23/23 across fibre optics and refraction. Both TIR conditions stated precisely, the cladding refractive-index calculation (1.52 × sin 62° = 1.34) done like a pro, critical angle logic applied correctly at 55°, and the diamond-sparkle explanation hit every marking point. Seismic waves were nearly as good — the P/S-wave distance calculation (378 km) was genuinely hard algebra and you nailed it. Your echo-method experiment description in Q7(a) was full marks: timing 20 claps to reduce reaction-time error is exactly the answer examiners want.

⚠️ The big finding: 16 of your 26 lost marks were never attempted

This is the most important thing in this whole report. Most of your lost marks weren't wrong physics — they were sub-parts (ii) and (iii) left blank:

Q2(c)(ii) — what happens to wavelength & frequency: not attempted (1 mark)
Q4(b)(ii) — the depth table for positions A, B, D, E: not attempted (4 marks)
Q4(c)(ii) — percentage error calculation: not attempted (2 marks)
Q6(b)(iii) — why the delay ruins phone calls: not attempted (1 mark)
Q6(c)(i)&(iii) — noise on analogue signal + one extra digital advantage: not attempted (2 marks)
Q7(b)(ii) — speed of sound from your own data: not attempted (2 marks)
Q7(c)(ii) — feature to fix echo problems: not attempted (1 mark)
Q7(d) — the whole airport noise question: blank (3 marks)

The fix is pure exam technique: before moving on from any question, count its parts — (i), (ii), (iii) — and tick each one off. You have 75 minutes for 80 marks, about 55 seconds per mark. An attempted answer can score; a blank never can. In Q7(b)(ii) you'd already done the hard part (0.47 s) — the speed was just 2 × 80 ÷ 0.47 = 340 m/s. Two free marks were sitting there.

⚖️ Two marks you should have claimed

Q2(c)(i): you wrote “6.0 / 0.50 = 12 km” — that IS the correct wavelength with the unit. The notation at the start was scrambled (“v = f × λ = v / f”), but the answer earns the mark.
Q5(a): you wrote “v = 10 × 0.020 = 0.20 m/s” — that matches the marking point exactly. Tick it!

When self-marking, compare your final answer to the marking point. If it matches, the mark is yours — even if your working looks messy.

🔧 Real physics gaps to fix (worth 8 marks)

1. Sonar calibration logic (Q4c) — you said overestimate; it's underestimate

Your instinct was right — “the echo returns quicker” — but follow it one more step:

Sound is really faster (1540 m/s) → echo returns sooner → measured time t is smaller
Sonar still computes depth = ½ × 1500 × t → smaller t × same 1500 = smaller depth → UNDERestimate
% error = (1540 − 1500) / 1540 × 100 ≈ 2.6%

Rule of thumb: if the assumed speed is too LOW, the calculated distance is too LOW.

2. Why sonar uses pulses, not continuous sound (Q4d)

You wrote “to conserve energy” — this is the classic trap. The real reason: with continuous sound, the outgoing signal and returning echo overlap, so you can't tell when the echo arrives. Short pulses leave silent gaps so each echo is detected cleanly and its travel time measured accurately.

3. Diffraction answers need BOTH halves of the comparison (Q5b, Q7d)

You said “the radio wavelength is quite large” — half the answer. The full version always compares wavelength to obstacle size, both ways:

Radio: λ ≈ 3 m, comparable to the building → diffracts around it → signal received
Light: λ ≈ 5 × 10⁻⁷ m, vastly smaller → negligible diffraction → sharp shadow

Same skill kills Q7(d): sound (λ roughly 0.3–3 m) diffracts over a 5 m wall, so the barrier only partly works — and lower frequencies (longer λ) diffract most.

4. Refraction direction (Q5c ii)

You correctly computed the new wavelength (1.4 cm) and kept f = 10 Hz — but the question also asked for direction: waves slowing down bend towards the normal. Slower = towards; faster = away.

5. Echo geometry in the concert hall (Q7c)

You calculated the direct sound (5 ÷ 340) but the echo path is stage → back wall → listener = 45 + 40 = 85 m:

Delay = (85 − 5) / 340 = 80 / 340 ≈ 0.24 s — way over the 0.05 s limit
Fix: absorbing panels / diffusing surface on the back wall

📊 Question by question

Q	Topic	Score	Note
1	Optical fibres & TIR	12/12	Perfect
2	Seismic waves	10/12	+1 under-marked; (c)(ii) blank
3	Refraction & dispersion	11/11	Perfect
4	Sonar	3/12	Table blank, calibration logic, pulses
5	Ripple tank	7/11	+1 under-marked; comparisons incomplete
6	EM spectrum & signals	6/10	Digital signals shaky; (iii)s skipped
7	Sound experiments	5/12	Great method; follow-through missing

🎯 Your action plan

This week: redo Q4(b)(ii), Q4(c)(ii), Q7(b)(ii) and Q7(d) on paper — all four are now fully within your ability, and they're worth 12 marks.
Revise two ideas: the sonar under/overestimate logic, and the “compare λ to obstacle size — both ways” diffraction sentence. They appear constantly in Cambridge papers.
Exam habit: tick off every (i)/(ii)/(iii) before leaving a question. Your blank sub-parts cost more than your physics mistakes did — fix this one habit and this same paper is a 68–70/80.
Digital signals (Q6c): 10-minute read of the study guide section — regeneration works because noise can't disguise a 1 as a 0; extra advantages: encryption, compression, multiplexing.

Bottom line: the physics understanding is clearly there — optics flawless, hard algebra handled. Finish every sub-part and claim your correct marks, and you're an A* candidate on this topic. 💪

📝 Personalised Feedback

🌟 What you're excellent at

⚠️ The big finding: 16 of your 26 lost marks were never attempted

⚖️ Two marks you should have claimed

🔧 Real physics gaps to fix (worth 8 marks)

📊 Question by question

🎯 Your action plan