The questions are on the physical optics test. Take it under real conditions before reading the mark scheme.
Quick answer: Physical optics explains light and sound as waves that add together. This mark scheme gives the model answers and the marks for the 8 questions of the AP Physics 2 test on physical optics, from double slit calculation to beats, with the equivalent answers to accept.
Marks are shown for each step. Method marks are given for the right relationship even if the arithmetic slips, but every final answer needs its unit.
Level: AP Physics 2 (algebra-based) · Curriculum: AP P2 Unit 14
Question 1 Double slit calculation (3 marks)
a. Δy = λL / d = 520 × 10⁻⁹ × 1.80 / 1.5 × 10⁻⁴ = 6.24 mm.b. y₃ = 3 × 6.24 = 18.7 mm.
Marking: formula (1), correct value with unit (1); order multiplied (1).
Question 2 Changing the set-up (2 marks)
a. Halving d doubles the spacing and halving L halves it, so the new spacing is equal to the old one.b. Δy = λL / d: factor 2 from d and factor ½ from L cancel.
Marking: correct prediction (1); reason using the formula (1).
Question 3 Oil film (3 marks)
a. The air–oil reflection flips the phase; the oil–water reflection does not.b. 2nt = λ/2, so t = λ / 4n = 580 / (4 × 1.45) = 100 nm.
Marking: correct interface (1); condition (1); answer (1).
Question 4 Standing wave on a string (2 marks)
a. Three half wavelengths fit in 1.2 m: λ = 2L/3 = 0.80 m.b. f = v / λ = 48 / 0.80 = 60 Hz.
Marking: one mark each.
Question 5 Closed pipe (3 marks)
a. f₁ = v / 4L = 340 / 3.4 = 100 Hz.b. Next is the third harmonic: 300 Hz.
c. A closed end is a node and an open end is an antinode, so only odd multiples of a quarter wavelength fit; 200 Hz would need an even number.
Marking: one mark each part.
Question 6 Moving source (3 marks)
a. f′ = 800 × 340 / 320 = 850 Hz.b. f′ = 800 × 340 / 360 = 756 Hz.
c. The source moves between emissions, so wavefronts ahead are closer together and wavefronts behind are further apart.
Marking: one mark each part.
Question 7 White-light fringes (2 marks)
a. The central fringe is white, because all wavelengths have zero path difference there.b. Fringe positions depend on wavelength, so each colour has its own fringe positions and the colours separate at higher orders.
Marking: one mark each part.
Question 8 Beats (2 marks)
a. f_beat = 516 − 512 = 4 Hz.b. Time between loud sounds = 1 / 4 = 0.25 s.
Marking: one mark each part.
