“nλ = d sinθ,” he said. “No approximation.”
The paper landed on his desk. He flipped through it calmly. Question 2: a satellite losing energy due to atmospheric drag. He smiled. Question 5: an alpha decay calculation. He checked the units—MeV to joules—before he even wrote the first line. Question 7: a design for measuring the viscosity of a liquid using only a marble, a graduated cylinder, and a stopwatch. No laser this time, but the same principle: use what you have, think from first principles.
He flipped to 2021 Paper 1, multiple choice. Question 17: A particle moves in a circle with constant angular speed. Which graph shows the variation of its acceleration with time? The obvious answer—a sine wave—was wrong. Centripetal acceleration for uniform circular motion is constant in magnitude, only direction changes. The graph should be a straight line. He’d chosen the sine wave in his first attempt. The mark scheme said: B (straight line). Common distractor: C (sinusoidal). He drew a star next to it.