This is the boss. No calculator — $c = 3.00 \times 10^8\ \mathrm{m/s}$, the angles are pinned at 30° so $\sin\theta = \tfrac{1}{2}$ exactly, and every value on the bench is chosen to fit in your head. One laser travels the whole length of the course: the wave equation gives its frequency, the index gives its speed and wavelength inside a block, Snell's law bends it at the face, and Young's slits paint what is left on the wall.

Every answer feeds the next. Write each line down as you go — one early slip rolls all the way to the wall, and the finish will trace exactly where the cascade started. Two things to hold onto: the frequency never changes, whatever the light passes through; and a block with parallel faces hands the beam back to the air with its original wavelength and its original direction, merely shifted sideways.

Everything from the chapter rides in it. 5★ under par here is the chapter crown. Clock in.