Sound and Light

Every wave obeys one rule: speed equals frequency times wavelength. Sound in air moves at a roughly fixed speed, so if you know the frequency of a tone, you can always back out its wavelength, and the reverse. Push frequency up and wavelength must shrink to keep the product the same. This single relationship is the one a background-knowledge question is really testing, no matter how the scenario is dressed up.

The ambulance siren is the classic case of relative motion changing what a listener hears. The siren itself never changes frequency. But while the ambulance is approaching, it's chasing its own sound waves, crowding them closer together in front of it, so the listener catches more wave crests per second and hears a higher pitch. Once it passes and moves away, it's outrunning the waves behind it, spreading them out, so the listener catches fewer crests per second and the pitch drops. The sudden flip happens right at the moment of passing because that's when the motion switches from toward you to away from you.

Light works differently because it isn't about motion, it's about speed changing at a boundary. Light travels slower in water than in air. When a ray crosses from air into water at an angle, the part of the wavefront entering first slows down before the rest, which swings the whole ray toward the normal, the imaginary line perpendicular to the surface. That bending is why a pencil in a glass of water looks broken at the surface: light from the submerged part bends before reaching your eye, shifting where it appears to be.