Sound is not a thing travelling through air β it is the air itself being squeezed and stretched, each patch shoving the next one and springing back. That is why it needs something to travel in, why its speed is a property of the material and not of the sound, and why it bends round a doorway when light does not. Pick a medium below and watch the molecules.
Both are waves, and there the resemblance ends. One is a wobble in a material; the other is a wobble in the electromagnetic field itself, which is why it can cross empty space. Lab 11 covers light β
| Sound | Light |
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The source keeps emitting at the same rate. But each new wavefront starts from a point further forward, so the crests ahead are packed closer together and the ones behind are spread out. Push the speed past the speed of sound and the source outruns its own wavefronts, which pile onto a cone.
The picture has no absolute scale and runs far slower than reality. What is exact is the only thing that matters here: the ratio between how fast the source moves and how fast its sound does. At 0.5 Mach the wavefronts ahead are squeezed into half the distance; at 1.0 Mach they have nowhere left to go.
Trap a wave between two ends and it meets its own reflection. At most frequencies the two cancel into a mess; at a few special ones they lock into a pattern that stands still, with points that never move (nodes) and points that swing hardest (antinodes). Those few frequencies are the note you hear.
Two waves in the same air simply add. When their frequencies are close, they drift in and out of step, so the sum swells and fades at exactly the difference between them β 440 Hz against 443 Hz throbs three times a second. Piano tuners work by slowing that throb to nothing.