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CED objectives

Electromagnetic Waves

▶︎  Watch it animatedinteractive step-through · ~3 min · optional ⚙︎  Open the appletSpectrum Bench · pump the chamber down and run the bell, then the lamp, and find where the felt ordering of the spectrum doubles back

An electromagnetic wave is an oscillation of electric and magnetic fields, mutually perpendicular and both perpendicular to the direction of travel, and it needs no medium because each field sustains the other. In vacuum every electromagnetic wave travels at $c = 3.00\times10^8$ m/s, so $c = f\lambda$ locks frequency and wavelength in inverse proportion. The spectrum runs from radio at the longest wavelengths through microwave, infrared, visible, ultraviolet and X-ray to gamma at the shortest, with red to violet inside the visible band.

Two errors dominate. Requiring a medium for light, which transfers the mechanical-wave rule to a field oscillation and leads to filling space with a thin something, or to expecting sound to reach us from the Sun. And ordering the spectrum by how dangerous or powerful each band sounds rather than by wavelength, which files X-rays as long waves because they penetrate deeply or puts gamma rays at the long end because they are the most energetic.

nothing material is moving: the fields sustain each other travel E B E and B are perpendicular to each other AND to the travel direction which makes light TRANSVERSE, and therefore polarizable the Sun is blindingly bright and completely silent: light crosses the vacuum, sound stops at the atmosphere
The two curves are field strengths, not displacements of anything. That is the whole reason no medium is required.
anchor both ends, then walk across radio microwave infrared visible ultraviolet X-ray gamma λ ~ kilometres λ ~ picometres λ DECREASES this way, so f and photon energy INCREASE red orange yellow green blue violet X-rays penetrate deeply because their wavelength is SHORT, not long the ordering is what is asked for, never exact wavelength ranges
Every band on this strip travels at the same speed in vacuum. The only thing the axis sorts is wavelength, and frequency follows from it.

The work

3 ways in · any order
Lesson
Electromagnetic Waves ›

Sorts waves by what is oscillating so light needs no medium, and orders the spectrum by wavelength with the applications following from it rather than from how powerful each band sounds.

Skill check · 10 scenarios
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Diagnostic
10-item topic check ›

Ten items spanning the failure modes of this topic: requiring a medium for light, and ordering the spectrum by how dangerous or powerful each band sounds. Take it cold to find which one is yours, or after the lesson to confirm it is not.

Not started · 10 items · ~15 min
Start diagnostic →
Targeted Practice
Drill a single misconception ›

Pick one of the failure modes you missed and drill it on its own. The round is adaptive: two correct in a row clears it for now and moves you to the next. Two in a row is a checkpoint, not proof: if the error resurfaces later, the misconception comes back.

Take the diagnostic to identify your misconceptions
Diagnostic first