3.2Electromagnetic Energy

• Needs2.4Models and Applications

  Finding frequency from wavelength means rearranging c = λν to ν = c/λ, and finding photon energy from wavelength means substituting c = λν into E = hν to get E = hc/λ — both are isolate-then-substitute literal-equation moves that the worked example performs explicitly but the cell-phone-frequency Check Your Learning expects the student to redo without that scaffolding.

• Alongside5.9Modeling Using Variation

  c = λν is used throughout this section as an inverse relationship — as λ increases, ν decreases for fixed c — to answer qualitative ranking questions (which region of the spectrum has the higher frequency) without solving an equation; naming this explicitly as inverse variation sharpens the chemistry reasoning, and the chemistry gives the algebra concept a concrete home, but a student can still get the rankings right from plain proportional reasoning.

No algebra turns up here. Most of the descriptive sections need none.