Practice question

**Visible light** λ≈400-700 nm, f≈4×10¹⁴ to 7×10¹⁴ Hz, narrow part of spectrum sensitive to human eye, produced by atomic transitions, used for vision, photosynthesis, optical fibers, central frequency ≈5×10¹⁴ Hz, different colors correspond to different λ. The document mentions that infrared waves are absorbed by materials, increasing thermal motion and heating the surroundings, which is useful for physical therapy. Using c = fλ, E₀/B₀ = c, I_d = ε₀ dΦ_E/dt, and spectrum classification λ = c/f, evaluation yields They heat tissues by increasing thermal motion, illustrating EM wave transverse nature and Maxwell's displacement current concept.