[Radiologic Physics] Ep 5 – Inverse Square Law

This episode opens with a review of wave-particle duality, establishing how x-rays can be described both as waves — characterized by wavelength and frequency — and as photons that interact with matter. Listeners explore how photon wavelength determines what size of matter it interacts with, from radio waves interacting with metal antennae down to x-ray photons interacting with atoms and electrons. Key terminology is introduced: radiopaque, radiolucent, attenuation, and intensity — along with the distinctions between reflection, transmission, attenuation, and absorption as x-rays and visible light pass through matter. The second half of the episode focuses on beam divergence and the Inverse Square Law. The cone-shaped geometry of the x-ray field is explained, showing why intensity decreases as photons spread over a larger area with increasing distance. The ISL formula (I₁/I₂ = D₂²/D₁²) is introduced and applied through worked clinical examples — including how doubling the SID reduces intensity to one-quarter and halving the distance quadruples it — with direct connections to SID adjustments, patient dose, and radiation protection practice. This episode aligns with the Safety content category — Radiation Physics and Radiobiology subcategory — of the ARRT Radiography Examination Content Specifications. Audio content is adapted from original instructional material developed by Professor Sanjay Arya, M.S., R.T.(R)(MR) for radiologic technology education. Part of the Radiologic Physics series — Quantum Imaging Lab. © 2026 Quantum Imaging Lab. All rights reserved.