Bare glass reflects about 4% of the light that hits it — annoying glare in a camera lens, and lost light in a solar panel. A thin transparent film deposited on the surface fixes this through interference: part of the incident wave reflects off the top of the film, part travels through and reflects off the film-substrate boundary, and the two reflected waves recombine. This simulator renders that geometry in real 3D — a coated slab, an incident beam and its two reflected components — and computes the exact Fresnel/thin-film reflectance R(λ) from the coating thickness, the coating and substrate refractive indices, and the incidence angle, plotting the full visible-spectrum curve live. Hit the quarter-wave preset to see how a single well-chosen MgF₂ layer drives reflectance at 550 nm almost to zero, exactly as it does on real camera and eyeglass lenses.