• $summary_{type}$: concept
• $word_{count}$: 160

When light reflects from a thin film (thickness t, refractive index μ), interference occurs between rays reflected from the top and bottom surfaces. The path difference is 2μt (geometric). A phase change of π (equivalent to λ/2 extra path) occurs when light reflects from a denser medium. For a film in air: constructive reflection when 2μt = (2n-1)λ/2 (bright), destructive when 2μt = nλ (dark). The conditions swap if the phase change situation differs (e.g., film between two different media). Colors in soap bubbles arise because different thicknesses satisfy the constructive condition for different wavelengths. Newton's rings (between a plano-convex lens and flat glass) have dark center in reflected light (t = 0 plus π phase change). Dark ring radii $r_n$ = $\sqrt{nλR}$. The rings get closer together at larger radii (r ∝ √n).