Figure 4. Structural color is the result of the constructive or destructive interaction of alternative, multiple pathways for light. 1, Interference through a single layer (thin layer) with negligible refraction at the boundary. Arrows show the two different pathways that a photon can take as it is reflected, each of a different length. 2, Interference through a single layer (thick layer), with a higher refractive index (shaded, ~1.5) than air (~1.0). Both the thickness and refractive index of the layer produce a longer path for light that is not reflected at the upper surface. 3, The many alternate paths that a photon could take through a multilayer reflector, comprised of alternating layers with high (white, ~1.0) or low (shaded, ~1.5) refractive index. 4, Another view of the many pathways that a photon could take through this multilayer (3), showing each pathway separately. With regular spacing and alignment at the nanoscale, this can produce a series of consistent differences (each corresponding to a multiple of the favored wavelength) and a constructive effect as a result. 5, Passage of light through the slits of a transmissive diffraction grating. As with the interference of light through reflection at multiple layers, this produces a series of possible pathways for each photon, each of different length. As shown here, red light, because of its longer wavelength, is favored over blue light at larger angles relative to the incident light. 6, Reflection of light from a reflective diffraction grating. The effect is similar to that seen in a transmissive grating (5), in that the interaction of photons with different points of reflection produces a series of different path lengths, favoring longer wavelengths (red) at lower angles of reflection relative to the incident light. 7-12, Relationship of relative direction of incoming light, wavelength, and spacing of reflectors to interference. When the difference between two alternative pathways is equal to an integer multiple of the wavelength of a photon, interference of light traveling on the two pathways is constructive ("additive"). When it is equal to an integer multiple of half of that wavelength, interference is destructive ("subtractive" as one path cancels out the other). Anything else is somewhere in-between.