Frequency hopping can be superimposed on other modulations or waveforms to enhance the system performance. Interference is the variation in intensity which can occur when two light waves are superimposed. Overlays are generally superimposed over the main price chart. On the medallion is a shield surmounted by an eagle superimposed over clouds. Thus, speckle and resonant Rayleigh-scattering are always superimposed to the incoherent emission. Occasionally, tracks are superimposed to imitate many instruments played simultaneously. Masks are images that are superimposed onto the camera in real-time. Interference occurs when one or more wavefronts are superimposed. The United States Note obligation now became superimposed over a gray numeral 2. The figures are sometimes simply superimposed without apparent order. There is an extended sequence where the film is superimposed over itself in reverse. Polychords are formed by two or more chords superimposed. Kanesh is the result of several superimposed stratigraphic periods. Additional information is both superimposed on and arranged around this graphic. The performance featured sweeping shots of iconic United States landmarks with contestants faces superimposed.