“The change in the apparent time interval between two events which arises from the motion of an observer together with the finite velocity of information about the events” (Gill 1965). Doppler effect is often used to mean frequency shifts (Doppler shift) of acoustic and electromagnetic waves because of relative motion between sources and observers. The relative magnitude of a Doppler shift is of order of the ratio of a characteristic speed (e.g., speed of a source) to a speed of propagation (e.g., speed of sound, speed of light). A shift to lower frequency (relative to a reference frequency) is sometimes called a red shift whereas a shift to higher frequency is sometimes called a blue shift, although no colorimetric meaning should be attached to these terms. Acoustic waves do not evoke sensations of color, nor do electromagnetic waves outside the visible spectrum, and even Doppler shifts of visible light are so small as to yield no visually perceptible color changes. According to classical theory, there is no frequency shift of electromagnetic radiation for motion of a transmitter perpendicular to the line between receiver and transmitter. But according to relativistic theory, even for this kind of motion there is a Doppler shift (transverse Doppler shift), although it is appreciably smaller than the longitudinal Doppler shift. See also Doppler frequency shift.