An illusion is a lack of correspondence between the perception of an object and the physical reality. Illusions are often the result of a misinterpretation by the brain of information received by the senses. Most commonly the sense involved is sight: one of the exploitations of optical illusions is the use by artists of perspective. Optical illusions may also have external causes, such as refraction, as in the observation of a stick held in water (see also Brocken specter). Examples of auditory illusions including beating and the apparent change in pitch of a railroad train's whistle as it passes (see Doppler effect). Rather different classes of illusion are hallucinations and eidetic images. The unconscious falsification of the memory of a past experience is also termed an illusion.