Sound and light share many properties. Early recording devices, such as film strip optical recorders, have taken advantage of these similarities by converting audio signals into light waves, printing those waves onto film strips, then converting the film's light waves back to sound during playback. Among the similarities of light and sound are spectrum, waves, propagation and the Doppler effect.
Light and sound operate within specific spectrums in terms of human perception. Perceivable sound exists within a specific spectrum of frequencies, around 20 cycles per second, known as Hertz (Hz) to 20 kHz. Twenty Hz represents the lowest bass, and 20 kHz the highest treble. Humans can perceive light within its visible spectrum, which ranges from 400 nm to 700 nm in wavelength, where 400 nm is seen as red and 700 nm as violet.
Both sound and light consist of and travel in waves. These waves differ in length and frequency but share the same physical characteristics. Each wave can be a complex combination of simple sine waves, which are smooth, symmetrical waves that oscillate at a specific frequency. Each wave features peaks and valleys, which represent the wave's power, and a frequency, which represents the wave's visible color or sonic tone.
Both sound and light waves travel in similar fashions. Although they differ in that sound waves travel within air pressure and light travels regardless of air or vacuums, they do exhibit similar characteristics as they encounter objects. Both light and sound can be reflected or absorbed by objects such as walls, which change the characteristics of the sound or light visible. Light waves can change color as they bounce from objects (which give items their visible color), and sound can change its tone as different frequencies are absorbed or diffused.
Both waves exhibit the Doppler effect. As the sound or light source changes its distance relative to an observer, the waves will be perceived as rising in frequency as the source comes closer and lowering as the source moves farther away. A sonic example is an ambulance siren sounding like its pitch rises as it speeds toward a listener and lowers in pitch as it drives away. Light is also subject to the same phenomenon, which allows scientists to measure the speed of distant stars and planets.