Simple Nature - Light and Matter

the story are more fittingly reserved for later in this course, but theend result was that a long series of experiments failed to detect anyevidence for the ether, and it is no longer believed to exist. Instead,light can be explained as a wave pattern made up of electrical andmagnetic fields.6.1.4 Periodic wavesPeriod and frequency of a periodic waveYou choose a radio station by selecting a certain frequency. Wehave already defined period and frequency for vibrations,T = period = seconds per cyclef = frequency = 1/T = cycles per secondω = angular frequency = 2πf = radians per secondbut what do they signify in the case of a wave? We can recycle ourprevious definition simply by stating it in terms of the vibrationsthat the wave causes as it passes a receiving instrument at a certainpoint in space. For a sound wave, this receiver could be an eardrumor a microphone. If the vibrations of the eardrum repeat themselvesover and over, i.e., are periodic, then we describe the sound wavethat caused them as periodic. Likewise we can define the periodand frequency of a wave in terms of the period and frequency ofthe vibrations it causes. As another example, a periodic water wavewould be one that caused a rubber duck to bob in a periodic manneras they passed by it.The period of a sound wave correlates with our sensory impressionof musical pitch. A high frequency (short period) is a high note.The sounds that really define the musical notes of a song are onlythe ones that are periodic. It is not possible to sing a nonperiodicsound like “sh” with a definite pitch.The frequency of a light wave corresponds to color. Violet is thehigh-frequency end of the rainbow, red the low-frequency end. Acolor like brown that does not occur in a rainbow is not a periodiclight wave. Many phenomena that we do not normally think of aslight are actually just forms of light that are invisible because theyfall outside the range of frequencies our eyes can detect. Beyond thered end of the visible rainbow, there are infrared and radio waves.Past the violet end, we have ultraviolet, x-rays, and gamma rays.Graphs of waves as a function of positionSome waves, light sound waves, are easy to study by placing adetector at a certain location in space and studying the motion asa function of time. The result is a graph whose horizontal axis istime. With a water wave, on the other hand, it is simpler just tolook at the wave directly. This visual snapshot amounts to a graphof the height of the water wave as a function of position. Any wavecan be represented in either way.o / A graph of pressure versustime for a periodic soundwave, the vowel “ah.”p / A similar graph for a nonperiodicwave, “sh.”q / A strip chart recorder.r / A water wave profile createdby a series of repeatingpulses.Section 6.1 Free Waves 351