Simple Nature - Light and Matter

An easy way to visualize this is in terms of a strip chart recorder,an obsolescing device consisting of a pen that wiggles back and forthas a roll of paper is fed under it. It can be used to record a person’selectrocardiogram, or seismic waves too small to be felt as anoticeable earthquake but detectable by a seismometer. Taking theseismometer as an example, the chart is essentially a record of theground’s wave motion as a function of time, but if the paper was setto feed at the same velocity as the motion of an earthquake wave, itwould also be a full-scale representation of the profile of the actualwave pattern itself. Assuming, as is usually the case, that the wavevelocity is a constant number regardless of the wave’s shape, knowingthe wave motion as a function of time is equivalent to knowingit as a function of position.WavelengthAny wave that is periodic will also display a repeating patternwhen graphed as a function of position. The distance spanned byone repetition is referred to as one wavelength. The usual notationfor wavelength is λ, the Greek letter lambda. Wavelength is to spaceas period is to time.Wave velocity related to frequency and wavelengthSuppose that we create a repetitive disturbance by kicking thesurface of a swimming pool. We are essentially making a series ofwave pulses. The wavelength is simply the distance a pulse is able totravel before we make the next pulse. The distance between pulsesis λ, and the time between pulses is the period, T , so the speed ofthe wave is the distance divided by the time,v = λ/T .This important and useful relationship is more commonly writtenin terms of the frequency,v = fλ .Wavelength of radio waves example 5⊲ The speed of light is 3.0 × 10 8 m/s. What is the wavelength ofthe radio waves emitted by KMHD, a station whose frequency is89.1 MHz?⊲ Solving for wavelength, we haves / Wavelengths of linear andcircular waves.λ = v/f= (3.0 × 10 8 m/s) /(89.1 × 10 6 s −1 )= 3.4 m352 Chapter 6 Waves