Like all various other waves (waves in a string, water waves, sound, earthquake waves…), light and electromagnetic radiation in general can be defined as a vibration (more general: a periodical change of a certain physical quantity) the propagates into space. The propagation is caused by the fact that the vibration at a specific location impacts the an ar next come this location. For instance in the instance of sound, the alternative rarefaction and compression of air molecules at a specific location outcomes in periodic changes in the regional pressure, i beg your pardon in turn causes the movement of surrounding air molecules towards or away from this location.

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Fig. 1: Formation and propagation the a wave in a string

The propagation is resulted in by the truth that the vibration in ~ a particular location influences the an ar next come this location. For instance in the situation of sound, the alternating rarefaction and compression of air molecule at a details location results in periodic changes in the local pressure, which in turn causes the movement of nearby air molecules towards or far from this location.

Fig. 2: Formation and also propagation that a compressionwave in air,a phenomenon colloquially referred to as sound

In the instance of an electromagnetic wave, the system of propagation entails mutual generation that periodically varying electric and also magnetic fields and is far more an overwhelming to know than sound. Yet, the result can tho be defined as a periodic readjust of a physical quantity (the stamin of the electric and also the magnetic field) propagating into space. The velocity that this propagation is normally abbreviated with the letterc (unit: meters per second, m/s) and depends top top the medium and also nature the the tide (see Tab.1 below).


Optical (electromagnetic)radiation

atλ=434nm at λ=589nm atλ=656nm
in vacuum 299792km/s (n=1) 299792km/s (n=1) 299792km/s (n=1)
in air 340m/s 299708km/s (n=1.000280) 299709km/s (n=1.000277) 299710km/s (n=1.000275)
in water 1500m/s 223725km/s (n=1.340) 224900km/s (n=1.333) 225238km/s (n=1.331)

Tab.1: Velocities of sound and also light in air and also in water. Because that optical radiation, the particular index that refraction is provided in parenthesis

In bespeak to describe the straightforward properties that a wave, the following quantities have been identified for all kinds the waves:

Theamplitudeis the preferably disturbance the the tool from that equilibrium.In the instance of a tide in a horizontal string,this worth is identical with half of the verticaldistance in between the wave’s crest and also itstrough.

Thewavelengthλis the street betweentwo adjacent crests (or troughs) and is givenin meters.

The period T of a wave is the moment thatelapses in between the arrival of 2 consecutivecrests (or troughs) at a certain locationX. Thisdefinition is the same with the statement thatthe period is the moment the vibration at X takesto complete a full cycle native crest come trough tocrest. The period of a tide is offered inseconds.

The frequency f that a tide is the number ofvibration cycles per second at a certainlocation X. The unit of frequency is Hertz (Hz)and 1 Hz is the reciprocal of 1 second. Together anexample, a wave through a duration T=0.25stakes ¼of a 2nd to finish a fullvibration cycle (cresttroughcrest) at acertain location and thus performs fourvibrations per second. Thus its frequency is f= 4 Hz. Native this example, that is obvious thatthe period of a wave totally defines itsfrequency and also vice versa. The relationbetween these quantities is provided byf=1/T.If us look at a tide as a process that isperiodical in an are and in time, we can regard thewavelength λas the distance between tworepetitions the the process in an are and the periodT together the ”distance” between two repetitions of development in time.

A straightforward relation between wavelength, frequencyand velocity results from the followingconsideration:

During the moment span, a crest demands to travel thedistance that one wavelengthλ indigenous locationX tolocationY.This time expectancy is the same with the wave’s periodT. And when a crest needs the time spanT totravel the distanceλ, that is velocityc quantities to

c = λ =λf

When a tide passes from one medium toanother, its frequency continues to be the same. If thevelocities of the tide in the 2 media differ, thewavelengths in the 2 media also differ together aconsequence.Since the frequency that a wave does not dependon the tool the tide is passing, that is moreconvenient to usage frequency rather ofwavelength come characterize the wave. Inacoustics, this is typical practice– in many casesthe pitch of sound is defined by itsfrequency instead of the wavelength in a certainmedium (for example air).

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In optics, the case is different: In many cases, wavelength is used rather of frequency back this leads to a details complication: for example, environment-friendly light has a wavelength the 520nm in vacuum, yet in water that velocity is smaller sized by a factor of 1.33 and thus, in water the same green light has a wavelength of just 520/1.33= 391.0nm. Hence, if we desire to characterize a wave by its wavelength, we likewise have to state the medium for i beg your pardon the actual wavelength worth is given. According to CIE regulations, which are applied throughout this tutorial, the hatchet “wavelength” describes “wavelength in air” uneven otherwise stated. However, when using the offered wavelength figures to irradiate passing with a medium other 보다 vacuum, one need to keep in mind that the light’s wavelength changes according come the following relation

λMedium= λVacuum = λAir×nAir
nMedium nMedium


nAir= cVacuum


nMedium= cVacuum

nMediumis referred to as the medium’s table of contents of refractionand is much more commonly provided to specify the opticalproperties that a material thancMedium.