Intro to Light

Recall that waves can be categorized into two major divisions:

Mechanical waves, which require a medium. These include sound, water and waves on a (guitar, etc.) string

Electromagnetic waves, which travel best where there is NO medium (vacuum), though they can typically travel through a medium as well. All electromagnetic waves can be represented on a chart, usually going from low frequency (radio waves) to high frequency (gamma rays). This translates to: long wavelength to short wavelength.

All of these EM waves travel at the same speed in a vacuum: the speed of light (c).

The standard wave velocity equation is still:




But for light, where c is the speed of light (3 x 10^8 m/s ):

c = f l


Still, f is frequency (in Hz) and l is wavelength (in m).  Note that, depending on your browser, the lambda (Greek symbol for wavelength) may show as an l.

(We will see during the next class that the speed of light, while constant in a vacuum, is NOT constant in other mediums.  That is, the speed of light does depend on what it travels through - and this causes refraction to occur, usually exhibited by bending of light.  More to come!)


General breakdown of e/m waves from low frequency (and long wavelength) to high frequency (and short wavelength):

Radio
Microwave
IR (infrared)
Visible (ROYGBV)
UV (ultraviolet)
X-rays
Gamma rays

In detail, particularly the last image:




Don't forget - electromagnetic waves should be distinguished from mechanical waves (sound, water, earthquakes, strings on a guitar/piano/etc.). 

ALL E/M waves (in a vacuum) travel at the SPEED OF LIGHT (c).


For E/M waves, the speed is the speed of light, so the expression becomes:
c = f  l

Also note that for a given medium (constant speed), as the frequency increases, the wavelength decreases.

Again, note that, depending on your browser, the lambda (Greek symbol for wavelength) may show as an l.





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