# Are EM radiation and EM waves the same thing?

1. Are EM radiation and EM waves the same thing? I have seen this topics treated separately in many books. It is still not clear to me whether EM radiation and EM waves are synonymous. Is there any difference?

2. Another question: When one says that EM waves are solutions of Maxwell equations on vacuum does this mean that there is no charge at all in any point of space?

EM waves are a special case of electromagnetic radiation, where typically the source is periodic, or near enough that there is a carrier wave, as with radio and television.

Maxwell's equations support a "sourceless" electromagnetic wave, as if it has existed forever.

Let's consider two cases of electromagnetic radiation which is not obviously wave-like:

For a brief, non-periodic motion you get pulses, which can be decomposed into a myriad of wavelengths. For example, ultrafast optical pulses, with a duration of a few femtoseconds, can be modeled as the travelling interference packet of a very large number of independent waves, all travelling in the same direction. If you separate such a pulse with a diffraction grating -- you actually obtain such a spectrum! See chirped pulsed amplification, especially the diagrams.

For a more spread-out source, consider that thermal radiation typically originates from a large collection of randomly oriented and stimulated miniature antennas, which can be statistically described. Due to the random phase factors it will not show much (if any) coherence, but can be separated into a thermal spectrum with an appropriate diffraction grating. Due to the random nature of the thermal generation, you should not expect to see any large scale wave behavior.

With an IR viewer one can still see an image; this is due to contrast variations, corresponding, for example, to variations in temperature of the object.

• Thank you for the answer. So is there EM radiation which is not a EM wave? What would be the main difference this radiation originated from a non periodic source would have compared to EM wave? – user115652 May 7 '16 at 17:50

Whether waves == radiation is somewhat a question of semantics, and thus a bit subjective, however...

In general wave equations support radiation, but not all solutions to a wave equation are radiative. Evanescent solutions are also called waves ("evanescent waves"), yet typically not considered to be radiation since they do not propagate in three dimensions.

A notable example of electromagnetic evanescent waves is surface plasmons. The near fields around an oscillating dipole are are also waves in some sense, but they are not propagating.

• Thanks Nanite! Does the following analogy make sense? Radiation is for EM wave as perturbations in the sea are for a sinusoidal wave. For instance some perturbation in the fluid could be composed by many natural modes or maybe they could be non linear and the final result would be a dispersion. While a sinusoidal wave would keep propagating. – user115652 May 7 '16 at 19:59

1.

I think, the difference of "EM waves" and "EM radiation" is the different focus while it both deals with electromagnetic fields.

The first thing I associate with "EM waves" are the things I associate with waves: wavelength, frequency, diffraction, interference, phase & group velocity. The differential equations of waves and for EM waves in general the Maxwell equations. It is pretty much the classic electromagnetic theory.

In contrast "EM radiation" deals mostly with the characteristics of energy transfer: Thermal and Black-body radiation, flux (watt, lumen), intensity, brightness, reflectivity. In case of ionic radiation it looks at the (harmful) exchange of radiation with biological tissue.

2.

Different charge distributions cause electric and magnetic fields which will need energy to build up and which may have different characteristics. It may be a static electric field. It may be a static magnetic field. I can create EM waves with any phase velocity I like (Hint: c is not a barrier, see anomalous dispersion). The thing is that there is no connection between the wavefront, But important is: I always need energy to sustain this specific field.

The Maxwell equations now show us if and only if the speed of a resulting wave is exactly c, then no further energy is needed to propagate the wave: The wave becomes self-sustaining.

The short answer, appropriate to A level or below, is that there is no difference. The term "EM waves" usually means "EM radiation" - ie visible light, IR, UV, X-rays, gamma rays, radio waves, etc.

The situation is a little more complicated because the same EM radiation can behave as waves or as particles (photons) in different situations. It also occurs as long trains of a single frequency or short pulses of a spread of frequencies, but that doesn't make them different phenomena.

However, all periodic solutions to Maxwell's Equations are also called "EM waves". EM radiation is only one particular solution - the only solution which is self-sustaining and travels through a vacuum. All other solutions require the presence of charges, conductors or dielectric media or "wave-guides" of some kind.

Your 2nd question is not clear. That EM waves (ie EM radiation) are solutions of Maxwell's equations in a vacuum means that they can propagate through empty space for an unlimited distance and do not require any medium composed of polarisable material. However, they do require electric charges in order to be emitted (radiated) or absorbed.

• Are you able to measure the wave characteristics of an thermic source - for example of an electric bulb powered from an DC current? – HolgerFiedler May 7 '16 at 18:35
• @HolgerFiedler : I am sorry, I do not understand how your question relates to my answer. – sammy gerbil May 7 '16 at 18:38
• Read Petter Diehrs answer. A radio wave is a modulated EM radiation and really has the characteristics of a wave. A thermic source doesn't have such characteristics. The only waves are the photons of any EM radiation with their oscillating electric and magnetic field components. – HolgerFiedler May 7 '16 at 18:42
• @HolgerFiedler : It seems to me that you and Peter are complicating the issue. I think the OP is asking if there is any difference in essence, not in form. – sammy gerbil May 7 '16 at 19:08
• @HolgerFiedler Sure, the coherence length of a thermic source is quite small, but you can observe all wave characteristics: diffraction and interference. What exactly do you think happens with EM radiation from a thermic sources if it passes a slit ? Something different than light ? – Thorsten S. May 7 '16 at 23:15

## protected by Qmechanic♦May 7 '16 at 20:01

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