What is Em wave: Definition and 224 Discussions

In physics, electromagnetic radiation (EM radiation or EMR) refers to the waves (or their quanta, photons) of the electromagnetic field, propagating through space, carrying electromagnetic radiant energy. It includes radio waves, microwaves, infrared, (visible) light, ultraviolet, X-rays, and gamma rays. All of these waves form part of the electromagnetic spectrum.Classically, electromagnetic radiation consists of electromagnetic waves, which are synchronized oscillations of electric and magnetic fields. Electromagnetic radiation or electromagnetic waves are created due to periodic change of electric or magnetic field. Depending on how this periodic change occurs and the power generated, different wavelengths of electromagnetic spectrum are produced. In a vacuum, electromagnetic waves travel at the speed of light, commonly denoted c. In homogeneous, isotropic media, the oscillations of the two fields are perpendicular to each other and perpendicular to the direction of energy and wave propagation, forming a transverse wave. The wavefront of electromagnetic waves emitted from a point source (such as a light bulb) is a sphere. The position of an electromagnetic wave within the electromagnetic spectrum can be characterized by either its frequency of oscillation or its wavelength. Electromagnetic waves of different frequency are called by different names since they have different sources and effects on matter. In order of increasing frequency and decreasing wavelength these are: radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, X-rays and gamma rays.Electromagnetic waves are emitted by electrically charged particles undergoing acceleration, and these waves can subsequently interact with other charged particles, exerting force on them. EM waves carry energy, momentum and angular momentum away from their source particle and can impart those quantities to matter with which they interact. Electromagnetic radiation is associated with those EM waves that are free to propagate themselves ("radiate") without the continuing influence of the moving charges that produced them, because they have achieved sufficient distance from those charges. Thus, EMR is sometimes referred to as the far field. In this language, the near field refers to EM fields near the charges and current that directly produced them, specifically electromagnetic induction and electrostatic induction phenomena.
In quantum mechanics, an alternate way of viewing EMR is that it consists of photons, uncharged elementary particles with zero rest mass which are the quanta of the electromagnetic field, responsible for all electromagnetic interactions. Quantum electrodynamics is the theory of how EMR interacts with matter on an atomic level. Quantum effects provide additional sources of EMR, such as the transition of electrons to lower energy levels in an atom and black-body radiation. The energy of an individual photon is quantized and is greater for photons of higher frequency. This relationship is given by Planck's equation E = hf, where E is the energy per photon, f is the frequency of the photon, and h is Planck's constant. A single gamma ray photon, for example, might carry ~100,000 times the energy of a single photon of visible light.
The effects of EMR upon chemical compounds and biological organisms depend both upon the radiation's power and its frequency. EMR of visible or lower frequencies (i.e., visible light, infrared, microwaves, and radio waves) is called non-ionizing radiation, because its photons do not individually have enough energy to ionize atoms or molecules or break chemical bonds. The effects of these radiations on chemical systems and living tissue are caused primarily by heating effects from the combined energy transfer of many photons. In contrast, high frequency ultraviolet, X-rays and gamma rays are called ionizing radiation, since individual photons of such high frequency have enough energy to ionize molecules or break chemical bonds. These radiations have the ability to cause chemical reactions and damage living cells beyond that resulting from simple heating, and can be a health hazard.

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  1. U

    Electromagnetic field vs electromagnetic wave "radiation"

    ( @jrmichler wrote to me: "SHEESH, more fearmongering. But please do not bring these conspiracy theories to PF." You write to wrong person, you and moderator of PF can send a email to scientists in WHO/International Agency for Research on Cancer (IARC) and ask them why they make this...
  2. Sibilo

    I Can EM waves stimulate neurons?

    I'm posting this topic after an invitation to do so. So considering that transcranial magnetic stimulation which operates in frequencies and therefore through EM induction can excite neurons, then can an EM wave also excite neurons?
  3. Hak

    B Frequency of an EM wave in Classical and Quantum Physics

    In Classical Physics, when a charged particle oscillates, it emits an electromagnetic wave, and the frequency of the wave depends on the frequency with which the particle oscillates. But in Quantum Physics, when an excited atom emits a photon, the energy of the photon depends on the amplitude of...
  4. LarryS

    I Transmission Line EM Wave vs EM Wave in Free Space

    According to Maxwell’s Equations, the speed an EM plane wave in free space, far from its source, is determined by the electric constant, ε0, and the magnetic constant, μ0, such that c = 1/√( ε0 μ0). The units of ε0 are capacitance per unit length and the units of μ0 are inductance per unit...
  5. R

    B Interpreting light as Maxwell's EM wave

    Sometimes I cannot imagine light as the popular Maxwell's blue and red electric and magnetic wave https://simply.science/images/content/physics/Electromagnetism/em_waves/emv.jpg but I found the image below to be the more accurate representation of how light actually looks like as waves...
  6. Leureka

    I How does EM wave geometrical attenuation affect atomic absorption?

    Let's say we have a point source of an EM wave in a vacuum of total energy E, and an absorber atom at some distance from this source, whose first excited state is at the energy B, with B < or = E. The energy of the wave is constant as a whole, but at each point around the source the energy...
  7. C

    A Penetration depth of a ion beam coupled with an EM wave

    A Ion source is a device that allows creating ion beams (e.g. argon ions) and to project them outside the device, for example to be further processed by a particle accelerator, or to irradiate materials or biological tissues etc. Now, suppose the ion beam is coupled with an EM wave, especially...
  8. T

    I EM wave propagation: respective phase of E and M field

    Hi alltogether, I have been confused about a certain topic of EM wave propagation: it´s clear to me that E and M field are perpendicular to each other (I know Maxwell´s equations well). But: sometimes you can find on the internet that both fields are in phase...
  9. LarryS

    I Is E/B = c for spherical EM Wave in Vacuum?

    In classical EM, consider an EM plane wave traveling in free space. The ratio of the amplitude of the electric field to the amplitude of the magnetic field is the velocity of the wave, the speed of light. Is the above also true if the wave is spherical, expanding from a point source, as in a...
  10. wnvl2

    I Applying Reisenbach Transf. to EM Wave in Microwave Oven

    There is no possible measurement, no matter how clever, that can measure the one way speed of light. It is a synchronization convention. In this topic I would like to apply this idea on a specific case. I have a microwave oven with width L. In this oven I have a standing wave. $$E(t,x)=E...
  11. H

    I Understanding the Expression for a Linear EM Wave Transmission?

    Hi, I have an expression in my textbook that I don't really understand. I have 2 questions regarding this expression for a linear EM wave## \tilde{\vec{E_{0i}}} = (E_{0x} \hat{x} \pm E_{0y} \hat{y}) e^{i(kz- \omega t)}## ## \tilde{\vec{E_{0t}}} = (\sum_j E_{oij} e_{pj}) \hat{e_p} ## ##...
  12. Boltzman Oscillation

    I How does the magnetic component of an EM wave affect surroundings?

    Hi all, as we all know EM waves are made up of magnetic and electric waves in a plane perpendicular to the direction of propagation. Given this, why don't I see conductors being affected when I shine light at them? Woulnt the magnetism cause a force? Is is that the force is too small? What am i...
  13. C

    I Equivalent formula for a Sound wave in a medium like an EM wave

    1.) In electromagnetics, wavelength in a medium is $$\lambda = \frac{\lambda_{0}}{n}$$, where $$n$$ is the refractive index. What is the equivalent formula for sound wave in a medium? 2.) Is there a reference sound velocity, like electromagetic wave speed in vacuum is $$c_{0} =...
  14. P

    Analogy between EM wave reflection and S-parameters

    It is well known that one can solve incident an reflective wave in homogeneous linear media by matching PDE boundary conditions. In the electrical engineering community, one solves similar problem using smith chart and scattering parameters for 1-dimensional propapation of TEM modes in...
  15. F

    I Why we know average speed of single photon equal speed of EM wave?

    Why we know that average speed of a single photon(in point particle view) equal the speed of EM wave?If average speed of a single photon smaller than c then there exist massive photons?
  16. A

    EM wave reflected or transmitted?

    As I learn in class, when EM wave goes from medium 1 to medium 2, there are 3 possibilities that can happen Totally transmitted (i.e when the angle of incident is 0 degree) Partially transmitted and reflected (i.e when the angle of incident is between 0 and critical angle) Total internal...
  17. E

    Average Energy density and the Poynting vector of an EM wave

    Hi, In Problem 9.12 of Griffiths Introduction to Electrodynamics, 4th edition (Problem 9.11 3rd edition), in the problem, he says that one can calculate the average energy density and Poynting vector as using the formula I don't really understand how to do...
  18. F

    I The Speed of Light: Comparing Photon and EM Wave Velocities

    Velocity of photon allways is c(photon is massless particle).While velocity of EM wave in medium < c.So does velocity of photon need not allways equal velocity of EM wave?
  19. F

    I Is there an uncertainty between amplitude and phase in an EM wave?

    Is there an uncertainty between amplitude and phase in classical quasi-monochromatic light?(E(t)=a(t)cos(phi(t)-omega_0*t))If it exist, what is the relation between classical and quantum uncertainty(delta I* delta phi>=1/2)?
  20. F

    I What is difference between EM wave and EM field?

    They say wave function is different to quantum field. Then what is the difference between EM wave and EM field?(By the way :Is that EM wave the wave function of photons?).It seem to me EM wave is the wave of EM field?
  21. Mr_Allod

    EM Wave Reflection and Transmission Between 3 Materials

    Hello there. I set up the problem like this, I have a wave incident from air on the anti-reflective coating consisting of: ##\tilde {\vec E_I} (z,t) = \tilde E_{0_I} e^{i(k_1z- \omega t)} \hat x## ##\tilde {\vec B_I} (z,t) = \frac 1 v \tilde E_{0_I} e^{i(k_1z- \omega t)} \hat y## This wave gets...
  22. C

    How Is the Energy Density of EM Waves Related to Capacitors and Inductors?

    The energy density of an EM wave is given as (1/2) ϵ E^2 + (1/(2μ)) B^2. This is derived from the energy density of the electric and magnetic fields of capacitors and inductors, respectively. But why should the energy density of the fields of capacitors and inductors be the same as that of...
  23. anuttarasammyak

    Energy transfer and conservation cases for pendulum motion and EM wave

    Let me ask a very primitive question. To and fro motion of pendulum under gravity tells us potential energy + kinetic energy = const. At the top points potential energy: max kinetic energy :0 At the bottom point potential energy: 0 kinetic energy :max EM wave is usually illustrated as...
  24. A

    I How to measure a 21 cm EM wave if it is Doppler shifted?

    we know that all emission from asctrophysical context is doppler shifted. So, how to make sure the doppler shifted 21 cm not contaminated by some other emission?
  25. mgkii

    EM Wave - basic question on energy conservation in a wave

    I've searched threads and can't find easy explanation - sorry if I'm missing something basic / have a basic understanding error! In the classic picture of an EM wave with the Electric and Magnetic components oscillating at 90 degrees to each other, both components cross the middle axis at the...
  26. U

    When is the propagation of an EM wave not reversible?

    By reversibility, if we turn the direction of the light propagation by 180 degrees, then the new propagation path follows the old propagation path. I suspect that when there is diffraction, the light propagation is not reversible?
  27. fisher garry

    Calculating the energy in an EM wave

    You don't have to read all this theory to answer my question. I added it just in case. Above they use the definition work energy theorem in vacuum to get to (8.12). Since it is in vacuum I would guess that one could use the equations for B and E field from EM-waves so that magnitude...
  28. gibberingmouther

    EM Wave Amplitudes and Fictional Force Field

    So, my game is coming along. My psychic energy shielding protects against EM radiation. The energy used for shielding gets depleted based on the type of EM radiation (the wavelength) and according to the amplitude of the radiation the energy shielding is exposed to. I can't find many numbers...
  29. S

    B Is the classical EM wave a single photon?

    Does a single photon travel in two different waves at once? If photons are particles like the Photoelectric Effect, Compton Scattering, and Blackbody radiation all suggest, how do polarizing filters block light completely? Is a particle from a radio antenna actually that large in size?
  30. A

    EM Wave Reflection and Refraction: Understanding the Fresnel Formalism

    For this question I want to clarify that 5k which is the electric field component perpendicular to the incident plane ( the xy plane) will be continuous for reflection and refraction , For which none of the options seems correct ,am I right?? The component of magnetic field perpendicular to...
  31. I

    Normal incidence of EM wave - p & s polarization convention?

    Hi. I'm reading a paper "Transmission of light through a single rectangular hole in a real metal" and the author refers to the incident light shown below as "p-polarized" without further specification. Note that ax > ay. Is there any convention in regarding a certain polarization as...
  32. P

    How Does Absorbing an Electric Field Affect the Magnetic Field Across a Surface?

    Homework Statement An electromagnetic wave is incident on a surface which absorbs all the electric field. Use Maxwell’s equations to determine the magnetic field on the other side of the surface. Homework Equations The Attempt at a Solution My initial thought was that ##B=0## as a varying B...
  33. J

    Phase Change and Reflection of Electromagnetic Waves

    Homework Statement Note : There are no minus signs in first two options . Homework EquationsThe Attempt at a Solution The wave is propagating in +z direction whereas the electric field is varying in x direction . On reflection , there is a phase change of π . Also wave starts traveling in...
  34. S

    Graphics illustrating vector potential for plane EM wave

    I'm looking for a diagram or animation that shows the vector potential A (in the form of arrows or whatever) superimposed on the E and B fields of a plane EM wave. Since A is not unique, maybe two or three versions of the diagram (including one with Coulomb guage). An animation with a slider to...
  35. D

    EM Wave Propagation Homework.Incident/Transmitted Power Density

    Homework Statement An E field with f = 2.45*10^9 Hz passes through a material with the following properties e_r = 10 u_r = 1 sigma = 1 (S/m) The Incident E field has peak magnitude of 300 V/m at the air to surface boundary. (a) *solved* Find the incident power density at the material...
  36. DoobleD

    EM wave generation using a single charge in a vaccum?

    I wonder if it is possible to generate a propagating EM wave by making a single charge oscillate in vacuum ? In practice, when we create EM waves, we (always ?) have opposite charges oscillating, like with a dipole antenna : In the dipole antenna case, I can see how the fields detach...
  37. F

    Is the polarization of electromagnetic waves definite or in superposition?

    Hi, In Classical electromagnetic wave.. does it have definite polarization when the EM wave leaves the sun for example? Or is it in superposition and the polarization only exist after measurement just like in QM? I don't understand the Maxwell Equation. Does Superposition in Maxwell Equation...
  38. L

    How to determine the direction of the E-field of an EM wave

    Homework Statement Consider a wave vector which hits a plain boundary between water and air. The wave vector hits the boundary with an angle α1 measured from the vertical axis. The magnetic field amplitude has a y-component only. Also, notice: The z-axis is the horizontal axis, the x-axis is...
  39. C

    I How do we interpret an EM wave using Quantum Mechanics?

    I know that if the intensity of a light beam with of a certain frequency varies it means the number of photons the light beam is composed of varies and not the individual photons energy. That would mean the E and B field amplitudes vary. This would mean that that the amplitudes of the E and B...
  40. A

    A question about radio receivers

    Could somebody explain me why it would not be sufficient for a radio receiver of an AM signal to simply consist in two elements: A very long antenna. A speaker/headphones. The set up would be as follows, the antenna is connected to the speaker and the other part of the speaker is grounded. My...
  41. Pushoam

    Variance of the EM wave equation under Galilean transformation

    For using Galilean transformation, I have to assume that speed of light w.r.t. ether frame is c. W.r.t. ether frame, E = E0 eik(x-ct) W.r.t. S' frame which is moving with speed v along the direction of propagation of light, E' = E0 eik(x'-c't') Under Galilean transformation, x' = x-vt, t' = t...
  42. Noaha

    Change in direction of electric field on conductor surface

    I would like to know why electric field of EM waves changes it's direction to opposite when hitting a conductor surface. I know that electric field inside a conductor is zero but I am not able to connect these two situations.
  43. T

    Nonreflective Coating and Emission Relation?

    Hi all, I've been able to find the answers to most of my questions in these forums, but this time I was not able to. So here goes my first post: I've been learning about thin film interference, and it all makes sense to me except for the correlation between destructive interference caused by...
  44. mktsgm

    EM wave penetration through walls....

    What are the factors that determine the property of wave penetration of bodies? For instance we can listen to fm radio from inside the walls of a room. But visible light is unable to penetrate the walls. Similarly some metals reflect some frequency but absorb some. How atomic orbitals play...
  45. sweet springs

    Spreading of electric current -- start signals in a circuit

    Hi I have a question about current start signals in a simple circuit such as explained in http://amasci.com/elect/poynt/poynt.html. Turning on a switch somewhere in the circuit, sphere of influences, i.e. motions of electrons in wire and generation of Poynting vector around the wire, start from...
  46. G

    Intensity of EM wave independent of frequency?

    Hi. I'm a bit puzzled that the classical formula for the intensity of a monochromatic, linear EM wave $$I=\frac{1}{2}\cdot c\cdot \varepsilon_0\cdot E_0 ^2$$ seems to be independent of frequency whereas I find for the energy of a mechanical wave (e.g. on a string with total mass ##M##)...
  47. Jackson Lee

    Is AC energy really transferred as EM wave?

    Hey, guys. There is an interesting question about EM waves. I know electricity is transferred on transmission line as electromagnetic wave. But electromagnetic wave is one type of transverse wave, then how is AC electricity like? I feel it hard to imagine it. Besides, conductors could be used...
  48. Tazerfish

    Phase difference between magnetic and electric field

    When solving the differential equations for an electromagnetic wave you get out that the electric and magnetic field oscillate in phase. But when considering a oscillating dipole, the electric and magnetic field at a point close to the dipole are a quater period out of phase. Can someone please...
  49. C

    B Classical EM Wave: Can Model be Proved Wrong?

    There is also a classical model of a photon available, which to me explains a particular behavior of it. Can this model be proved to wrong ?
  50. M

    What causes oscillations in EM wave fields?

    I've been trying to understand what an electromagnetic wave is, and have spent quite a while now reading around and piecing different bits of information together to try and get an answer. I haven't yet found an answer to my title question. It might just be because I have a lack of...