# What is Em waves: Definition and 226 Discussions

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. ### I Do EM waves have negative frequency inside negative-index materials?

The speed of light in a vacuum, ##c##, is defined as positive. The refractive index of a material, ##n##, can be positive or negative. The dispersion relation for light inside the material is given by $$\omega=\frac{c}{n}|\mathbf{k}|.$$ The magnitude of the wavevector, ##|\mathbf{k}|##, must...
2. ### I Oscillating charged particles and E.M waves

If I were to tie a friend of mine adjacent to the oscillating charge and make him oscillate in parallel to my oscillating charged particle such that to him the oscillating particle is at rest, would he observe the generation of electromagnetic waves.
3. ### B What is the analogy for EM waves traveling in vacuum?

So, is water for water waves, what is the vacuum for EM waves traveling in vacuum. I know the analogy can't be exactly perfect because water molecules oscillate in the presence of water waves, but in vacuum nothing seems to oscillate? Or the vacuum oscillates in some way? And no I am not trying...
4. ### I Are spherical transverse waves exact solutions to Maxwell's equations?

In this paper in NASA https://www.giss.nasa.gov/staff/mmishchenko/publications/2004_kluwer_mishchenko.pdf it claims (at page 38) that the defined spherical waves (12.4,12.5) are solutions of Maxwell's equations in the limit ##kr\to\infty##. I tried to work out the divergence and curl of...
5. ### I EM waves have no mass but they do have momentum?

I was studying radiation and came across an article: https://www.wtamu.edu/~cbaird/sq/2014/04/01/light-has-no-mass-so-it-also-has-no-energy-according-to-einstein-but-how-can-sunlight-warm-the-earth-without-energy/#:~:text=In summary, all objects with,not the only massless object. Which said...
6. ### I Frequency of EM waves 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 magnitude of...
7. ### I Check out this YouTube video on EM waves in free space

I've always had difficulty grasping why the electric and magnetic fields are in phase in EM waves in a vacuum. Of course, Maxwell's Equations imply that is the case, but I had a hard time intuitively visualizing it. Then I found this short video on YouTube. I would appreciate your opinion...
8. ### B Cryogenic storage dewars and EM waves

I was researching cryogenic storage dewars and read that, "All dewars have walls constructed from two or more layers, with a high vacuum maintained between the layers. This provides very good thermal insulation between the interior and exterior of the dewar, which reduces the rate at which the...
9. ### Finding path difference of EM waves received by radio telecopes

My attempt: I think ##x## is the path difference so by using trigonometry, I got ##x=d \sin \theta## But my teacher said the answer is ##d \cos \theta## What is my mistake? Thanks Edit: Sorry, I found my mistake. My ##x## is not the path difference
10. ### EM Waves Generated by an LC Circuit?

I am not an electrical or electronic engineer. I am trying to understand how a simple, series LC circuit running at it's resonant frequency can generate EM waves. I believe, based on what I have read, that the frequency of the generated EM waves will be the resonant frequency of the circuit...
11. ### Exploring the Propagation of Electromagnetic Waves: Visualizing the Concepts

Hi everyone, I m not a physicist and I don't really speak english... please forgive me if I write any "rubbish". I'm quite curious, and I was wondering how do electromagnetic waves travels. I mean, from a "point" source, they propagate in every direction (I've been told) so I tend to imagine...
12. ### Polarization of EM waves is preserved after reflection/refraction -- Why?

Hello, here's my question: during the usual derivation of Fresnel's equations, it is assumed that an incident EM wave (plane monochromatic) is transverse electric or magnatic and that it keeps this nature after reflection and transmission. How can this be proven? Thank you!
13. ### Electromagnetism -- What do charges have to do to create EM waves?

I am not sure, but I think the answer is between acceleration and velocity.
14. ### Frequency of EM waves produced by linearly accelerating charges

I was wondering about EM waves produced by linearly accelerating charges, as opposed to oscillating charges. With oscillating charges, the frequency of the wave depends on the frequency of the oscillation of the charge. But what determines the frequency of the wave produced by a linearly...
15. ### EM waves, longitudinal EM propagation?

Hey, after doing some reading I stumbled across a few fundamental questions.1) Do all EM waves across the EM spectrum , if they travel through space have their E field and B field amplitudes exactly equal and in phase and shifted 90 degrees from one another? If the answer is yes then... 2) In...
16. ### Frequency of TV signal from EM waves

Having some trouble with this question I believe phenomenon behind this one is that the student is passing between nodes (minimum displacement) to antinodes (maximum displacement) which explains why the signal weakens and strengthens continuously. Hence the ans to this is option C? For the...
17. ### Understanding EM Wave Reflection: Wavelength Changes Explained

Problem Statement: 1) The wavelength of an electromagnetic wave after reflection at angle on a surface [A]. remains same as the wavelength perpendicular to the surface . remains same in the free space @ [C]. increases in actual direction of propagation [D]. decreases in actual direction of...
18. ### Can a Tiny Antenna Transmit Long Wavelength EM Waves?

Can you transmit em wave with antenna much smaller than its wavelength? For example. ELF antenna is very long. Can you make one small enough to fit in the pocket by the device constructing the long wavelength part by part?
19. ### I What will eventually happen to all EM waves / photons?

I'm not an expert in this matter, and at best only aware of some superficial facts and a layman's understanding of them. So please forgive me for any ignorant mistakes in my thoughts, and kindly point them out to me. Going by the Lambda-CDM model, the expansion of the Universe will eventually...
20. ### Do intensity flickers result from the nature of EM waves?

Since light intensity is proportional to the amplitude of the EM wave, and wave amplitudes undulate up and down, does this result in natural intensity flickering of observed light? For visible light, the frequency is extremely high, but it might be more easily observable in ELF waves.
21. ### From Maxwell's equations to EM waves

Hi, I just finished studying Maxwell's equations. Based on my understanding, when you solve maxwell's equation, you get the wave equation and it simplies to in a charge and current-free region. I understand that these two equations are similar to an equation of a wave in space. What I am...
22. ### Are all EM waves produced by accelerating charges?

So, a static charge at rest produces an electric field, but no magnetic field. A charge moving with constant velocity produces both electric and magnetic fields. Why is it that accelerating charges are the source of all electromagnetic radiation? How would one go about showing this using...
23. ### Very Low Frequency EM Waves (now: Magnetic Launcher)

Is it possible to make very low frequency EM waves, say 10 KHz, without an antenna the size of Texas? Something more human sized? Thanks.
24. ### Why Do Electromagnetic Waves Propagate Independently of Their Source?

Homework Statement Use Maxwell's equations to elaborate an coherent explication of why electromagnetic waves propagate independently of the source that produces them. Homework Equations Maxwell's equations in vacuum: ##\nabla * E=0## ##\nabla * B=0## ##\nabla \times E = -\frac{\partial...
25. ### A Is energy contained in matter wave equals hv like EM waves?

h is plank constant and v is frequency. I was using this to derive the TDSE. But I ran into problem because to substitute k^2 in E=h^2/8mpi^2 * k^2, I can use single derivative of psi squared or double derivative, both of which tend to give the correct answer. So, is my assumption of energy...
26. ### How does the velocity of air molecules affect EM waves?

I would like to know how exactly or if the velocity of air molecules affect the light i.e electromagnetic waves passing through it. Ignoring the effect of pressure and/or temperature differences in the air which might also affect the light (due to changes in refractive index).
27. ### Why is there interference if EM waves don't interact?

Radio waves pass through everywhere without interacting with each other and that enables us to hear different phone calls and radios without disturbance. However, we do hear some noise sometimes because different signals interfere with each other. How are both of the last two statements true (if...
28. ### Superposition of Plane EM Waves Using Complex Notation

Homework Statement I have a simple problem relating to the superposition of plane EM waves that I'd to try out using complex notation. Could anyone run through the work to see if my understanding is right? Many thanks in advance! The incident E bit of the wave is \vec{E}_I = E_0 \sin(ky -...
29. ### EM waves in phase and E/B = c question

My textbook (Serway and Jewett, Physics for Scientists and Engineers) says that Emax/Bmax = E/B = c. And that E and B are in phase. My question is, if they are in phase, they both reach zero at the same time. At that point, E/B = 0/0 and not c. I know I am missing something, but not sure what...
30. ### What charges generate EM waves in free space?

as we know light travel in vacuum because of oscillation of electric and magnetic field and both are perpendicular to each other. But i don understand how these electric and magnetic fields get generated in vacuum. as electric filed can vary by oscillating charge and that generate varying...
31. ### Confusion about the Energy Density of EM waves

I am a bit confused about the energy density in an EM wave. why do we take the Peak value of E vector while calculating the energy density? Like if the E field is ##E_0 Sin(kx-wt)## what is the energy density of the EM wave(Magnetic + Electric)? is it A) ##\frac {e_0E_0^2}{2} ## or B) ##...
32. ### Cell Phone EM Waves - Health Risks & Physics Explained

Homework Statement A cell phone sends and receives electromagnetic waves in the microwave frequency range. a) Explain the physics of how an oscillator creates these waves. b) Research the possible side effects of using cell phones. citing at least three websites that you consider reliable...
33. ### Two States of Polarization of EM Waves

I am studying about the cavity radiation inside a metallic cube. In the textbook it states that there are two independent waves corresponding to the two possible states of polarization of electromagnetic waves. What does it mean by this? (My current assumption is the phase change of the waves)...
34. ### How is the orientation of EM waves related to the wavelength and antenna height?

A while ago, I saw an explanation of radio waves. Overall, it's a decent vid. I bumped into some nasty problems. Basically, the image shows that the wavelength of a EM-wave corresponds to the height of the antenna. The visualisation however can't be right since the wave cannot go upwards- the...
35. ### Experiment to measure EM waves

Okay, let me prelude this by saying I only have an -EXTREMELY- limited understanding of classical physics, and zero knowledge of quantum mechanics. This is really just my asking a few questions in regards to an idea I had for an experiment, and what would be a good and accurate way to...
36. ### B Local/Proper Time of EM Waves: Explained

This may be a dumb question, but some of these other thread got me wondering: is there a concept of local/proper time for electromagnetic waves? I imagine the only 'clock' that could measure time (ticks) at the speed of light would be the field oscillations.
37. ### I EM Waves in a Rotating Frame: Questions & Answers

Hello there, I have a question (two very similar questions) about the time and phase delay between rotating objects. I want to describe two extreme cases here: I would appreciate any helps. Case 1 Imagine two observers (people with telescopes maybe) in space that are standing thousands of...
38. ### EM waves -- Energy Calculation

Homework Statement Homework EquationsThe Attempt at a Solution I didnt understand much the question. Should I use, ##υ=εE^2## ? Then I ll take rms value of that E.But I am not sure that is that the max value.Is it max value ? I didnt understand part (B)
39. ### Propagation of EM Waves in metamaterials

We have materials that have negative effective permittivity and permeability. In such materials, when the product of permittivity and permeability is negative, solving the wave equation yields a wave with a purely imaginary wave number. Does this mean complete attenuation of the wave ?
40. ### EM Waves - Would the bulb glow?

Homework Statement A long, thin steel wire is cut in half, and each half is connected to a different terminal of a light bulb. An electromagnetic plane wave with E and B moves past the wire as shown. Part B: Suppose the wire were oriented parallel to the y-axis, as shown above. WOuld the bulb...
41. ### Want accurate cartoons of EM Waves

If you were trying to explain to kids how EM Waves really look in nature, what would you show? None of these common graphics below are perfect. If you wanted to show comprehensive depiction of transverse EM Waves from the sun, from a laser pointer, from a radio / wifi transmission tower, how...
42. ### Orthogonal Polarisation in EM waves and Interference

I've been trying to get my head around Polarisation and how it achieves orthogonality. I'm not sure if this should be in Physics or Electrical Engineering Section. (Mods can move this where appropriate) I know that 2 EM wave with linear polarisations where one wave is shifted by π, they would...
43. ### Why do EM waves of longer wavelengths spread out more?

Why do longer wavelengths spread out more than shorter wavelengths? What is the physics principle/law which explains why radio waves spread out more than optic waves in free space?
44. ### Are light waves/ EM waves damped?

We can't see objects from objects far away from us. Why? I think light waves damps! When it reaches our eyes it's amplitude is too small to be visualised! Is this true? If indeed EM waves are damped then why? If not please give a suitable definition for the mentioned phenomena too !
45. ### Induced potential - when do you have to consider EM waves?

There's a long conductor carrying a 60 hz AC current. There's a second conductor parallel to the first current carrying conductor, and a hundred meters away from it. I want to know what the electric potential induced by the changing B field is in the second conductor. Theoretically I could...
46. P

### Em waves and magnetic field lines

So i know that magnetic field lines are closed, in an electromagnetc wave how can be these lines closed? i cannot picture this in my head
47. ### EM waves and traditional method of transmission

Hey, guys. We all know power of AC currents is transferred via EM waves, but we seldom use that to calculate power in AC system. The reason for this is wavelength is very long. I want to know if there is really existed a transmission line which is 500km, just the length of EM wavelength, then...
48. ### Metals touching, produce EM waves

Can you help me on this question I had for years? Every time I touch two metals together (holding them with my fingers) and place a shortwave SSB radio nearby, I hear clicks and noises on the radio. Why is that happenning? Has anyone observed this phenomenon before?
49. ### Charge at constant velocity emitting EM waves?

So far I have came to know that when a charge is accelerated the electric field magnitude around the charge changes and the effect is not felt instantaneously. The change in magnitudes of electric and magnetic field travels outwards at speed of light creating the so called EM wave. So the EM...
50. ### How exactly are high energy EM waves harmful for us?

I've always read in my Physics textbooks that high energy EM waves like x-rays and gamma rays, if our body is exposed to them for a long time, can damage the skin significantly. However, how does that happen at an atomic level? As far as I'm concerned, the thing that differentiates a high...