Questions tagged [electromagnetic-radiation]

Propagating solutions to Maxwell’s equations in classical electromagnetism and real photons in quantum electrodynamics. A superset of thermal-radiation.

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7 votes
3 answers
905 views

If we have a cosmic microwave background should't we also have a cosmic radio wave background?

I'm a layman in physics, but here is what I understand: What we see in the sky with naked eyes is a map of electromagnetic waves in the frequency visible to the human vision. But that kind of ...
23 votes
4 answers
6k views

How could 19th century people know light is electromagnetic waves?

From Maxwell's equation, we can find out that certain waves exist. However, it's unclear to me why 19th-century people thought that what they had called light is a wave. As far as I know, 19th-century ...
5 votes
1 answer
220 views

Boundary condition on $\mathbf{B}$ to describe resonant cavities from waveguides

In Jackson- Classical Electrodynamics when resonant cavities are discussed (8.6, page 252) (but also at page 7 here or at page 19 here) the explanation is made by saying that the solution is the same ...
0 votes
1 answer
31 views

What are the sources of electric and magnetic fields that produce lights?

Electromagnetic waves are formed as a consequence of the interference of electric and magnetic fields. I wonder where are these fields coming from that form the light? Does the fields have to be ...
1 vote
2 answers
43 views

Questions regarding light being depicted as a sine wave [duplicate]

Light waves are often depicted as sine waves... Why is it so? What's actually waving? And what does the sine wave signify and represent? What does wavelength of light mean/signify? and how is it ...
2 votes
1 answer
179 views

Can two semi-infinite plane waves undergo perfectly constructive interference?

This is building off of a question I asked here. When discussing the linked problem with some friends, the consensus seemed to be that the reason two identical semi-infinite plane waves cannot ...
0 votes
0 answers
17 views

Complete set of commuting observables for the non-interacting light field

The Hilbert space for the non-interacting electromagnetic field is the bosonic Fock space $\mathcal{B}(L^2(\mathbb{R}^3) \otimes \mathbb{C}^2)$. Here, $L^2(\mathbb{R}^3)$ is the space of square-...
0 votes
0 answers
18 views

Does it require energy to generate an electric field as well as a magnetic field?

It is known that the electric field as well as the magnetic field carriers energy, so I wonder is the energy of the fields generated by a charge originated from the kinetic energy of the charge?
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0 answers
20 views

Shape of things and EM waves

What is it that directs EM waves out in a particular direction, (EM waves in space)? As in what generates these and is the shape of the star influential in the EM Wave direction?
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0 answers
23 views

Monochromatic intensity [closed]

Let monochromatic intensity based on wavelength be denoted by $I_y$ and that based on frequency be denoted by $I_v$. If $v_1=2v_2$ and $I_{v2}=2I_{v1}$, what is the relationship between $I_{y1}$ and $...
0 votes
1 answer
173 views

Electric field from the intersection of two spheres with different charges

This question is repeated a lot, but I for two spheres with different charge densities, sphere one with radius a centered at the origin and charge density $\rho_1$, and sphere two with radius centered ...
0 votes
2 answers
83 views

Over what distance might this device be able to interfere with car radios?

I've been reading recently about a mysterious and possibly hokey medical device of the 1930s. It was a variable-frequency radio emitter, invented by a Dr. Rife, which was supposed to kill various ...
0 votes
1 answer
150 views

Why is the magnitude of the electric field dependent only on $x$ and $y$ coordinates?

So I am reading a book on electromagnetism and in it they are assuming that the magnitude of the electric field is dependent only on $x$ and $y$ coordinates in a waveguide and I wanted to know why ...
2 votes
1 answer
287 views

What is the mean free path (MFP) of a Cosmic microwave background radiation (CMBR) photon?

If the IGM (inter-galactic medium) had a massive particle (proton) every cubic metre, or a carbon grain every cubic kilometre, what would be the MFP of a CMBR photon? The coherence length of the ...
0 votes
2 answers
1k views

Spacing between primary maxima of $N$-slit diffraction pattern and single-slit envelope

As far as I know, in the double-slit diffraction pattern, the spacing between primary maxima is determined by the equation of double-slit interference pattern, and the intensities of primary maxima ...
0 votes
1 answer
158 views

Cardboard and IR transmittance

Does infrared radiation transmit (not absorb & re-radiate) through cardboard in the IR-B or IR-C bands? I know IR-A does not transmit well but am curious about the longer wavelengths and can't ...
0 votes
2 answers
160 views

Origin of electromagnetic radiation in the universe

I don't know if the total electromagnetic radiation in the universe is finite or not. But either way: what is the origin of the electromagnetic radiation in the universe? Can we say that all of it ...
3 votes
2 answers
71 views

Why the sunset is red and a possible experiment?

Before downvoting: I have read some of the answers and will cite them as well. Not trying to debunk physics or create conspiracies. I love physics but I don't understand it. Background This is more or ...
2 votes
1 answer
244 views

How does Hertz's radio wave experiment show there are both magnetic and electric fields in a radio wave?

Could somebody check whether my understanding of this experiment is correct, because I feel like I assume Maxwell's results. When a spark forms across gap, a spark is also created across the ...
0 votes
0 answers
24 views

Electric field from an oscillating electric dipole

In the case of two charged spheres connected by a wire and separated by a distance d, when we drive the charges back and forth in the wire (by applying an oscillating electric field), we then have a ...
0 votes
1 answer
46 views

Where the E-M radiation is coming from?

In the case of two charged spheres connected by a wire and separated by a fixed distance $d$, if the charges are changing periodically, then this system (also referred to as an electric dipole) ...
2 votes
4 answers
150 views

Details of how we see and percieve objects and images

I am a high school student and I have a question related to the concept of object and image I know where incident rays intersects this point is called object but here if this is the case here what ...
2 votes
1 answer
200 views

Physical significance of growth rate in plasma

Let us say that we have a dispersion relation curve and associated instability curve as shown below for a magnetised plasma, which have been formulated through kinetic theory. The frequencies and ...
0 votes
2 answers
42 views

Question about the electric dipole radiation [duplicate]

I can understand why an oscillating electric dipole emmits electromagnetic rdiation-charges are constantly accelerating. But how come fixed electric dipoles with varing charges also emmit E-M ...
0 votes
1 answer
22 views

What are the speeds of light inside a retarder?

A retarder introduces a phase shift between the field components traveling along the fast axis and the slow axis. But at what speeds are they actually traveling? Is the speed of light along the fast ...
1 vote
2 answers
320 views

Is EM radiation flux an extensive or intensive property?

Is electromagnetic radiation flux (measured in watts per square metre) an extensive or intensive property? Can the fluxes from two sources be somehow combined at a target and what are the rules?
0 votes
0 answers
34 views

Why does the Planck curve drop below the Rayleigh-Jeans curve for blackbody radiation when Planck quantized the energy?

This has been a research topic of mine for days now. I understand the Rayleigh-Jeans law and how it leads to the ultraviolet catastrophe. I have been searching for a clear, conceptual explanation of ...
0 votes
1 answer
3k views

Angular separation of the lines in a diffraction grating problem

This question is about using a diffraction grating to view the emission spectrum of sodium. Light from a sodium discharge tube is incident normally upon a diffraction grating having 8.00 x 10^5 lines ...
3 votes
3 answers
175 views

What is electromagnetic radiation (when it is not a 'wave' of EM fields or interacting locally as a photon)?

My questions are related to the question asked at Are EM radiation and EM waves the same thing?. My background is in math (my Ph.D. thesis was in geometric analysis), and I have only taken basic ...
1 vote
1 answer
13 views

Diffrence between thermionic emission and photoelectric emission

Thermionic emission involves heat energy to excite the electron and remove it. In the photoelectric effect, a beam of light is involved. As per my understanding heat and photons, both are energy. Heat ...
0 votes
1 answer
173 views

Mode expansion inside a waveguide

In the classic book by Collin (Foundations for Microwave Engineering, 2nd ed.), the author postulates on p. 278 that, given a waveguide structure that stretches along the $z$ axis with open ends (see ...
2 votes
1 answer
165 views

What is the difference in application for near infrared camera data capture as opposed to infrared data capture?

I am trying to understand why I would use a near infrared as opposed to an infrared camera for data capture of vegetation health monitoring.
4 votes
1 answer
250 views

Sunscreen protection, SPF, and amount applied: relationship?

In the article How SPF Changes with How Much Sunscreen You Use (last updated Dec 29, 2017), the author says that insufficient sunscreen disturbingly decreases protection exponentially, in other words, ...
3 votes
1 answer
50 views

Wave nature of light [duplicate]

In a light source (like filament of bulb) there are various atoms and as such the things are deemed to be randomised. So there must be a phase difference between each of the wavetrains emitted by the ...
3 votes
6 answers
220 views

Why do microwave ovens need to have a resonant cavity?

Why is it considered necessary for the microwave oven to be a resonant cavity? If it wasn't resonant then the energy wouldn't have anywhere to go but in the food as well. So I don't understand the ...
29 votes
5 answers
34k views

What exactly is a quantum of light?

I am currently trying to learn some basic quantum mechanics and I am a bit confused. Wikipedia defines a photon as a quantum of light, which it further explains as some kind of a wave-packet. What ...
2 votes
1 answer
69 views

A doubt about Kirchhoff's scalar diffraction theory

Scalar diffraction theory assumes a scalar wave equation of the form $$\frac{1}{c^2}\frac{\partial^2u}{\partial t^2}=\nabla^2u$$ where $u(\vec{r},t)$ is a scalar field called the optical disturbance. ...
1 vote
4 answers
970 views

Why does the electric field and not the magnetic field remains in the same direction after reflection from a medium?

I was reading up about reflection and transmission when an electromagnetic wave is normally incident on a surface. I came across this figure : My question is why is the direction of electric field ...
3 votes
4 answers
3k views

What can change a photon's frequency?

I found this question: Is it possible to apply force to a light particle? As it states, gravity can change the frequency of light by changing its momentum. My question regards other phenomena that ...
0 votes
0 answers
21 views

Is it better to spread the coil to maximize magnetic strength in air gap of a C shape iron core

See the 2 example below of a C shape Iron Core 99.8% iron. in both cases, assume that: the wire length is the same, say 1000 feet of copper wire. The amp being used is the same The N turns are the ...
0 votes
0 answers
20 views

How to calculate Tesla value at the end of a solenoid when iron surface expands?

assumptions: The core and top cylinder are made of pure iron 99.8% with a permeability ( µ ). To calculate the Tesla value at point B (at then end of the solenoid), we use: $$B =\frac{\mu\cdot N\cdot ...
0 votes
1 answer
48 views

Superposition of two electromagnetic waves

If an electromagnetic wave in isolation with vector potential $A^1_{\alpha}$ satisfies the wave equation $\Box A^1_{\alpha}=0$, how do we construct the total electromagnetic wave that results from ...
1 vote
1 answer
345 views

How is it that 62" is the best length for 72Mhz receiver?

According the this article (which was published somewhere in 2005-2008 I think, but it's still comes up first in Google) the best length for a 72Mhz receiver antenna is 62" (~157.5cm). This puzzles me ...
1 vote
2 answers
342 views

Does direct current in a wire have electromagnetic radiation?

The starting point is a wire with no current. We now turn on a DC power source and a current starts flowing. There was an accelerating/changing current during 'power on'. This causes 1 ...
2 votes
2 answers
49 views

Speed of light vs wavelength due to gravitational lensing - straight path versus curved path

Non-physicist asking. Note: I see that there are similar questions but none that I can find that addresses this specific query. Exaggerated diagram Question If the two beams of light leave the ...
6 votes
3 answers
289 views

Which Fresnel coefficients should be used at normal incidence?

There are two sets of Fresnel coefficients depending on the polarisation of the incident light. At normal incidence these equations converge except there is a phase factor of $\pm1$ For the reflection ...
1 vote
3 answers
75 views

How a dipole should behave in order to produce an electromagnetic field?

I am trying to understand what an electric dipole "has to do" to produce electromagnetic waves. I know that an oscillating electric dipole will produce EM waves and by oscillating electric ...
1 vote
1 answer
55 views

Phase and group velocities in QFT / Quantum Optics

How does one define phase and group velocities in QFT? More precisely, are they referring to the mode structure of the field or do they characterize excitations? The question is motivated by question ...
55 votes
13 answers
40k views

Why and how is the speed of light in vacuum constant, i.e., independent of reference frame?

I was told that the Galilean relative velocity rule does not apply to the speed of light. No matter how fast two objects are moving, the speed of light will remain same for both of them. How and why ...
1 vote
1 answer
18 views

Induced charge density on perfect conductor

Assuming there is a perfect conductor at $x=0$ in $\mathbb{R}^3$ and a plane EM wave $\vec{E}_i(\vec{x},t)=\vec{E_i^0}e^{i(kx-\omega t)}$ is coming from $x=- \infty$. We know, that the wave $\vec{E_r}(...

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