Radiation Definition and 1000 Threads

why or how does an object emit radiation of different wavelength for a given temperature?

Why must the charged particle that leads to Cherenkov radiation travel faster than the phase velocity of light not the group velocity of light? One of the sides of the triangle that is used to define cosθ is v=c/n i.e. the phase velocity. I don't see why it's one rather than the other. Thanks!

for RF, how does this mechanism work? http://i.imgur.com/Crx4Amx.png how does the conductor do this? and why does the unpolarized RF not seep through the spaces between the conductive bars?

I would love some clarification on a gamma ray process. This is what I understand so far: Electrons are accelerated at 19MeV at a cathode which is releasing gamma ray photons with an energy of 1.9MeV. Is it possible to generalize the emission number of photons (roughly) to be equal to the...

Textbooks say that by measuring the stopping potential V0, we can determine the maximum kinetic energy with which electrons leave the cathode: eV0 = Kmax However, as I know, when we apply the stopping potential, the electrons will decelerate and radiate parts of their energies. Thus, the...

Ive been reading lately about the Planck constant and have been trying to learn more about it if possible. The way I understand it, the constant came about from experiments with black body radiation. Planck noticed that the experimental data fit the equation e=hf. My first question: how...

Hi everyone I realized that I had a mistaken belief about the meaning of the term greenhouse effect. I thought that it refers to the phenomenon whereby there is an exchange of heat due to light but without there being a direct air contact between the body being heated and the surrounding...

Hi, https://www.physicsforums.com/attachment.php?attachmentid=60066&stc=1&d=1372909704 I would like to ask a question on thermal radiation. In the attached image, if all three surfaces \{ dA,d{S_1},d{S_2}\} are black body surface, and d{S_2} = d{S_1} = S, then does d{S_2} receive the same...

I’m searching for a book about the applications of electromagnetic radiation. A more specific topic I’m interested in is radio antennas but I’m searching for a book concerning more than 3 different applications (eg X-rays, microwave ovens and radio antennas). It should include equations and the...

Homework Statement "How could you show that a radioactive source was only emitting alpha and gamma radiation?"The attempt at a solution This really stumped me...obviously they have different penetration distances, and different strengths of ionisation, but I don't know how to show that only...

As far as I know, Cherenkov radiation occurs when a charged particle travels through a medium at a greater phase velocity than the speed of light in that medium. Molecules in that medium are polarized and after de-excitation emit radiation. But there are a couple of things about Cherenkov...

I am trying to resolve some long standing problems I have encountered with blackbody radiation. Namely, the derivation of the radiation energy flux equation $$J=\sigma_{B} T^4$$. I understand the derivation of the energy density of photons in "a box". $$U/V=const. T^4$$ I do not understand the...

I'm looking for the general formula for Angular Momentum radiated away from quadrupolar electromagnetic radiation. I searched the usual books (Jackson, Landau-Lifchitz, ...) and just found the usual dipolar contributions. Using dimensional analysis and energy radiated away, I found this...

Hi everyone, I am unsure about how to solve this problem. I believe i have the right equation but I'm confused about how I can find Q/T with the information I am given. Is there another equation I can use? Homework Statement Assume the steel in the body of an automobile has an emissivity of...

Hi, Iam working with a High purity Germanium detector and have an issue with the dead time. I let the detector do a background measurement over the weekend, about 55h. When I checked the measurement 60h had passed and the measurement was not finished due to dead time about 20%. At the start...

Hi, please could someone help me as I am struggling to understand the classical blackbody theory. I will briefly summarize what my textbook says. It says that if the classical model was correct then a tennis ball should be emitting radiation in mainly the ultraviolet region. The idea at the time...

Homework Statement My questions relate to a worksheet given which includes a data table displaying results from putting a source of beta radiation near a Geiger-Mueller tube, and recording the Beta radioactivity through the shielding. It started off with the radiation going through nothing...

I have a question regarding the parameters that reduces the Planck distribution to the Rayleigh-Jeans distribution. According to the Planck distribution, the average energy in a unit volume in the \nu frequency mode of a blackbody radiation field is <U> = \frac{h\nu}{e \frac{h\nu}{KT} - 1}...

Homework Statement a) Show that for photons of frequency \nu and wavelength \lambda : 1) d\nu = - c d\lambda / \lambda^{2} 2) u(\lambda)d\lambda = - u(\nu)d\nu 3) u(\lambda)d\lambda = u(\nu) c d\lambda / \lambda^{2} b) Show that the Rayleigh-Jeans spectral distribution of blackbody...

Homework Statement Estimate the minimum proton lifetime from the fact that you haven't yet died from internal radiation. Use the following conditions: 1.) At a dosis of Dmax = 2 Greys per year (Jkg-1 yr-1) you would already be dead. 2.) The human body is made mostly of water. 3.) The total...

(I am not sure what is the best subforum for this doubt, please move the thread if necessary) When we study the interaction of the electromagnetic waves with a charged particle (let's say an electron) we can find two totally different approaches in the literature. If we are talking about light...

Hey all, my first post here. I'm 17 years old and am taking physics at A level. (Just finished my end of year exam today) So the understanding I have of light is that it is the oscillation of an electric field and of a magnetic field perpendicular to each other. I've come to the conclusion...

If what used to be visible light when the universe was smaller and younger has been stretched due to the Doppler effect so that it is now microwaves and detectable as the Cosmic Microwave Background Radiation that can be found everywhere, why hasn't the gamma or x-rays of the same period not...

According to Maxwell's Laws of electromagnetism , the orbital frequency (the number of complete orbits per second) of an electron will match the frequency of the emitted radiation. if an electron spirals into the nucleus, what would happen to the frequency of the emitted radiation...

According to Maxwell's Laws of electromagnetism , the orbital frequency (the number of complete orbits per second) of an electron will match the frequency of the emitted radiation. If an electron spirals into the nucleus, what would happen to the electron's orbital frequency? This is what...

Anyone know if there are any graphical simulations online for the field of a charged harmonic oscillator, or better yet maybe some kind of paper on it?

I don't understand this: According to what modern physicists believe to be true, there is entropy that slowly converts all energy of the universe into heat that cannot do any work. Than this heat is radiated as infrared light into space. Correct? Besides infrared heat radiation, start also...

So I'm coming up on my senior year and I'm taking ionizing radiation next semester and I was wondering exactly what is taught in an ionizing radiation course. I've heard the class can be pretty difficult, almost as bad as heat transfer. Although heat transfer wasn't too bad for me, I made an A.

does every electromagnetic radiation consist of photon?

can anyone tell me the applications of radiation pressure? i have manged to find out one application i.e in solar sails but still my instructor is demanding more applications. i have also found an abstract of an article its about Adiabatic electron thermal pressure fluctuations in tokamak...

At 110Kev they block above 90 percent but I wonder about other lower frequencies specifically microwaves and radio waves?

Homework Statement (i) What is the phase velocity of EM radiation in free space? (ii) What is its group velocity? Homework Equations The Attempt at a Solution Using the definition as per this page: http://physicsdaily.com/physics/Phase_velocity (i) Vp = (C^2)/Vg (ii)...

After performing this experiment, we will get different peaks on the computer screen, where on x-axis lies thickness of the absorber and on the y-axis the nb of channels. What do these represent exactly? Why do we have a high peak and a medium sized one? What are the significances of each...

Second law of thermodynamics states that heat does not flow spontaneously from cold to hot bodies. But a cool fluorescent bulb is perfectly capable of heating something that had already started out being warmer than the bulb itself. Is this not a contradiction to the law ? :confused:

Hi, I've tried several times to get my head around this, but every way I look at it Hawking Radiation doesn't seem to fix the thermodynamic issues it was supposed to solve. People who are a lot smarter than me seem to believe it's real, so can someone please point out where I'm going wrong...

I have some questions regarding thermal radiation spectra of different objects. Is thermal radiation of objects due to electromagnetic radiation alone? Does my desk which is at room temperature emit as a blackbody with the proper factor for emissivity? Where is the "cavity" that the radiation is...

I was wondering whether anyone could provide an explanation as to why when two particles are created near the event horizon, only the negative energy particle is captured by the black hole, and the positive particle travels outward and is seen as Hawking Radiation. I have read numerous posts...

I was reading an article about Cherenkov radiation in wikipedia. If the electron moves slower than light the EM waves in the medium are destructive and if they travel faster than light waves are constructive .Why so?When the electron travels faster than light the radiations cannot interfere...

What law is being obeyed by emr traveling out from a source eg a magnetic field does not get emitted from a magnet, why does a magnetic field get emitted (eg travel outwards at speed of light) from the alternating magnetic field present in a light bulb for example? I would appreciate any help...

Homework Statement I've attached the problem sheet and my attempt at the problem. I've completed all the parts except for the last part, where I have to calculate the count rate of the radioactive source. The answers should be 6000 counts for 185MeV and 4650 counts for 1MeV. I'm not sure...

The solution of Einstein's field equations for a simple black hole show a slowing of time as you get close to the black hole. Time stops at the event horizon. An observer in flat spacetime far from the hole would see an astronaut fall slower and slower as he approaches the event horizon. It...

Homework Statement Hey all, I am working through a derivation that my professor gave me to do and I am having trouble understanding one of the formulas that he gave me, namely: "EM waves can be described as: E(x, t) = Eosin(2πx/λ)*sin(2πc/λ)" The the Eo*sin(2πx/λ) term makes sense to...

Homework Statement The intensity of sunlight reaching the Earth is 1360 \frac{W}{m^2}. a.) Assuming all sunlight is absorbed, what is the radiation pressure force on the earth? Give your answer in Newtons. F_r= b.) Give your answer as a fraction of the suns gravitational force on the earth...

Question: I'm looking to estimate my total extra exposure (dose above background) by taking into account account roughly how long I was near each source, the strength of the source, and its approximate distance from me. I believe it's useful to use the following equation...

Question: I'm looking to estimate my total extra exposure (dose above background) by taking into account account roughly how long I was near each source, the strength of the source, and its approximate distance from me. I believe it's useful to use the following equation...

Obtaining the correct black body spectrum is one of the great triumphs of quantum physics. It is also touted as a total failure of classical physics, since with very basic assumptions, namely thermal equilibrium and the validity of Maxwell's equations, one gets the well known divergent...

The equation for the change in momentum for total absorption of electromagnetic radiation is ΔU/c and that for total reflection is (2ΔU)/c. How could the momentum of electromagnetic radiation change if it is massless and travels at a constant speed c? Is the ΔU just a representation of a change...

Planck's radiation law: "Act of desperation" Why did Max Planck refer to his own radiation law (http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=35774&fbodylongid=1696) as an act of desperation. Was it because it was only mathematical and that he didn't really understand why the...

Question: Recently I've conducted an experiment measuring the dose rate (in μR/h) from various samples (Ba-133, Cs-137, Co-60), at varying distances. In determining the efficiency of paper as a shield, it is required to convert the activity of the source to units of dinsintegrations/second...

How is the search for dark matter, dark energy and gravitational radiation going, as far as i can make out there has been no direct detection and they remain conjectures