# Why only higher energy photons can affect atoms even though their speed is same? [closed]

I understood that a baseball moving at the cosmic speed limit can do lot of harm, but since photons are not comparable to a baseball so irrespective of their high speed they do not rip anything in their way. Now I want to know that why every photon, whether its of RADIO WAVES, X-RAYS or VISIBLE RAYS have same speed, even if their energy is different? Is it only because photons (of different waves) do not have REST MASS so they can achieve the cosmic speed limit? But there is another fact that they do have momentum and momentum does increase with energy then why their speed does not change?

• Photons don't have any mass. Their momentum and energy is tiny. See physics.stackexchange.com/q/2229 – Brandon Enright Nov 10 '15 at 4:15
• I suggest you to do the math,$E=hf$ ,because value of $h$ is very small ,so energy is very small. – Paul Nov 10 '15 at 5:00
• youtube.com/watch?v=AK11yIBDqk8 (a video of photons doing exactly that) – Dan Nov 10 '15 at 11:22
• Why would you expect they rip up everything? – ACuriousMind Nov 10 '15 at 14:21
• @ACuriousMind i thought they have such a high speed and even if they do not have mass, all of the photons do have momentum. so they should just tear up everything which comes in their way. But I have read that only high energy rays show the photoelectric and Compton effect etc. I cannot understand that if every TYPE OFphoton does have same speed then why only HIGH ENERGY photons affect the atoms... – shivani Nov 13 '15 at 6:43

It's because photons are massless and so they can and must move at the speed of light with any none zero amount of momentum. This means that even though they move at the cosmic speed limit they can have very little amounts of momentum.

• So does this momentum increase or decrease with their energy? – shivani Nov 13 '15 at 6:52
• When the momentum of a photon increases the energy of the photon increases. – Anders Gustafson Nov 14 '15 at 6:34

Size is relevant. Even weak photons can be absorbed by knocking an electron to a higher energy level. If you were that small you would be ripped apart.

• yes you are right. size matters... – shivani Nov 13 '15 at 6:08
• One word of caution: While this answer is in principle correct, when one speaks of energy levels of electrons one often means eigenvalues of the nonrelativistic Hamiltonian. These energies are usually so far apart that you need visible or UV light in order to do excitations. For some systems with higher spin (e.g. triplet $O_2$) there might however be tiny splittings due to local magnetic fields induced by the environment, and excitations over these tiny splittings might e.g. also be done by radiofrequency photons. – LLang Nov 13 '15 at 6:43
• I have not read about these excitation in detail... – shivani Nov 13 '15 at 6:57

thats an interesting question, but photonic fields are fundamental and cannot really be compared to say a baseball. If you throw a baseball at near the speed of lightit would probably break a lot of stuff, but photons don't really effect things very much unless they are high energy. Note that high energy photons like x-rays actually do have the ability to "rip stuff apart" but they do that on short length scales. Sunlight is also capable of giving you a sunburn. However, most low intensity fields (especially at long wavelengths) will not interact at all with matter.

• 'Will not interact with matter' you say. Perhaps you should instead say that they do not significantly affect the surrounding matter? Neutrinos on the other hand do not interact much with matter. – Horus Nov 10 '15 at 5:24
• what-if.xkcd.com/1 – Stop Harming Monica Nov 10 '15 at 13:41

Let me turn your question around. A radio wave photon, with thousandths or millionths of an eV in energy, travels at $c$. Why should it be able to rip anything apart? Its energy is very small compared to any chemical bonds. That says it has only three options-be absorbed, reflect, or scatter. Reflection imparts the most momentum into the object, but it is so small it won't deflect the atom it scatters off significantly. Not quite so fast, but the air molecules that strike you all the time are traveling very fast by conventional measure, but they don't rip you apart. They carry more energy than an AM radio photon.

• Photons in radio waves could be from different energy and frequency, this depends from the temperature of the antenna rod. To say that the modulated from the antennas generator frequency is the frequency of the involved photons is not correct. – HolgerFiedler Nov 10 '15 at 6:02
• @HolgerFiedler Ross gave a range of 5-6 orders of magnitude for the energy of the photon. The precise energy of the photons doesn't matter, just that it is small by everyday standards. Do you disagree with that assertion? – Kevin Driscoll Nov 10 '15 at 6:52
• @KevinDriscoll It is a comment as a reminder, that the involved photons are infrared photons. Radio waves are a radiation of a hugh number of infrared photons. The frequency of the radiated photons can be the same for MHz waves or for GHz waves for example. – HolgerFiedler Nov 10 '15 at 7:12
• @HolgerFiedler what do you mean? There's no way you could add waves in some range of frequencies (IR in your comment) to get a wave of completely different frequency (RF in your comment). Fourier transform is unique, you can't trick it. And the only thing which depends on antenna rod's temperature is its thermal radiation, which is completely separate from RF emission. – Ruslan Nov 10 '15 at 13:31
• @Ruslan Radio waves are made from a huge number of photons, which were emitted from accelerated electrons. Use a shorter then needed for some generator frequency antenna rod, when the efficiency of the radiation will decreases, but the wavelength of the oscillated radiation will stay the same. The radio wave length depends from the generator frequency and the velocity of light in air. That's way it is not correct to talk about EM radiation from infrared to gamma in the same manner as about radio waves. – HolgerFiedler Nov 10 '15 at 13:45