suppose you have a flashlight in one hand and a mirror in the other. turn on the flashlight and photons of light will travel towards the mirror. BUT, when the photons hit the mirror, they will reflect. This implies that while hitting the mirror, the photons decelerate to 0 m/s and continue to decelerate to -c (negative speed of light). In other words, light reflecting off of a surface must reach 0m/s. But doesnt this defy the second postulate of special relativity (light always travels at the speed of light)?
2007-12-23 09:20:25 · 8 answers · asked by Hi 2 in Science & Mathematics ➔ Physics
We must understand quantum mechanics to fully understand the physical processes required for reflection.
When we actually consider reflection we get a theory roughly similar to the one i will present.
The photons hit the mirror which we cna safely assume is made of baryonic matter. This matter absorbs the photon and then by calcultaing the S-matrix we can predict that the matter of mirror is most likely to emit a photon in the opposite direction to which the photon hit the matter.
The above explanation has been tremendously simplified but the basics remain the same and the photon only exists before it is absorbed and after it is emmitted always travelling at c.
2007-12-23 10:53:00 · answer #1 · answered by Anonymous · 0⤊ 0⤋
If we are going to consider this phenomenon in such detail we have missed many stages. Let us consider a photon, a particle of light which is emitted by the flashlight. When it approaches the edge of the mirror refraction not reflection happens first. The speed of light in vacuum is constant at c. But when light enters a medium its trajectory changes. Hence light appears to take more time when traveling in a medium, it simply has to cover more ground however small. When it hits the back of a mirror to the photon the material is like a net. Higher the energy of a photon the smaller(wavelength) it is and may just get through the net. We hardly ever observe the effects of such photon through naked eyes. The ones that cannot get through impact it like a tennis ball on net and are bounced back. These bounced back photons go through everything I have stated above in reverse and exit as reflected rays of light. Now when these particles impact the back of a mirror they do impart some energy to the material. This energy is not their kinetic energy. Remember photons are packets of energy. Hence the only thing that will happen is photons loose some energy.
I agree that with a particular frame of reference it may seem the photon has changed from c to -c. Particles like photon do not exactly behave as balls. To give you an idea if you truly want to imagine it the way I have described above think of a photon as table tennis ball and the net being made up of football field sized circles with over sized footballs balls moving in crazy orbits. That is how small a photon is and its interaction though I have simplified it here is more complex and for bouncing back photons will involve lot more interaction with the material itself.
To find out more about photons look at the link below
2007-12-23 10:29:28 · answer #2 · answered by Anonymous · 1⤊ 0⤋
Give 10 points to "The Man" above. Light, in a vacuum travels at the speed of light, but it is a wave, and it reflects off of the mirror the way a water wave would reflect off a wall.
Let me add that quantum mechanics means that the reflection is within the uncertainty associated in position and momentum (i.e., velocity) associated with photon. So in other words, whether or not a light photon is traveling at c the instant it reflects, (or even exactly what instant it reflects) is something we will never know.
2007-12-23 10:34:49 · answer #3 · answered by Frst Grade Rocks! Ω 7 · 1⤊ 0⤋
i hafta go with Adam B on this one.
the photons don't really decelerate then change directions and bounce back in the opposite direction. the photons will hit the reflective surface of the mirror, be absorbed, the absorbed energy will excite an electron to a higher state, the electron will "fall" back to a lower energy state, and in doing so release the excess energy in the form of another different photon. the point in question of its speed where it seems it may violate the light speed limit rule we don't actually have light. the photon is not a photon at that moment.
2007-12-23 18:16:54 · answer #4 · answered by lalowawrzyniec 1 · 0⤊ 0⤋
Look at it as a wave. Before or after the wave hits the mirror the speed of the wave is the speed of light. On the moment that the wave hits the mirror, the direction of the wave is not well defined, but the speed is still the speed of light.
Compare it with a wave in the water that is reflected by a riverside wall. The speed of the wave is constant throughout the reflection.
2007-12-23 09:45:26 · answer #5 · answered by Anonymous · 2⤊ 0⤋
The second postulate of SR is NOT "light always travels at the speed of light." The second postulate of SR is "There exists an invariant speed, c."
Light travels at c in a vacuum, but not in a medium such as air, or a mirror.
2007-12-23 09:33:13 · answer #6 · answered by Anonymous · 0⤊ 0⤋
Light slows down in anything other than a vacuum.
The refractive index of a material [n sub i] is c/[speed in that material].
Cerenkov radiation [that blue glow around the core of a nuclear reactor] is the optical equivalent of a sonic boom; it is caused by particles moving faster than the speed of light in that medium.
But just to make your head hurt, there have been several very strange results in advanced experimental physics in the last few years involving light:
1] Some researchers prepared a medium such that light traveled at 17 M/S ! http://www.aip.org/enews/physnews/2000/split/pnu495-2.htm
2] There are materials that are usually opaque, but if you hit them with an intense enough laser they become transparent. Some guys took such material, illuminated it with a sufficiently intense laser, and shot a pulse of light into it. Before that pulse had traversed the sample, they turned off the laser, making the sample opaque. Later they turned the laser back on, and out came the pulse. They 'froze' light. This link kinda touches on it http://www.sciencentral.com/articles/view.php3?type=article&article_id=218392702
and http://www.sciam.com/article.cfm?chanID=sa006&articleID=000BAEB1-B2AA-1C6F-84A9809EC588EF21
is more directly related.
3] My favorite is the superluminal experiment of Wang et al: http://scienceworld.wolfram.com/physics/Superluminal.html
they shot a a light pulse into a sample and it came out before it went in....
Does your brain hurt yet ?
2007-12-23 11:14:40 · answer #7 · answered by redbeardthegiant 7 · 1⤊ 0⤋
Military Grade Tactical Flashlight : http://FlashLight.uzaev.com/?tFJr
2016-07-10 22:29:32 · answer #8 · answered by Anonymous · 0⤊ 0⤋