What is Light?

Bravo25

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So lets get technical. I posted a thread in the LED forum about how well my L4 reflected off from reflected material at considerable distance. That brought some good answers, and a couple of more questions.

1. What exactly is light?

2. How far do light particles (for lack of a better term) travel? Are they like water ripples that eventually fade out, or like the Energizer bunny that just keeps going, and going?
 

stockwiz

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well.. might as well cover the basics /ubbthreads/images/graemlins/smile.gif the most broad definition would be: Electromagnetic radiation of any wavelength.

Wavelengths of light (Electromagnetic radiation)

Radio: Wavelengths longer than infrared and very low energy.

Infrared: Wavelengths longer than the red end of visible light and shorter than microwaves (roughly between 1 and 100 microns). Little infrared radiation reaches Earth's surface, but some can be observed by high-altitude aircraft or telescopes on tall mountains.

Optical/Visible: Electromagnetic radiation at wavelengths visible to the human eye. We perceive this radiation as colors ranging from red (longer wavelengths about 700 nanometers) to violet (shorter wavelengths about 400 nanometers).

Ultraviolet: Wavelengths shorter than the violet end of visible light. Earth's atmosphere blocks most ultraviolet light.

X-rays: Very short wavelengths and very high-energy; X-rays have shorter wavelengths than ultraviolet light but longer wavelengths than gamma rays.

Gamma rays: The highest energy, shortest wavelength electromagnetic radiation. Usually, they are thought of as any photons having energies greater than about 100 keV (kiloelectron volts).

I would assume light waves/particles would travel forever until blocked by some object of matter or gravitation, whether it be a planet, an astroid, or a black hole. We can see stars, galaxies millions of light years away with sophisticated devices.. although the human eye cannot detect these small amounts of radiation, they are there and all around us.

I enjoy science, so I'll enjoy some of the experts throwing in what they have to say on the subject. /ubbthreads/images/graemlins/smile.gif
 

js

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Light is photons. Photons are subatomic particles that have zero restmass and that travel like waves and interact like particles. For our purposes as flashlight users, light may be defined as electromagnetic radiation that falls within a certain range of wavelengths, and which is described by J.C. Maxwell's equations, or, in most cases, by classical optics. Unfortunately, to get technical, you have to ... er ... get technical. Which means Mathematical equations and physics and experimentally derived data. Transmission, reflection, and so on, is a complicated (in general) thing to deal with. Plus, you have the added complication of light travelling not just through one medium (such as vacuum or air) but two or more: glass, air, etc. You end up with matrices and all sorts of other fun stuff. None of which really answers the question you asked. Light is a mystery. All of the universe is a myserty, despite the self-assured answers that scientific materialists my throw at you.

2. In a perfect vacuum, light will eventually spread out to the point where the particle nature of light becomes apparant. i.e. to the point where there are so few photons near each other that a light detector will start to register little "blips" or "light quanta". At the point where a single photon is travelling along (like a wave!) it becomes less and less likely (with distance) that a detector in a certain place will actually "catch" ANY of the light from a very very faint far distance light source, unless it collects light over a long time.

However, again, for our purposes, this case never applies, and light can be thought of as a wave. And in air, light does get deflected, refracted, and absorbed, in addition to the simple geometric spreading out, which goes (intensity)as the inverse square of the distance. So with distance, light fades and spreads out, but an individual photon either gets absorbed (and destroyed) or it doesn't--keep in mind that another photon may be immediated created going in a different direction. Stuff at this quatum electrodynamical level is REALLY, REALLY, far removed from everyday experience, and is very difficult to calculate. The index of refraction of glass is still too dificult to calculate from first principles, i.e. from QED. It's just too complicated a situation for us right now. So you have to change theories to something more suited to the situation. Same reason you wouldn't try to calculate the trajectory of a base ball from quantum mechanics.

So the answer is that light fades out, but photons are like an energizer bunny that just keeps going and going and going, until someone gets annoyed and kills him.
 

PeterW

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....... don't ask hard questions!!

As a professional physicist, we consider it as waves or particles depending on the situation at hand and which makes more sense.

I know that sounds like buck passing, but that's the way it goes...... uncertainty rules!

Cheers


PEterW
 

lightnix

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On the other hand, the "Copenhagen Interpretation" of quantum theory says that light is neither wave nor particle, but exists in an in between state, known as a wavicle (or wave function) until it is observed, at which point it "collapses" into "being" one or the other, according to how it is observed /ubbthreads/images/graemlins/thinking.gif

Scarily enough, the same seems to be true of matter. For example: electrons, which are generally accepted as being tiny particles, can easily be made to exhibit wave phenomena in the laboratory /ubbthreads/images/graemlins/naughty.gif

Sleep well /ubbthreads/images/graemlins/wink.gif
 

woodsman

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1. What exactly is light?

To prevent my mind from exploding I'll just make it as simple as it really is, the way I like to think about everything.
Does that make me simple minded? /ubbthreads/images/graemlins/grin.gif

Answer: The RESULT of God saying, "let there be light." /ubbthreads/images/graemlins/grin.gif
 

Bravo25

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[ QUOTE ]
Eugene said:
almost but not quite entirely unlike dark.

[/ QUOTE ]

Maybe your right, but a room can filled by light, It can't be filled with darkness. Darkness is only an absence of light.
 

Bravo25

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While I am still trying to wrap my mind around exactly what light is here is one more.
We know that light at some level travels for infinity. This is presumed by astronomist who use light from stars to measure time, and distance. Since there are billions of stars, planets, and other anomolties reflecting light in all directions, why is space dark? There certainly is no absence of light in space. Is it only a percieved darkness?


....... don't ask hard questions!!

Got to. Most of the easy ones I can figure out for myself /ubbthreads/images/graemlins/wink.gif
Besides you know the quest for knowledge, deeper understanding, all of that.
 

js

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[ QUOTE ]
Bravo25 said:
While I am still trying to wrap my mind around exactly what light is here is one more.
We know that light at some level travels for infinity. This is presumed by astronomist who use light from stars to measure time, and distance. Since there are billions of stars, planets, and other anomolties reflecting light in all directions, why is space dark? There certainly is no absence of light in space. Is it only a percieved darkness?


....... don't ask hard questions!!

Got to. Most of the easy ones I can figure out for myself /ubbthreads/images/graemlins/wink.gif
Besides you know the quest for knowledge, deeper understanding, all of that.



[/ QUOTE ]

Space is dark because there is very little in it to refract light. Light that does not reach your eye is not seen or sensed. So there is a bunch of light flying all over the place in the vacuum of space, but only what ends up in your eye is seen. In the atmosphere of earth, any light travelling in any direction is refracted and reflected and absorbed and so on, so that some of that light ends up in your eye. The air ends up affecting the blue light the most, which is why the sky is "blue".

At any rate, when it comes right down to it, space is not a perfect vacuum, and so if you look long enough and carefully enough, you will see that space is not absolutely "black". But this would need to be done by a telescope in orbit.
 

js

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[ QUOTE ]
lightnix said:
On the other hand, the "Copenhagen Interpretation" of quantum theory says that light is neither wave nor particle, but exists in an in between state, known as a wavicle (or wave function) until it is observed, at which point it "collapses" into "being" one or the other, according to how it is observed /ubbthreads/images/graemlins/thinking.gif

Scarily enough, the same seems to be true of matter. For example: electrons, which are generally accepted as being tiny particles, can easily be made to exhibit wave phenomena in the laboratory /ubbthreads/images/graemlins/naughty.gif

Sleep well /ubbthreads/images/graemlins/wink.gif

[/ QUOTE ]

As a trained physicist, and as one who is in daily contact with world class particle physicists, I can tell you that the Copenhagen interpretation is NOT universally accepted by scientists. Especially is this the case when it comes to physicists who have studied quantum measurement theory, and solid state physics.

Without dredging up a rather unexciting argument/conversation, let's just say that there are plenty of us physicists who do NOT agree with the whole simple minded "observation" collapsed wave-function. For starters, what constitutes an "observation"? Some people I've talked with actually seem to have this idea that we can "look" and see this stuff and that the human consciousness is what constitutes the observation. Hogwash.

Consider the famous Shrodinger's Cat paradox. In a box is a cat, a radioactive isotope, a detector, and a mechanical device which dispenses poison gas when the detector gets a radiation count. The idea is that the radioactive atom(s) has an amplitude to decay, thus causing a gamma ray to strike the detector, but this may or may not happen, and in fact, until an "observation" takes place BOTH things (decay/ non-decay) happen and exist in an "in-between" state, or in parallel universes, if you really want to screw things up. And because of this, the cat, whose life is directly tied to the decay probability, is also both alive and dead. The problem with this muddle headed paradox, is that THE RADIATION DETECTOR IS THE OBSERVATION! As soon as the detector gets a count, the wave function is collapsed, and thus the cat is either dead or not. Except in very, very special situations, no macroscopic object can be in a superposition of states or eigen values. The interaction of the gamma ray and the gieger counter constitutes an irreversible interaction, a change in the universe, that in no way can be completely undone. This is why it is an observation, and it has nothing to do with the consciousness of either a human or a cat. Too much mysticism has crept into QM, in exactly the wrong way.
 
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