Light's Reach: How Far Does 1 Candela's Luminosity Go?

[zeromodz]

Like say I have one candela of luminosity. How far away do I have to go until the photons of this light are completely undetectable?

 

[Andy Resnick]

Keep going until the signal-to-noise ratio at the detector is equal to 1.

 

[Drakkith]

Given a PERFECT detector with ZERO noise or interference, there isn't a maximum distance. As long as one photon from the candle hits it, it would detect it. The further away you go the less photons per unit of time will hit the detector.

In real life it's like Andy said. You will reach a point where you cannot detect it with your detector due to sensetivity and noise and such.

 

[russ_watters]

As long as one photon from the candle hits it, it would detect it.

...which is only a matter of waiting long enough.

 

[Drakkith]

...which is only a matter of waiting long enough.

Yep, just like I said. =)

 

[AtomicJoe]

Given it travels as a wave how can it be in so many places at the same time, before it is detected?

 

[Drakkith]

Given it travels as a wave how can it be in so many places at the same time, before it is detected?

Light travels in "packets of energy" called Photons, the particle of light. The light bulbs you see everywhere emit trillions upon trillions of photons per second. Only when a photon hits the right point in your eye can you see it, even though many many more are actually entering it.

ALL matter AND light has wavelike and particlelike properties. You'd have to look up more about Quantum Physics to learn about it all.

 

[AtomicJoe]

Light travels in "packets of energy" called Photons, the particle of light. The light bulbs you see everywhere emit trillions upon trillions of photons per second. Only when a photon hits the right point in your eye can you see it, even though many many more are actually entering it.

ALL matter AND light has wavelike and particlelike properties. You'd have to look up more about Quantum Physics to learn about it all.

But the double slit experiment says it travels as a wave, and not like a particle, thus it must (the wave) be in all directions at once.

 

[russ_watters]

The double slit experiment is more complicated than that. Light may show similarities to a wave but it is not a wave.

 

[AtomicJoe]

The double slit experiment is more complicated than that. Light may show similarities to a wave but it is not a wave.

Not sure what your point is, however it does travel like a wave and a wave moves in all directions at once.

I think light is a wave and not a particle at all.

 

[russ_watters]

Well sorry, but both of those statements are just plain factually wrong.

 

[twofish-quant]

Like say I have one candela of luminosity. How far away do I have to go until the photons of this light are completely undetectable?

Define "detect."

For example, if at noon you are looking at a star in the sky, you will be getting photons from that star hitting your eye. Unfortunately, you won't be able to notice that star because it will get lost the noise of the sun.

 

[pallidin]

...and a wave moves in all directions at once...

Where did you get that idea?
A "pressure" wave can be made to do this, but not a singular photon wave.

 

[sophiecentaur]

There is an answer in terms of classical information theory. Given a narrow enough bandwidth (enough time) then you can detect a signal in any arbitratry amount of noise and interference (sunlight and other light pollution).
However, once you get down to energy flux densities so low that you need to consider individual photons (not necessarily little bullet-type particles but quanta of Energy) you are in the realms of probability and statistics. But, still, there is no real lower limit.
I believe our eyes, when fully dark adapted and in 'pitch dark' conditions can be shown to detect individual photons and, of course, photomultipliers can, too.
Extending this to the "how far away can I go?" question, however, implies doing it in the open - when light interference would come into play. Designing a telescope with zero flare is hard / impossible so you can't eliminate stray light completely.

 

[Drakkith]

Not sure what your point is, however it does travel like a wave and a wave moves in all directions at once.

I think light is a wave and not a particle at all.

Like Russ said, that is incorrect. There is overwhelming evidence that light has properties of BOTH waves and particles. Look up the photoelectric effect.

 

[russ_watters]

Like Russ said, that is incorrect. There is overwhelming evidence that light has properties of BOTH waves and particles. Look up the photoelectric effect.

Heck, you can stick with the double-slit experiment. It can be performed by firing one photon at a time at the slits. When most people find out what happens for the first time, it blows their minds!

 

[Drakkith]

Heck, you can stick with the double-slit experiment. It can be performed by firing one photon at a time at the slits. When most people find out what happens for the first time, it blows their minds!

Yes, but that still demonstrates the wavelike properties of the photon. The photoelectric effect is one of the most basic demonstrations of the particle like properties of the photon, as Einstein himself explained!

 

[AtomicJoe]

Well sorry, but both of those statements are just plain factually wrong.

The statements.

s1) Not sure what your point is, however it does travel like a wave and a wave moves in all directions at once.

s2) I think light is a wave and not a particle at all.

For s1 the double slit experiment shows it travels as wave does it not?

As for s2, well I am entitled to be wrong in my thinking I guess, but I find it easier to think of a particle as a special type of wave rather than accept a particle went through two slits at the same time. So if I was forced to choose I go for a wave. Maybe the answer is that is is 'something else'.

 

[AtomicJoe]

Like Russ said, that is incorrect. There is overwhelming evidence that light has properties of BOTH waves and particles. Look up the photoelectric effect.

Well I looked it up but I am slightly confused as to what I should be looking for.

 

[AtomicJoe]

Yes, but that still demonstrates the wavelike properties of the photon. The photoelectric effect is one of the most basic demonstrations of the particle like properties of the photon, as Einstein himself explained!

Definition/Summary
When a metal surface is irradiated, it ejects electrons whose kinetic energy can be measured. This electron emission only happens when the irradiating light is above a certain angular frequency . This frequency threshold is found to be independent of the intensity of the radiation. The kinetic energy of the electrons is found to be linearly related to the frequency of light after the threshold, with

OK you say this is a demonstration of the particle properties, however looking at the highlighted text in bold, isn't it evidence of wave like properties?
After-all, particles do not have frequencies!

 

[russ_watters]

s1) Not sure what your point is, however it does travel like a wave and a wave moves in all directions at once.

1. Light has some similarities to waves, but, also behaves in distinctly unwavelike ways.
2. A wave - any wave - does not need to move in all directions at once.

s2) I think light is a wave and not a particle at all.

Absolutely not.

As for s2, well I am entitled to be wrong in my thinking I guess, but I find it easier to think of a particle as a special type of wave rather than accept a particle went through two slits at the same time. So if I was forced to choose I go for a wave.

Yes, you are certainly entitled to be wrong if you want, but given a choice, I'd think you'd want to be right! That's why most people come here, after all - to learn what is right.

Maybe the answer is that is is 'something else'.

The answer to how light travels is, in fact, "something else" besides a (classical) particle and a wave. Light travels in photons, which have properties like waves and particles, but are not either. They are "something else".

After-all, particles do not have frequencies!

Particles do have frequencies, but that's besides the point. The point is that a single photon knocks-loose a single electron, demonstrating clearly that light is quantized (travels in discrete packets).

In the double-slit experiment, it is possible to fire single photons at a time and watch them impact a single spot on a detector, also clearly demonstrating a particle-like behavior.

 

[Drakkith]

The statements.

s1) Not sure what your point is, however it does travel like a wave and a wave moves in all directions at once.

s2) I think light is a wave and not a particle at all.

For s1 the double slit experiment shows it travels as wave does it not?

As for s2, well I am entitled to be wrong in my thinking I guess, but I find it easier to think of a particle as a special type of wave rather than accept a particle went through two slits at the same time. So if I was forced to choose I go for a wave. Maybe the answer is that is is 'something else'.

1. It demonstrates the wavelike properties of the photon, yes. But you cannot look SOLELY at that one experiment. You must look at all of the available evidence.

2. You can think what you like. Just realize that when you tell people that they will most likely say that you are wrong because the available evidence supports them.

3. Light, and all matter as well, is neither a particle NOR a wave only. It all has properties of both. You don't have to "choose" one or the other.

 

[FtlIsAwesome]

Didn't the double slit experiment show that light must behave as both a particle and a wave for it to make the observed interference pattern? Like it behaves as one of them when it passes through one slit, then it behaves as the other when it passes through the other slit.

 

[Drakkith]

Didn't the double slit experiment show that light must behave as both a particle and a wave for it to make the observed interference pattern? Like it behaves as one of them when it passes through one slit, then it behaves as the other when it passes through the other slit.

Not quite. The double slit experiment shows that even when a SINGLE photon is fired at TWO slits, it interferes with itself and produces and interference pattern after a large number of single photons are detected over time and the pattern is observed.

You would THINK that the photon would have to go through one slit OR the other. But that is incorrect. Also, with a single photon at a time going through the slits, you would think that it would be impossible for it to interfere, since there aren't any other photons to interfere with. However it has been shown that the photon will interfere with itself.

 

[FtlIsAwesome]

Ah, the slits are side-by-side not one after the other. Right.

Now that you've jogged my memory, I think I read it like this: A single photon behaving as wave goes through both slits and interferes with itself, but when it hits the detection plate (made of silver?) it behaves as a particle.
The culiminative effect of photons over time contribute to the pattern observed.

 

[russ_watters]

The culiminative effect of photons over time contribute to the pattern observed.

Yes - the photons might individually land anywhere, but as you watch large numbers over time, you find they follow the probability distribution of the interference pattern.

 

[AtomicJoe]

. Particles do have frequencies, but that's besides the point. The point is that a single photon knocks-loose a single electron, demonstrating clearly that light is quantized (travels in discrete packets).

Well waves definitely have frequencies (too), I am not to happy about particles having frequencies, however that is rather irrelevant to the point.

The point is, demonstrating 'it' has a frequency does not seem to prove much really does it as that is a property possessed by both waves, and particles.

It like saying, which man committed the murder, of the one in the blue hat, well both men have blue hats!

Then we get to the point that a single electron is released, well I can say the wave had enough energy to release a particle can't I?

 

[Dr Lots-o'watts]

The shorter the wavelength, the more it behaves as a particle.

Dim UV can knock off electrons from metals, but blinding red light carrying the same energy or more cannot.

If it were a totally classical wave, it wouldn't matter whether the energy was carried as frequency or amplitude. The fact that it does matter, suggests a particle nature.

 

[AtomicJoe]

The shorter the wavelength, the more it behaves as a particle.

Dim UV can knock off electrons from metals, but blinding red light carrying the same energy or more cannot.

If it were a totally classical wave, it wouldn't matter whether the energy was carried as frequency or amplitude. The fact that it does matter, suggests a particle nature.

I don't see that, turning up the light just means more waves are emitted per second, so to me it proves nothing.

 

[Drakkith]

Then we get to the point that a single electron is released, well I can say the wave had enough energy to release a particle can't I?

Except that like Dr Lots o watts said, a huge amount of deep red light cannot eject electrons, while a single photon of UV light can.

Also, electrons ALSO behave like particles and waves. How would you explain that?

 

[Drakkith]

I don't see that, turning up the light just means more waves are emitted per second, so to me it proves nothing.

It proves that each "wave" is a distinct entity in itself that must be emitted and absorbed as a whole and has a set amount of energy. In a classic wave, this is not the case.

 

[Dr Lots-o'watts]

No matter how you phrase it, turning up the light won't knock off electrons. Only shortening the wavelength will.

How would YOU (atomicjoe) explain that experimental fact?

 

[AtomicJoe]

It proves that each "wave" is a distinct entity in itself that must be emitted and absorbed as a whole and has a set amount of energy. In a classic wave, this is not the case.

Isn't it? Maybe not. I am not too sure what 'classical wave theory' is. I don't know if I had an understanding of the theory as such, just something you came across in physics and answered a few questions about, I don't recall much about energy.

However, I see little difference between a quantised particle and a quantised wave, so it does not necessarily suggest particle behaviour to me, could be either.

 

[AtomicJoe]

No matter how you phrase it, turning up the light won't knock off electrons. Only shortening the wavelength will.

How would YOU (atomicjoe) explain that experimental fact?

Well I would just say the waves have to be the right frequency, that seems a bit more logical than saying the particles have the right frequency, because the idea of particles having a frequency is a bit 'odd', we normally speak of the frequency of waves not particles. (well I do).

I mean there are two ways of looking at it for the energy from light, you can say the wave has a bigger amplitude, or that more waves of a smaller amplitude are emitted per second.

Turning it around somewhat you could say when you turn up a light it emits bigger particles of light, or maybe that's not a good analogy?

So to me the photo-electric experiment is not very helpful either way.

 

[russ_watters]

Then we get to the point that a single electron is released, well I can say the wave had enough energy to release a particle can't I?

If light were really purely a wave, it would be possible to cut down the intensity by any amount, including an amount where it wouldn't be enough to cut loose an electron.

I see little difference between a quantised particle and a quantised wave, so it does not necessarily suggest particle behaviour to me, could be either.

Well, one exists, one doesn't: there is no such thing as a "quantized wave".

I mean there are two ways of looking at it for the energy from light, you can say the wave has a bigger amplitude, or that more waves of a smaller amplitude are emitted per second.

Except that you're now saying that there is a minimum amplitude to a wave. That doesn't jive with how waves work.

Moreover, this contradicts what you said earlier about waves being continuous and omni directional - while they may not need to be omni directional, neither are they completely discrete. Ie, with the double-slit experiment, there is no way for a wave-based light to create a single dot on a detector.

 

[Drakkith]

Well I would just say the waves have to be the right frequency, that seems a bit more logical than saying the particles have the right frequency, because the idea of particles having a frequency is a bit 'odd', we normally speak of the frequency of waves not particles. (well I do).

I mean there are two ways of looking at it for the energy from light, you can say the wave has a bigger amplitude, or that more waves of a smaller amplitude are emitted per second.

Turning it around somewhat you could say when you turn up a light it emits bigger particles of light, or maybe that's not a good analogy?

So to me the photo-electric experiment is not very helpful either way.

The right frequency? No, they have to be above a certain frequency. ANY frequency above that can eject an electron from the surface of a materiel. NO frequency under that will eject one however.

Amplitude has NO meaning with light. It is a proven fact, like the photoelectric effect shows, that only a change in frequency can raise or lower the energy of a photon.

 

[AtomicJoe]

If light were really purely a wave, it would be possible to cut down the intensity by any amount, including an amount where it wouldn't be enough to cut loose an electron. .

Well by your definition of a wave yes, but I using a different idea of a wave, a wave pulse if you like.

Well, one exists, one doesn't: there is no such thing as a "quantized wave". .

I don't really have a problem with the concept.
A tsunami could be considered a quantised wave for example.
Albeit a large one.

Except that you're now saying that there is a minimum amplitude to a wave. That doesn't jive with how waves work.

Well yes from a particular source there could be. Like dropping pebbles all the same size into a pond.

Moreover, this contradicts what you said earlier about waves being continuous and omni directional - while they may not need to be omni directional, neither are they completely discrete. Ie, with the double-slit experiment, there is no way for a wave-based light to create a single dot on a detector.

I don't think I said they were continuous as such, omni directional yes.

Creating a single dot is, I admit, somewhat more problematic, on the same scale as you getting your particle through two slits at the same time.

I have never seen the experiment done myself, it takes some believing!

 

[AtomicJoe]

The right frequency? No, they have to be above a certain frequency. ANY frequency above that can eject an electron from the surface of a materiel. NO frequency under that will eject one however.

Amplitude has NO meaning with light. It is a proven fact, like the photoelectric effect shows, that only a change in frequency can raise or lower the energy of a photon.

So what happens when you increase the frequency?

Is the electron released with more energy?
Or is there some 'spare light' left over?

Also if it has no amplitude, how can it have a frequency?

 

[russ_watters]

Well by your definition of a wave yes, but I using a different idea of a wave, a wave pulse if you like.

Well, you're starting to make up a definition that sounds more and more like how light really works and less and less like the actual definition of wave. While that's an improvement, it would be better if you'd just acknowledge that you're no longer describing the (not my) definition of a wave. Words have specific meanings and you're really not entitled to make them up as you go along.

I don't really have a problem with the concept.
A tsunami could be considered a quantised wave for example.
Albeit a large one.

That's not what the word "quantized" means.

Well yes from a particular source there could be. Like dropping pebbles all the same size into a pond.

That's just not how it works.

I don't think I said they were continuous as such, omni directional yes.

Waves are continuous, which is why they cause interference patterns.

 

[Drakkith]

So what happens when you increase the frequency?

Is the electron released with more energy?
Or is there some 'spare light' left over?

Also if it has no amplitude, how can it have a frequency?

As you increase the frequency, the electrons are ejected with more and more energy. There is not spare light left over.

Don't know about the amplitude thing though. I just know I've never heard of amplitude of a single EM wave mattering for anything.

 

[sophiecentaur]

Like Russ said, that is incorrect. There is overwhelming evidence that light has properties of BOTH waves and particles. Look up the photoelectric effect.

The photoelectric effect does not indicate particulate nature. It tells us that the energy is quantised. These two are entirely distinct properties.
The word 'particle' is used by people because it is less intellectually demanding.

 

[AtomicJoe]

Well, you're starting to make up a definition that sounds more and more like how light really works and less and less like the actual definition of wave. While that's an improvement, it would be better if you'd just acknowledge that you're no longer describing the (not my) definition of a wave. Words have specific meanings and you're really not entitled to make them up as you go along.
That's not what the word "quantized" means. That's just not how it works. Waves are continuous, which is why they cause interference patterns.

There are different types of waves, for example a shock wave, which is more like a quantised pulse. I am sure you can have a un-continuous wave for example I am sure the photoelectric will work in reverse?
I mean surely the process which releases the single photo for the double slit experiment is the effect in reverse. A single particle detected through the slit would not be enough to be called a pattern.

 

[sophiecentaur]

A shockwave is not "quantised". What energy is associated with the energy of the individual molecules or the modes of bulk movement? Do you know what is meant by a quantum?

 

[zeromodz]

The further away you go the less photons per unit of time will hit the detector..

So if you were to take the limit as r goes to infinity, the light will contain zero photons? What happens to the photons as they travel. Why do you get less photons per unit of time if all photons travel at the same speed?

 

[AtomicJoe]

A shockwave is not "quantised". What energy is associated with the energy of the individual molecules or the modes of bulk movement? Do you know what is meant by a quantum?

It mean amount, if is the same root as quantity.

 

[Drakkith]

So if you were to take the limit as r goes to infinity, the light will contain zero photons? What happens to the photons as they travel. Why do you get less photons per unit of time if all photons travel at the same speed?

No. Individual photons are emitted in random directions. Because of this, the further away you are, you less photons you will detect per second for example. They spread out if you will. If you had a perfect detector and no interference and nothing to block the light, AND you had an infinite time to wait, you could go ANY distance from an object and eventually detect a photon from it.

 

[Drakkith]

It mean amount, if is the same root as quantity.

From wikipedia on Quantum:


In physics, a quantum (plural: quanta) is the minimum amount of any physical entity involved in an interaction. Behind this, one finds the fundamental notion that a physical property may be "quantized," referred to as "the hypothesis of quantization".[1] This means that the magnitude can take on only certain discrete values. There is a related term of quantum number. An example of an entity that is quantized is the energy transfer of elementary particles of matter (called fermions) and of photons and other bosons

A shock wave is NOT quantized. An example of quantization is the energy levels of electrons in atoms. They can ONLY have certain values, integers of N. It CANNOT have 1/2 N for example.

For light this means that the entire photon is "One entity". It can only be absorbed or emitted as a whole unit. You cannot split it in half.

A classic wave, like in a shock wave, can be split, absorbed, and will transfer any amount of the energy behind it to something else. This is NOT the case for light.

 

[sophiecentaur]

It mean amount, if is the same root as quantity.

The word has a much more specific meaning in QM. A quantum is a tiny amount of energy- the minimum energy that takes part in the interaction between an EM wave and an atom, molecule or arrangement of molecules. Some people refer to it as a particle by I like just to stick to the energy definition.