Photon Detection: Learn About Interaction, Detection & Angles

[Curious]

Hello, I have always been curious about physics and decided to try this forum. However I am a bit concerned due to my lack of knowledge on the subject, so I’m hoping my questions won’t be unacceptable for the forum.

I have always wondered about photons and how they interact with the world. If I understand things to some degree, you can’t detect a photon as it ‘passes-by’, but that they have to directly comes into contact with an object, such as the Rods or Cones in the human eye before they can be seen.

If this is true, is there a range of angles at which a photon must be within in so that they can be detected?

Thanks.

 

[jbriggs444]

If this is true, is there a range of angles at which a photon must be within in so that they can be detected?

It has to come from somewhere within your field of view and hit your eye. It has to be part of a bunch of other photons so that collectively they can excite a particular rod or cone on your retina. The point on your retina that is illuminated will depend on the angle of arrival.

 

[Curious]

It has to come from somewhere within your field of view and hit your eye. It has to be part of a bunch of other photons so that collectively they can excite a particular rod or cone on your retina. The point on your retina that is illuminated will depend on the angle of arrival.

Well a photon 'comes' in the form of a wave, so depending on where it might be within it's phase or cycle, would the angle of incidence need to be combined with the directional angle of it's origin to know if it will be detected?

 

[Curious]

If a photon were in a form of a 'Ray' the angle of incidence seems rather straightforward, however once I start thinking about it as a wave then I'm not so sure anymore?

 

[tech99]

A receiving antenna can only extract half the incident energy, so I imagine that a maximum of half the photons can give their energy to the moving of electrons. As the angle deteriorates, I suppose a smaller proportion of the photons will donate their energy.

 

[davenn]

If a photon were in a form of a 'Ray' the angle of incidence seems rather straightforward, however once I start thinking about it as a wave then I'm not so sure anymore?

don't confuse yourself, the photon doesn't travel as a little wavy line, as in your diagram,
so that makes your following comment incorrect

Well a photon 'comes' in the form of a wave, so depending on where it might be within it's phase or cycle, would the angle of incidence need to be combined with the directional angle of it's origin to know if it will be detected?

photons DONT travel like little bullets radiating out from the source

At a very basic level, without getting into QFT ( Quantum Field Theory) and QED ( Quantum Electrodynamics) which is way above my head.
Photons are quantum packets of energy that mediate the electromagnetic wave. In other words, they carry the energy of the EM wave.
They don't exist at any particular position along the wave until they are detected, then they are absorbed.

 

[Curious]

Oh man, now I'm really confused. So an electron returns to a lower energy level emiting a photon. It travels through space in some mass-less form radiating outwards from an atom. As it heads away, does it travel in a straight line until it encounters an object?

 

[Asymptotic]

Oh man, now I'm really confused. So an electron returns to a lower energy level emiting a photon. It travels through space in some mass-less form radiating outwards from an atom. As it heads away, does it travel in a straight line until it encounters an object?

Somebody on this forum - can't recall who (but you know who you are ... thanks) - linked out to an article on the nature of matter and energy on Matt Strassler's blog. You may find his "Fields and Particles: With Math" series interesting and useful. Although photons don't make a big appearance until article #7 it's best to start at the beginning.

 

[Curious]

Thanks to all who have responded to my question, it's much appreciated. As I thought, my understanding of physics is very rudimentary and well wrong. Makes me wish I'd taken more physics related courses. I'm quite interested in reading the Fields and Particles link provided above as I am very interested in understanding more about photons.

There are just things which get caught in the back of my mind that cause confusion. If light has no mass, how does gravity act upon it, or is that all the result of curved space-time.

Another I can't get past is the imposed speed limit of matter. If a photon has a spin and a measurable dimension, then when the direction of the spin lines up with the direction of the photon, then the surface of the photon would be moving faster then the speed of light which shouldn't be possible - should it?

 

[Nugatory]

Another I can't get past is the imposed speed limit of matter. If a photon has a spin and a measurable dimension, then when the direction of the spin lines up with the direction of the photon, then the surface of the photon would be moving faster then the speed of light which shouldn't be possible - should it?

Quantum mechanical spin is not rotation, and a photon has neither a size nor a surface, so there's no problem here.

 

[ZapperZ]

Hello, I have always been curious about physics and decided to try this forum. However I am a bit concerned due to my lack of knowledge on the subject, so I’m hoping my questions won’t be unacceptable for the forum.

I have always wondered about photons and how they interact with the world. If I understand things to some degree, you can’t detect a photon as it ‘passes-by’, but that they have to directly comes into contact with an object, such as the Rods or Cones in the human eye before they can be seen.

If this is true, is there a range of angles at which a photon must be within in so that they can be detected?

Thanks.

I need to go back to your very first post, because there's something here that I think that you are missing.

This issue is NOT just simply confined to photon and its detection. Everything that is detected HAS to interact with something else. We detect an electron when that electron's properties interact with something else. This could be the electron's EM field, the electron's spin, the electron's momentum, etc. Those properties, when one of them interact with something else, it is only then that we can detect that electron. A neutrino is extremely hard to detect because it interacts extremely weakly with something else. Because of this, we hardly know that a billion of them are passing through our bodies all the time.

The moral of the story here is, for everything, including photons, no interaction means no detection.

Zz.

 

[Curious]

Yikes, the more questions I ask, the more I realize how misinformed I really am. I know that this is largely due to my lack of knowledge, but I'm starting to wonder if some of the information I am reading might not be all that accurate. And well there is also the very strong possibility that I'm taking things a bit too literally.

Sorry it took me a bit of time to find the reference, but in regards to the photons 'surface' remark, well that came from a website I was reading where it was discussing photon spin:

("We know that the linear velocity of light is not infinite, so we must assume the speed of spin is also not infinite. If it is not infinite, it must take time. If it takes time, then it will be stretched by the linear motion. While the surface of the photon is spinning, the photon as a whole is moving some linear distance x."

"the circumference is 4 times the diameter, so a point on the surface of the photon travels 8.8 x 10-23 m.")

When reading information such as this, I form a picture in my mind of a photon with shape, size and form spinning complete with a surface.

 

[ZapperZ]

Yikes, the more questions I ask, the more I realize how misinformed I really am. I know that this is largely due to my lack of knowledge, but I'm starting to wonder if some of the information I am reading might not be all that accurate. And well there is also the very strong possibility that I'm taking things a bit too literally.

Sorry it took me a bit of time to find the reference, but in regards to the photons 'surface' remark, well that came from a website I was reading where it was discussing photon spin:

("We know that the linear velocity of light is not infinite, so we must assume the speed of spin is also not infinite. If it is not infinite, it must take time. If it takes time, then it will be stretched by the linear motion. While the surface of the photon is spinning, the photon as a whole is moving some linear distance x."

"the circumference is 4 times the diameter, so a point on the surface of the photon travels 8.8 x 10-23 m.")

When reading information such as this, I form a picture in my mind of a photon with shape, size and form spinning complete with a surface.

You really shouldn't be reading crap like this. I mean, can't you spot already a silly math mistake in there? If "circumference" is the circumference of a circle, it is 2πr, and r=d/2. So the circumference is πd, not 4d.

It is why we require only valid, authoritative websites to be used as sources. Otherwise, we will be correcting this type of crackpot mistakes all the time.

Rule of thumb: don't read webpages or blogs of unknown persons. If it doesn't have a valid authority standing behind it, you should read it with utmost care.

Zz.

 

[Curious]

Unbelievable! You know I had questioned the math right off the bat knowing π is roughly 3.14 not 4, but with my lack of knowledge on the subject, the last thing I wanted to do was to start placing questions on top of questions contradicting others when my own understanding is extremely limited.

At times it's difficult with so much information out there to know what's worth reading and what is not. I can't begin to tell you how many sites that even I have passed by due to lame and incorrect information. Unfortunately the site I referenced above did manage to pull me in, at least to some extent.

With all of the poor sites I was encountering, I decided to try and find a forum in order to get more accurate information, so I came here. I've only been a member for a day or so and it's already been quite an eye opener.

 

[Curious]

Instead of a human eye, when a photon hits a CCD, does it's kinetic energy get converted into an EMF by colliding with an electron and setting it into motion?

 

[tech99]

Instead of a human eye, when a photon hits a CCD, does it's kinetic energy get converted into an EMF by colliding with an electron and setting it into motion?

As people have mentioned, a photon is not something like a bullet. But any EM wave will apply an electric field to an electron, which will cause it to move.

 

[Curious]

tech99 - sorry about that, I guess it's going to take me a little bit of time to undo my incorrect concepts/thinking of particles flying around colliding with one another. I tend to picture things like a billiard game with the balls being particles hitting anything in there path.

 

[Khashishi]

If a photon were in a form of a 'Ray' the angle of incidence seems rather straightforward, however once I start thinking about it as a wave then I'm not so sure anymore?

Photons don't travel as rays. Geometric optics uses rays, but it is an approximation, useful when objects are much larger than the wavelength of light.
https://en.wikipedia.org/wiki/Geometrical_optics
Light actually travels as a wave and shows diffraction. Even a laser beam has a finite size and angular spread. A photon is a quantum of light, so it moves in the same way. Some cartoons show photons as little wave packets which sum together to make the light wave. These are incorrect. You can't really picture what a single photon looks like, since you only see them via their interaction with a screen, which collapses their position into a dot.

You might think that a cone shaped beam of light has many photons traveling in different directions, and it looks like a continuous cone because you are averaging over billions of photons. If this is the case, then you are wrong. Each photon is spread over the cone and spreads over all directions. It only appears to go in one direction after it collapses due to some measurement.

There are just things which get caught in the back of my mind that cause confusion. If light has no mass, how does gravity act upon it, or is that all the result of curved space-time.

It's the result of curved space-time. Here's a good video.

Another I can't get past is the imposed speed limit of matter. If a photon has a spin and a measurable dimension, then when the direction of the spin lines up with the direction of the photon, then the surface of the photon would be moving faster then the speed of light which shouldn't be possible - should it?

Spin doesn't work like that. Spin is associated with circularly polarized light, which is more of a corkscrew shape of the electric and magnetic fields.

 

[Curious]

Khashishi - thanks for the video.

I've been chewing on all of the answers I have gotten here on the forum and it has caused me to rethink so much of what I thought I understood.

I had assumed that photons had to collide with an electron for it to be absorbed, which further had me believing that light focused on an object could have atoms other then those on the surface to become excited. With photons being so small, I thought they could pass by many atoms in a material before actually hitting an electron exciting atoms below the surface. Now all of that along with other misguided thinking of mine has changed.

If collisions don't take place between photons and an electrons, then what exactly causes a photon to excite an atom and become absorbed? Is it the proximity of the EM field of the photon in relationship to the electron, or perhaps it's when an electron passed through the approaching photons EM field. I know the last two statements sound almost identical, but the difference I'm thinking of here is who or what initiates the interaction, the photon or the electron?

 

[Khashishi]

Photons are indistinguishable so it doesn't help to think about the EM field of this particular photon. You simply have the EM field. Regarding proximity, the atom is sitting in the EM field, so it has a probability of interacting at any time. You can't say where a particular photon is. The wavelength of the oscillations in the EM field are large compared to the atom, so the atom essentially sees an electric field that switches direction back and forth, which is essentially an oscillating dipole. The electron is restricted to several orbitals. If you multiply an orbital by the dipole field, you get a superposition of orbitals. This means there's a probability of transitioning to these other orbitals by the Born rule.

 

[Khashishi]

Unfortunately, you can't really picture what's going on classically. However, the correspondence principle says that QM approaches classical in some circumstances. Highly excited atoms (Rydberg states) can approach the behavior of a planet orbiting a star. If you apply a strong oscillating force on the planet, you can throw it out of orbit or make it escape (ionization). It's actually possible to make crude quantitative predictions of QM processes using a classical model (keyword: classical trajectory monte carlo).

 

[Curious]

So are electrons drawn towards the EM field as the photon nears, or does the oscillation prevent any attraction from taking place, leaving the orbital path responsible for bringing the electrons into the field? I know you said above that the atom is siting within the EM field, but there must be interim steps as time progresses and particles approach one another, right?

It would seem that If electrons are being drawn towards the photon's EM field, then a substantial amount of radiation would be enough to alter the electron cloud to such a degree that the atom itself would be affected, a change it's normal vibrations - or begin to heat. Maybe - or am I totally off in left field once again.

 

[lekh2003]

When reading information such as this, I form a picture in my mind of a photon with shape, size and form spinning complete with a surface.

Whenever you start asking questions regarding photons and detection, you inevitably reach the field of quantum mechanics and the most confusing part of physics you can venture into. Photons experience a wave particle duality, which means sometimes in some experiments they are a wave and in others they are a particle. This seems very odd to someone without an understanding in quantum physics, and I also struggle with the concept.

Not only is the very concept mind-bending, the terms used confuse you even more. Initially, people thought of these elementary particles (photons, electrons, etc.) as a ball with shape, size and other physical features. This is usually not true. Mind how I said usually, since sometimes it is. Welcome to quantum physics, we have no idea whether or not something is.

As for the spin, this is a quantum mechanical term with no relation to physical spin. Unfortunately, I still do not fully understand it, but I do know for a fact that it isn't what you picture.

If you want to venture and ask about the detection of photons and angles, you must understand that observing things in the quantum world is not as simple as watching a ball bouncing around. The very act of watching the photon bounce around causes the photon to change its behavior. It is like the photon is shy and tries to act differently when it is watched. No one knows what actually happens, but we make guesses.

 

[ZapperZ]

So are electrons drawn towards the EM field as the photon nears, or does the oscillation prevent any attraction from taking place, leaving the orbital path responsible for bringing the electrons into the field? I know you said above that the atom is siting within the EM field, but there must be interim steps as time progresses and particles approach one another, right?

It would seem that If electrons are being drawn towards the photon's EM field, then a substantial amount of radiation would be enough to alter the electron cloud to such a degree that the atom itself would be affected, a change it's normal vibrations - or begin to heat. Maybe - or am I totally off in left field once again.

Again, there is a problem here because you are mixing MANY different things together.

Are you asking about ONE single photon interaction, of MANY photons interaction?

Here's the deal. We KNOW that EM waves can interact with atoms. We have experiments for it. We also know that light interacts with charged particles. We have experiments for it. Heck, particle accelerators depend on that interaction.

HOWEVER

These are considered to be interaction with MANY, MANY, MANY photons (i.e. when the EM waves are often considered interacting classically).

It is unclear if you are asking if ONE photon can somehow affect an atom's orbital, or change it's structure, etc... or if you're asking if EM waves in general (i.e. when there are MANY, MANY, MANY photons) can do that. Those are two different and separate scenarios!

It is almost impossible to answer a question when the question itself is vague. I see people here trying to address your question, and yet, I don't see any level of clarity in the question itself. When that happens, you'll get answers (many, MANY different answers) that may not even address what you actually had in mind, and worse still, you may not even be aware that the answer you've received and may accept isn't even relevant to the question that you asked in the first place!

This is what commonly happens when you try to pry deeper and more carefully into physics. As physicists, we are fully aware that once we ask Mother Nature some of the most basic and fundamental questions, She will ask her back "Now young man or young woman, what EXACTLY do you mean by that?" As someone who is just learning about these things, you may want to start and put a bit more consideration into the questions that you form. It is a very good practice.

Zz.

 

[jerromyjon]

It would seem that If electrons are being drawn towards the photon's EM field, then a substantial amount of radiation would be enough to alter the electron cloud to such a degree that the atom itself would be affected, a change it's normal vibrations - or begin to heat. Maybe - or am I totally off in left field once again.

You might find it easier to break things down into simpler examples of quantum behavior, like the Stern-Gerlach apparatus shows how quantum spin is detected...

And two-slit diffraction patterns show how quantum objects follow wave-like trajectories...

Once you understand the basic properties you can begin to form a deeper understanding of quantum interactions and how they are probabilistic rather than deterministic...