@Rasbindu

By  detect I mean mentally respond to the light signal.

Human subjects were placed in a dark room and subjected to small pulses of light. The subjects reported each time they perceived light. The conclusion of the experiment was that humans can see (detect) a single photon of light. Repeats of the experiment confirmed that humans can see less than 10 photons. A pulse of more than 10 photons is necessary for a human to see one photon. Light is lost in the passage through cornea, lens, eye fluid, and the covering of the retina. The conclusion that a single photon was detected (seen, reported) depends on knowing how many photons impinged upon the cornea and knowing (assuming, estimating) how many photons reached a photo-receptor in the cornea. 

Yes. By detect I mean see or the mental experience of the sensation of light.

As Hanno noted my statements referred to the difference between perceiving light and perceiving color. 

Detector converts electromagnetic wave energy into another kind of energy through absorption. Absorption can be treated classically, if we have a lot of photons, but intrinsically individual act of absorption means disappearance of a single photon, and in single photon detectors it is important, of course. If no absorption/emission happens, mostly wave properties are important

Probably this is correct even in case of biphotons. One can think of interference in terms of biphotons, but all possible effects are revealed only after detection

In semiconductor detector, the energy quanta of photon should match the band gap to allow a transition of electron from valence band to conduction band. Photomultipliers, or detectors that operate based on photo cathode , function according to photoelectric effect. Electron gains higher energy from incident photon leading to the momentum rise after overcoming a certain work function of photocathode. In fact, absorption is a quantum mechanic concept where an atom (molecule) transition takes place or electron promotes to eject from atom( molecule) : ionization is basically an interaction of photon with atom( molecule) too. All deals with photon as a particle, what about the contribution of electromagnetic wave property of light?

In any case, I stress, that for detection, light energy (completely or partially) MUST BE CONVERTED to another kind. This is not principle of the universe, but it is the principle of man's interaction with the universe. And light energy can be converted to another kind via absorption of any kind, independently on the way we further deal with absorbed energy

Parviz, you understand correctly that in the wave-particle duality it's usual that propagation is mostly a wave behavior, and absorption/emission is mostly a particle type process.

When you measure light intensity by a thermal sensor, you do not care how the light energy is transformed to heat, like you do not count water molecules when measuring height of sea waves. Just power is a function of wave amplitude... I mean, there were light measurements before photons were invented. And those methods still are in use.

Similarly, in radio you rarely hear about photons, even in antennas, though a radio wave certainly consists of low energy photons...

Vladimir,

I perfectly agree with you. In the wave-particle duality it is usual that propagation is mostly a wave behavior, and absorption/emission is mostly a particle type process.

Alexandr,

It is not essential that always photon is absorbed during detection process. In Compton scattering ( for gamma photons ranging MeV ), the photon energy reduces and electron energy rises without photon absorption. Furthermore, the gamma photon may generate a number of ion- electron pairs in atmosphere without full absorption.

It there's a lecture I gave in 2010 in Algiers on the works of Ibn Al Haitham in 9-10th century and you can download from my profile. Maybe you are interested in the context of optics and visual perception.

Dear Benamar

Ibn Al Haitham and Imam Jafar Sadegh proposed very interesting comments on Optics in middle centuries. Is it possible to add highlighted parts of your lecture as an answer here?Then, I will insert the Imam Jafar Sadegh ideas too. It is intersting to know that 12 centuries ago the true ideas in Optics was available.

Parviz

you rightly pointed out difference between interaction light with matter and wave propagation. Usually, when when there is interaction between quantum packets or secondary wave in classical terms wave propagation manifests. While interaction of the packets with matter can be better represented by light quantum i.e. photons.

Parviz has pointed out that human eye is also a photon detector like other light detectors. In that sense when we either photographed or view with eye we only detect photons. So propagation of light as waves is only a concept like probability waves!!

I´m following with interest. Some short remarks.

- Compton scattering is impossible for "normal" light because low photon energy.

- Propagation of light can well be described by propagation of wave packets, that means in time and space compact em packets.

- Photon absorption can sufficiently be understood by energy absorption , if you prefer, quantum phenomena. Absorption is absorption, that means complete energy transfer to an optical detector system,

- In our eyes there are the optical receptors.

- In these receptors absorbed energy is transferred to a chemical, physical, biological signal for our nerves and brain.

Dear Hanno

Finally do you agree that photo-detection is entirely based on photon(particle) concept of light and not due to its wave effect ?

Thats what I´ve learned and understood. My central point is the energy transfer to the mentioned signals in our receptors.

Dear Hanno,

Raman effect which is considered to be optical analogue of Compton effect can be observed with " normal light" having lower photon energy than X-rays.

Question is whether the energy is transferred as photon or wave?

Dear Rasbindu,

Raman effect is well known, the main difference to compton effect is the missing ionisation. You are right, raman effect, the inelastic scattering of light, transfers energy. So in principle, our receptors can get energy signals.

The question, which you repeat "wave or photon" is no matter of our receptors. It´s a general question for em physics. For me photons are compact em- wave packets, so I don´t have to decide your problem.

Dear Hanno

The performance of a precise optical detector is based on photon counting while we have no measure how to sense the wave .Photon is the building block of light that exhibits ultimate sensivity of a detector. To my knowledge, the optical detectors deal with photons. The eye receptors follow the same rule to detect the light photon by photon. The amount of energy (eV) that the atomic or molecular transitions can absorb lies in optical spectrum. Most of optical detectors are based on photon absorption.

However this question is open for new ideas and opinions.

Human optical receptors

"Does it mean that light is particle?"

Answer: Yes, but only briefly on the occasion of its birth or death (or at high intensities)

There is no way of detecting a single photon twice, so we cannot provide evidence that any single photon travels from A to B. The wave desription of light accurately describes the behaviour of light wherever there is no change of state or nonlinear interaction. It accurately describes propagation, even through multiple slits. Conversely the quantum description af light accurately describes the creation of photons and their detection, their birth and death, but nothing between except at high intensities where noninear effects can occur in dielectrics. So photons don't propagate, unless we choose to believe they do through faith alone, with zero evidence.

Light is energy that propagates as waves. The particle description is just something that makes us feel more comfortable about the quantum mechanics of light. Imagine a source of light so dim that it is equivalent to a single photon every thousand years: When is the particle?

Light propagates as wave and interact as photons. The detector counts the photons while it does not sense the wavelet nature of light. I would see no contradiction, if we assume photon propgation. This happens in laser cavity in which stimulated emmission deals with coherent photons in birth and during propagation .

When ever light interacts with matter it manifest its photon nature and whenever it interacts with itself it exhibits wave nature.

Light propagates as a wave and interacts as a photon. To me,  there is no contradiction  if we assume photon traveling in the medium too. For instance, the birth of coherent  photons by stimulated emission in laser cavity and the round- trip in the resonator are among the examples.

I am a chemist so I am not as good as the scholars specialized in physics. I learned from physicists that light has wave properties & particle properties. This dual wave-particle nature of light is shown in deBroglie equation: 

wavelength= Planck's constant/ (mass X velocity).  This relationship leads to an important conclusion: any moving object has a wavelength associated with it. When a person movies, there is a wavelength. It is detection requires an "unavailable" instrument , as far as I know. So the nature of light is an issue & its interaction followed by its detection is another issue. Am I right ?

A photon is an energy packet characteristic of its origin. The packet energy is electromagnetic with an electromagnetic frequency corresponding to the amount of energy. The packet interacts with matter or other photons in accordance to its energy/frequency. Interactions that occur without change in energy are called reflections, refractions, diffractions, penetrations, or scattering depending upon our point of view. Interactions that occur with change in energy are called adsorption or scattering. Names for adsorption and energy loss scattering differ depending on energy or point of view. None of the interactions require the photon to be a particle or a wave, but an energy packet with an electromagnetic frequency. Many descriptions of photon interactions require a continuous wave or are treated as a continuous wave. These wave-like descriptions depend upon a continuous stream of photons.

A single photon can be detected by many photo-detection methods. Since a single photon is what is emitted or absorbed by an atom and corresponds to quantum levels of the atom, we refer to the photon as a particle or wave packet. The human eye can detect a single photon. It can detect a few photons. It can detect many photons at once, but unless the photon abundance reaches a level we call bright, we see only black and white.

Color vision is the same type of detection as seeing black and white. A photon is absorbed and a sensation is produced. Color vision is different in that it is not a simple detection-sensation event, but a sensation event from multiple detections. The detection of light is simple physics. A photon is absorbed and a sensation is produced. Color vision is more complicated. There is an absorption reaction with a pigment, several pigment reactions are combined, and the combination is a color sensation.

Our usual point of view is the bright point of view. Our daily processes require a continuous stream of photons. It is easy to adopt the wave description of the process. Knowledge of the single photon process is easily incorporated into the bright point of view. Many of our daily processes require the bright point of view. When confronted with some processes like color vision, the process may be blurred even when it is bright.

Dear Joseph,

Your statement "The human eye can detect a single photon. It can detect a few photons. It can detect many photons at once, but unless the photon abundance reaches a level we call bright, we see only black and white" needs further elaboration. I think by 'detection' you mean converting an input into an output. While, usually it is confused with detectivity that is confirming presence or absence of a signal. In that sense it is subjective and unless an observer confirms the presence of single photon he cannot say that the photon is detected.

Dear Rasbindu,

interesting remarks. I think also , detection is not the same as interaction. You are right, to produce a measurable signal, you must exceed the threshold of your detector system. If you stay below this energetic defined threshold, you have interactions but no measurable signal. If detecting colors you have to absorb enough energy to start the pigment reactions. I understood Josephs statement exactly in this way, when he talked about color detection.

@Rasbindu

By  detect I mean mentally respond to the light signal.

Human subjects were placed in a dark room and subjected to small pulses of light. The subjects reported each time they perceived light. The conclusion of the experiment was that humans can see (detect) a single photon of light. Repeats of the experiment confirmed that humans can see less than 10 photons. A pulse of more than 10 photons is necessary for a human to see one photon. Light is lost in the passage through cornea, lens, eye fluid, and the covering of the retina. The conclusion that a single photon was detected (seen, reported) depends on knowing how many photons impinged upon the cornea and knowing (assuming, estimating) how many photons reached a photo-receptor in the cornea. 

Yes. By detect I mean see or the mental experience of the sensation of light.

As Hanno noted my statements referred to the difference between perceiving light and perceiving color. 

Dear Hanno and Joseph,

Thank you for your remarks.

Dear Joseph

Interesting remarks. However, any detector records a signal when signal to noise ratio SNR=1 or even more. It arises from this fact that most of detectors suffers an inherent noise level due to thermal effects leading to further generation of e-h in semiconductors. It is measured by  NEP that denotes noise equivalent power.

Here, eye performance may be different from silicon detectors. What about the eye SNR and NEP ?!

To my knowledge, if eye requires 10 photons to sense 1 photon, then this deals with efficiency different from SNR and NEP.

Dear Parviz Parvin,

I read an interesting study described  in the attached document. According to this the SNR is about 5

@Parviz

A detection of one photon means that a the detector responds to one photon. This is not the measure of efficiency. Replace the retina with a silicon detector that can detect a single photon and the requirement for 10 photons to sense 1 is the same.

Detection does not mean useful information. Useful information is derived from a signal that is easily distinguished from the noise. Rasbindu Parvin indicated that the Rose criteria for optics is for an SNR of 5. This is a minimum SNR. Most applications want an SNR greater than 10.

Vision requires many photons to form an image. Imaging goes well beyond detection. My remarks may have taken us astray from the original question of particle-wave duality. My intent was to show that detection (response) or change in state is a photon process. Imaging, especially color imaging requires bright conditions (a multitude of photons). Bright conditions allow description of the process as a wave.

Detection or a change in state recorded by the system is a photon (EM energy packet) process. Imaging and diffraction studies require multiple photon interactions that can allow description as waves. There is no particle-wave duality in the process. There is a particle-wave duality in the description.

Dear Rasbindu

SNR=1 gives out the NEP. Surely, we would like high SNR greater than unity. In the case of remote sensing SNR=1.5 is min value that was reoprted. I agree that SNR =5 for human eye and SNR=1.5 for very sensitive detectors such as PMT, MCT cooled etc.

Thanks for your remark.

Joseph

Again, any detector counts photons. I do not know any detector to measure the wave properties. If it is please let me know.