What is the relation between electromagnetic wave and photon? At the end of this nice video (https://youtu.be/XiHVe8U5PhU?t=10m27s), she says that electromagnetic wave is a chain reaction of electric and magnetic fields creating each other so the chain of wave moves forward.
I wonder where the photon is in this explanation. What is the relation between electromagnetic wave and photon?
 A: You report that in the video it is stated that an electromagnetic wave is "a chain reaction of electric and magnetic fields creating each other so the chain of wave moves forward."
I disagree with this view. There is just one wave, that of the vector potential or more generally of the four potential. The electric and magnetic fields are just derivatives of the vector potential and do not "create each other".
Rejecting this explanation we then arrive at your deeper question: "What is the relation between electromagnetic wave and photon?"
Until a few years ago I shared the opinion of Willis Lamb, that the photon is a fictive particle. I finally changed my mind because such an explanation cannot account for low intensity diffraction experiments. Indeed, how can a single atom or molecule absorb a wave that is much larger that it? Note that I don't intend to fork off a discussion on this here but want to give my interpretation. This is that the vector potential describes the probability of a photon being absorbed, just like the Schrödinger and Dirac wave functions do for an electron. Indeed the Maxwell equations in vacuum can be written as a wave equation that closely resembles the Klein-Gordon equation. This interpretation implies that the photon indeed exists as a particle, much smaller than an atom and at least as small as a nucleon. 
A: In this link there exists a mathematical explanation of how an ensemble of photons of frequency $\nu$ and energy $E=h\nu$ end up building coherently the classical electromagnetic wave of frequency $\nu$.
It is not simple to follow if one does not have the mathematical background. Conceptually watching the build up of interference fringes from single photons in a two slit experiment might give you an intuition of how even though light is composed of individual elementary particles, photons, the classical wave pattern emerges when the ensemble becomes large.


Figure 1. Single-photon camera recording of photons from a double slit illuminated by very weak laser light. Left to right: single frame, superposition of 200, 1’000, and 500’000 frames.

A: In order to understand the wave particle dualism you have simply to understand what time is:
In 1905, the Newtonian unique time concept was replaced by a twofold time concept of observed coordinate time and proper time - the observed time is relative and observer-dependent, and it is derived from the intrinsic proper time of the observed particle ("The time measured by a clock following a given object"). Proper time is the more fundamental time concept.
You can understand the wave particle dualism if you consider the simplest case of a photon, that is a photon moving at light speed c. The spacetime interval of such photons (which corresponds to their proper time) is zero. That means that the event of emission and the event of absorption are adjacent in spacetime, the emitting mass particle is transmitting the momentum which is called photon directly to the absorbing mass particle, without any spacetime between them. That means that the particle characteristics are transmitted directly without need for any intermediate massless particle.
However, for observers the zero spacetime interval is not observable, e.g. between Sun and Earth are observed to be eight light minutes, even if the spacetime interval of the path of the photon is zero. In spite of the direct transmission of a momentum between two mass particles, observers observe an electromagnetic wave which is filling the gap of eight light minutes.
In summary, particle characteristics are transmitted directly according to the principles of spacetime intervals and proper time, whereas the wave is transmitted according the principles of the observed spacetime manifold.
Now you will ask: What about photons which are moving slower than c (through gravity fields and through transparent media)? The answer is that here quantum effects such as nonlocality are implied. But it is important to notice that the limit case of photons in vacuum moving at c may be explained and understood classically, without need for any quantum theory.
A: In 1995 Willis Lamb published a provocative article with the title "Anti-photon", Appl. Phys. B 60, 77-84 (1995). As Lamb was one of the great pioneers of 20th century physics it is not easy to dismiss him as an old crank. 
He writes in the introductory paragraph: 

The photon concepts as used by a high percentage of the laser community have no scientific justification. It is now about thirty-five years after the making of the first laser. The sooner an appropriate reformulation of our educational processes can be made, the better.

He finishes with these comments:

There is a lot to talk about the wave-particle duality in discussion of quantum mechanics. This may be necessary for those who are unwilling or unable to acquire an understanding of the theory. However, this concept is even more pointlessly introduced in discussions of problems in the quantum theory or radiation. Here the normal mode waves of a purely classical electrodynamics appear, and for each normal mode there is an equivalent pseudosimple harmonic-oscillator particle which may then have a wave function whose argument is the corresponding normal-mode amplitude. Note that the particle is not a photon. One might rather think of a multiplicity of two distinct wave concepts and a particle concept for each normal mode of the radiation field. However, such concepts are really not useful or appropriate. The "Complementarity Principle" and the notion of wave-particle duality were introduced by N. Bohr in 1927. They reflect the fact that he mostly dealt with theoretical and philosophical concepts, and left the detailed work to postdoctoral assistants. It is very likely that Bohr never, by himself, made a significant quantum-mechanical calculation after the formulation of quantum mechanics in 1925-1926. It is high time to give up the use of the word "photon", and of a bad concept which will shortly be a century old. Radiation does not consist of particles, and the classical, i.e., non-quantum, limit of QTR is described by Maxwell's equations for the electromagnetic fields, which do not involve particles. Talking about radiation in terms of particles is like using such ubiquitous phrases as "You know" or "I mean" which are very much to be heard in some cultures. For a friend of Charlie Brown, it might serve as a kind of security blanket.

A: The photon dilemma
It is postulated by Planck that energy is quantized. Owing to classical electromagnetic theory  light is an electromagnetic field. This field satisfies a wave equation traveling at the speed of light. Hence, light is an electromagnetic wave. Light consists of photons; and thus each photon carries a unit of energy. This behavior is demonstrated by the photoelectric and Compton effects.  Since light is an electromagnetic energy, photons must also carry electromagnetic field and a unit of it. While photons are quantum objects, light still governed by Maxwell's classical theory. The photon model is not critically consistent with Maxwell equations, since it has a dual nature. In fact light as a wave is well described by Maxwell. Recall that Maxwell's equations don't involve the Planck's constant, and thus can not describe the particle nature of the photon. A complete Maxwell's equations should involve this missing element. In quantum electrodynamic paradigm, the photon is brought to interact with the electrons by invoking the idea of minimal coupling where  electrons and photons exchange momentum. The photon appears as a mediator between charged particles.
At the same time while a moving charged particle has its self electric field, and magnetic field  that depend on the particle velocity, the photon, the carrier of the electromagnetic energy is void of these self-fields because it has no charge and mass. Thus, a charge-less photon can't have electric and  magnetic fields accompanying its motion.
The appropriate Maxwell's equations should then incorporate the photon linear momentum as well as its angular momentum. In such a case the new Maxwell's equations can then describe the dual nature of the photon. Like electric charge, the angular momentum is generally a conserved quantity. The question is how one can correct for these photon proprieties? One way to achieve that is to employ quaternions that generically allow many physical properties to be joined in a single equation. This is so because the quaternion algebra is so rich, unlike the ordinary real numbers.
To this end we employ the position-momentum commutator bracket, and invoked a photon wavefunction. This wavefunction is constructed from the linear complex combination of the electric and magnetic fields.
The outcome of the bracket yields three equations defining the photon electric and magnetic fields in terms of its angular momentum. These equations turn out to be very similar to those fields created by a moving charge. Thus, the electric and magnetic fields of the photon doesn't' require a charge for the photon. It is intriguing that the photon has no charge and mass but has electric and magnetic fields as well as energy. These fields should also satisfy Maxwell's equations. Doing so, yields additional electric and magnetic charge and current densities for the photon. The emergent Maxwell's equations are now appropriate to describe the photon as a quantum particle. These additional terms in Maxwell's equations are the source in describing the photon quantum electrodynamics behavior. Some emergent phenomena associated with topological insulator, Faraday's rotation effect, Hall effect and Kerr's effect could be examples of this contribution terms to Maxwell's equations.
Here are the quantized Maxwell's equations incorporating the photon linear and angular momentum. These are the electric and magnetic fields due to the photon as a particle:
\begin{equation}
\vec{L}\cdot\vec{E}=-\frac{3\hbar c}{2}\,\Lambda\,, \qquad\qquad \vec{L}\cdot\vec{B}=0\,,
\end{equation}
and
\begin{equation}
\vec{B}=-\frac{2}{3\hbar c}\,(\vec{L}\times\vec{E})\,,\qquad\qquad\vec{E}=\frac{2 c}{3\hbar}(-\Lambda\,\vec{L}+\vec{L}\times\vec{B})\,.
\end{equation}
And these are the new Maxwell's equations:
\begin{equation}
\vec{\nabla}\cdot\vec{E}=-\frac{4c}{3\hbar}\,\,(\vec{B}-\frac{1}{2}\,\mu_0\vec{r}\times\vec{J})\cdot\vec{p}+\frac{2}{3\hbar c}\,\vec{E}\cdot\vec{\tau}+\frac{\partial \Lambda}{\partial t}\,,\qquad
\vec{\nabla}\cdot\vec{B}=\frac{4}{3\hbar c}\,\,\vec{E}\cdot\vec{p}+\frac{2}{3\hbar c}\,\vec{B}\cdot\vec{\tau}\,,
\end{equation}
and
\begin{equation}
\vec{\nabla}\times\vec{B}=\frac{1}{c^2}\,\frac{\partial\vec{E}}{\partial t}+\frac{2}{3\hbar c}\left(\Lambda\vec{\tau}+\vec{B}\times\vec{\tau }-\frac{\vec{P}}{\varepsilon_0}\times\vec{p}\right)-\vec{\nabla}\Lambda\,,
\end{equation}
\begin{equation}
\vec{\nabla}\times\vec{E}=-\frac{\partial\vec{B}}{\partial t}-\frac{2c}{3\hbar}\left(\mu_0\vec{J}\times\vec{L}+\frac{\vec{\tau}}{c^2}\times\vec{E}+2\Lambda\,\vec{p}\right)\,,
\end{equation}
where
\begin{equation}
-\Lambda=\frac{1}{c^2}\,\frac{\partial\varphi}{\partial t}+\vec{\nabla}\cdot\vec{A}=\partial_\mu A^\mu\,.
\end{equation}
In the standard electrodynamics $\Lambda=0$ represents the  Lorenz gauge condition.
A: Both the wave theory of light and the particle theory of light are approximations to a deeper theory called Quantum Electrodynamics (QED for short). Light is not a wave nor a particle but instead it is an excitation in a quantum field.
QED is a complicated theory, so while it is possible to do calculations directly in QED we often find it simpler to use an approximation. The wave theory of light is often a good approximation when we are looking at how light propagates, and the particle theory of light is often a good approximation when we are looking at how light interacts i.e. exchanges energy with something else.
So it isn't really possible to answer the question where the photon is in this explanation. In general if you're looking at a system, like the one in the video, where the wave theory is a good description of light you'll find the photon theory to be a poor description of light, and vice versa. The two ways of looking at light are complementary.
For example if you look at the experiment described in Anna's answer (which is one of the seminal experiments in understanding diffraction!) the wave theory gives us a good description of how the light travels through the Young's slits and creates the interference pattern, but it cannot describe how the light interacts with the photomultiplier used to record the image. By contrast the photon theory gives us a good explanation of how the light interacts with the photomultiplier but cannot describe how it travelled through the slits and formed the diffraction pattern.
A: What are photons?
Photons get emitted every time when a body has a temperature higher 0 Kelvin (the absolute zero temperature). All bodies, surrounding us (except black holes) at any time radiate. They emit radiation into the surrounding as well as they receive radiation from the surrounding. Max Planck was the physicist who found out that this radiation has to be emitted in small portions, later called quanta and even later called photons. Making some changes in the imagination of how electrons are distributed around the nucleus, it was concluded that electrons get disturbed by incoming photons, by this way gain energy and give back this energy by the emission of photons. And photons not only get emitted from electrons. The nucleus, if well disturbed, emits photons too. Such radiations are called X-rays and gamma rays.
What is electromagnetic radiation?
EM radiation is the sum of all emitted photons from the involved electrons, protons and neutrons of a body. All bodies emit infrared radiation; beginning with approx. 500°C they emit visible light, first glowing in red and then shining brighter and brighter. There are some methods to stimulate the emission of EM radiation. It was found out that beside the re-emission of photons there is a second possibility to generate EM radiation. Every time, an electron is accelerated, it emits photons. This explanation helps to understand what happens in the glow filament of an electric bulb. The electrons at the filament are not moving straight forwards, they bump together and running zig-zag. By this accelerations they lose energy and this energy is emitted as photons. Most of this photons are infrared photons, and some of this photons are in the range of the visible light. In a fluorescent tube the electrons get accelerated with higher energy and they emit ultraviolet photons (which get converted into visible light by the fluorescent coating of the glass). Higher energy (with higher velocity) electrons reach the nucleus and the nucleus emits X-rays. As long as the introduced energy is a continuous flow, not one is able to measure an oscillation of EM radiation.
What are EM waves?
Using a wave generator it is possible to create oscillating EM radiation. Such radiations are called radio waves. It was found out that a modified LC circuit in unit with a wave generator is able to radiate and that it’s possible to filter out such a modulated radiation (of a certain frequency) from the surrounding noisy EM radiation.

So the wave generator has a double function. The generator has to accelerate forward and backward the electrons inside the antenna rod and by this the photons of the radio wave get emitted, and the generator makes it possible to modulate this EM radiation with a carrier frequency. It has to be underlined that the frequency of the emitted photons are in the IR range and sometime in the X-ray range. There is an optimal ratio between the length of the antenna rod and the frequency of the wave generator. But of course one can change the length of the rod or one can change the frequency of generator. This changes the efficiency of the radiation to the needed energy input only. To conclude from the length of the antenna rod to the wavelength of the emitted photons is nonsense.
What is the wave characteristic of the photon?
Since the electrons in an antenna rod are accelerated more or less at the same time, they emit photons simultaneous. The EM radiation of an antenna is measurable and it was found out that the nearfield of an antenna has two components, an electric field component and a magnetic field component. This two components get converted in each other, the induce each other. At some moment the transmitting energy is in the electric field component and otherwise the energy is in the magnetic field component. So why not conclude from the overall picture to the nature of the involved photons? They are the constituents which make the radio wave.

A: The reason Maxwell theory cannot describe the photon is that the radiation
phenomena is mutual energy phenomena.
It is not a self-energy phenomena.
traditional solutions of electromagnetic fields make mistakes here. Mutual energy phenomena includes, mutual energy theorem, mutual energy flow flow theorem, mutual energy principle. All this relates mutual inductance. Self-energy phenomena includes self-energy flow (Poynting vector energy flow), self-energy principle(self-energy flow do not carry energy). All this relates to the self-inductance.
Energy conservation law

How to describe mutual energy radiation phenomena? Maxwell equations
4 formula should be add another formula which is the energy conservation
law. Assume there are $N$ current sources:$\boldsymbol{J}_{i}$,
$i=1,...N$. The corresponding fields are $\xi_{i}=[\boldsymbol{E}_{i},\boldsymbol{H}_{i}]$,
One current $\boldsymbol{J}_{i}$ will offer another current $\boldsymbol{J}_{j}$
some power,
\begin{equation}
P_{ij}=\iiint_{V}(\boldsymbol{J}_{j}\cdot\boldsymbol{E}_{i})dV\label{eq:1}
\end{equation}
The above is the power current $\boldsymbol{J}_{i}$ lost. This power
is received by the current $\boldsymbol{J}_{j}$.
\begin{equation}
P_{ji}=\iiint_{V}(\boldsymbol{J}_{i}\cdot\boldsymbol{E}_{j})dV\label{eq:2}
\end{equation}
is the power current $\boldsymbol{J}_{j}$ give to the current $\boldsymbol{J}_{i}$.
When $\boldsymbol{J}_{i}$ lost some energy this energy, it will be
received by current $\boldsymbol{J}_{j}$. Hence, the total energy
will not change, that means,
\begin{equation}
\intop_{t=-\infty}^{\infty}(P_{ji}+P_{ji})dt=0\label{eq:3}
\end{equation}
Consider all $N$ current sources, there is,
\begin{equation}
\sum_{i=1}^{N}\sum_{j=1,j\neq i}^{N}\intop_{t=-\infty}^{\infty}dt\iiint_{V}(\boldsymbol{J}_{j}\cdot\boldsymbol{E}_{i})dV=0\label{eq:4}
\end{equation}
This formula is self-explanatory (introduced by Shuang-ren Zhao).
It should add to the Maxwell equations.
Mutual energy principle
Another formula which should
also add to Maxwell equation which is the mutual energy principle
(introduced by Shuang-ren Zhao)
\begin{equation}
-\sum_{i=1}^{N}\sum_{j=1,j\neq i}^{N}\iint_{\Gamma}(\boldsymbol{E}_{i}\times\boldsymbol{H}_{j})\cdot\hat{n}d\Gamma=\sum_{i=1}^{N}\sum_{J=1,j\neq i}^{N}\iiint_{V}(\boldsymbol{J}_{i}\cdot\boldsymbol{E}_{j}+\frac{\partial}{\partial t}(\boldsymbol{E}_{i}\cdot\boldsymbol{D}_{j}+\boldsymbol{H}_{i}\cdot\boldsymbol{B}_{j}))dV\label{eq:5}
\end{equation}
The mutual energy principle can be derived from Maxwell equations
by adding some conditions. The conditions are the Maxwell equation
must established as pairs. In each pair there are solution for transmitting
antenna and receiving antenna. Or pair for emitter and absorber. This
means assume the receiving antenna and absorber also radiate waves.
This also means assume the radiation is a mutual energy phenomena,
that receiving antenna and absorber must also join to the radiation
theory.
$R$ is the set of the solution of retarded wave. $A$ is the set of the solution of the advanced wave. $R\cup A$ is the set of the solutions of Maxwell's equations. $R\cap A$ is the set of the solution the mutual energy principle. $R\cap A$ is the solutions of physics. $R\cap A$ is possible the solution of physics, but is also possible a invalid solutions.

We also can build the electromagnetic field theory by adding the above
mutual energy principle formula, then the above descriptions can be
derived from the mutual energy principle.
Assume both energy conservation law and the mutual energy principle
are accept by us as two new axioms. From these two laws we can prove
that,
\begin{equation}
\sum_{i=1}^{N}\sum_{j=1,j\neq i}^{N}\intop_{t=-\infty}^{\infty}dt\iint_{\Gamma}(\boldsymbol{E}_{i}\times\boldsymbol{H}_{j})\cdot\hat{n}d\Gamma=0\label{eq:6}
\end{equation}
$\Gamma$ is the boundary of the volume $V$. It can be chosen as
big sphere with radius as infinity. This means there should no mutual
energy flow go to the outside of our universe. This is clear a correct
theorem. In order the above formula as 0, the two electromagnetic
fields $\xi_{i}=[\boldsymbol{E}_{i},\boldsymbol{H}_{i}]$ and $\xi_{j}=[\boldsymbol{E}_{j},\boldsymbol{H}_{j}]$
must be one is retarded wave and another is advanced wave. The retarded
wave reaches the surface at a future time. The advanced wave reaches
the surface at a past time. The electromagnetic fields will not nonzero
in the same time in the surface, hence the surface integral will be
0.
We can assume the current sends the retarded wave as transmitting
antenna or emitter. The current sends advanced wave as receiving antenna
or absorber. Hence, transmitting antenna and emitter must radiate
the retarded wave. The receiving antenna and the absorber must radiate
advanced wave.
about the advanced wave, Wheeler and Feynman have the absorber theory.
John Cramer has the transactional interpretation of the quantum mechanics.
The mutual energy flow theorem, photon is the mutual energy flow
From the mutual energy principle the mutual energy flow theorem can
be derived. Assume $N=2$, the mutual energy flow theorem is,
\begin{equation}
-\intop_{t=-\infty}^{\infty}dt\iiint_{V_{1}}(\boldsymbol{J}_{1}\cdot\boldsymbol{E}_{2})dV=(\xi_{1},\xi_{2})=\intop_{t=-\infty}^{\infty}dt\iiint_{V_{2}}(\boldsymbol{J}_{2}\cdot\boldsymbol{E}_{1})dV\label{eq:7}
\end{equation}
The flowing is the mutual energy flow:
\begin{equation}
(\xi_{1},\xi_{2})=\intop_{t=-\infty}^{\infty}dt\iint_{\Gamma}(\boldsymbol{E}_{1}\times\boldsymbol{H}_{2}+\boldsymbol{E}_{2}\times\boldsymbol{H}_{1})\cdot\hat{n}d\Gamma\label{eq:8}
\end{equation}
$\Gamma$ is any surface which separates the volume $V_{1}$ and $V_{2}$. See the following figure for the shape of the mutual energy flow.

$\iint_{\Gamma}(\boldsymbol{E}_{1}\times\boldsymbol{H}_{2}+\boldsymbol{E}_{2}\times\boldsymbol{H}_{1})\cdot\hat{n}d\Gamma$ is the mutual energy flow. The mutual energy flow is defined in contrast
to the self-energy flow: $\iint_{\Gamma}(\boldsymbol{E}_{1}\times\boldsymbol{H}_{1})\cdot\hat{n}d\Gamma$
$\iint_{\Gamma}(\boldsymbol{E}_{2}\times\boldsymbol{H}_{2})\cdot\hat{n}d\Gamma$.
$(\xi_{1},\xi_{2})$ is the mutual energy go through the surface $\Gamma$
through the mutual energy flow. Mutual energy flow do not decrease
like wave. The amplitude of the wave will decrease when it propagates.
The mutual energy flow will not decrease when it propagates. The mutual
energy flow is very thin when it is radiated or received. The mutual
energy will be thick between it its source and sink. Hence, the mutual
energy flow looks very like a photon. We can say the photon actually
is the mutual energy flow.
According these theory, the self-energy flow or self-energy radiation
do not transfer energy in space. Self-energy flow is the normal wave
solution of Maxwell equations (the Maxwell equation only for one current
source). This wave are canceled by time-reversal waves. there are
two kinds of time-reversal waves corresponding to retarded wave and
the advanced wave.
The following is a figure of photon. The emitter sends the retarded wave, the absorber sends the advanced wave. The retarded wave and the advanced wave either are reactive wave or they collapse back. The mutual energy flow bring the photon energy from the emitter to the absorber.

Wave collapse
In quantum mechanics waves collapse, this can be shown as,

In the mutual energy theory, the wave collapse actually is done by a wave backward-collapse process and a mutual energy flow process:

Summary, (1) photon is not a wave, but it is the mutual energy flow.
The mutual energy flow is built by the retarded wave sends from an
emitter and the advanced wave sends from an absorber. (2) there are
4 waves, retarded wave, advanced wave and two time-reversal waves.
All 4 waves cancel each other. However, the mutual energy flow survive.
(3) Wave collapse can be described by the two phenomena: The energy
is transferred through the mutual energy. The retarded wave and the
advanced wave is canceled by the time-reversal waves.
If this theory is interesting, the details can be google searched
by the keyword mutual energy principle'' or mutual energy flow'',
``self-energy principle''.
A: 
electromagnetic wave is a chain reaction of electric and magnetic fields creating each other so the chain of wave moves forward.

It's clear this doesn't have to be true. @my2cts described that. Classical theory says that an accelerated charge creates a field that takes effect at greater and greater distances, moving at lightspeed. If the charge isn't moving then you get a plain electric field whose effect moves at lightspeed. If it's accelerated then the result of that does the same.
We don't have to assume that the magnetic field moving at lightspeed creates an electric field, which creates the magnetic field. They're both created by the original charge.
Still, there might be uses for the idea. We can explain stuff without it, but it still might have some use.

I wonder where the photon is in this explanation. What is the relation between electromagnetic wave and photon?

The photon is a different concept.
When they invented radiation, they didn't know about atoms. So it made sense to think of electric charge as a fluid, like fluid water or fluid heat. It might have any shape, and as it flowed it might have currents or eddies that flowed at different rates.
But then they found out about atoms and electrons. Matter is quantized. Charge is quantized. It was easy to apply the old theory, though. Instead of charge as a fluid, they could think of it as a sum of a finite number of point charges.
One of the reasons to give up classical physics was that it couldn't explain everything. It couldn't explain why electrons didn't fall into the atomic nucleus. It was replaced by quantum mechanics which doesn't explain anything, but only describes what happens. If we apply classical physics without explaining why electrons don't fall, we get the following:
Electrons sit around, not having to move. But they are continually moved by electric and magnetic fields. When an electromagnetic wave comes through, it makes electrons oscillate, it pushes them sideways back and forth. Usually when that happens, they get oscillated and doing that, they radiate. So for a transparent material, the electrons absorb light and re-radiate it, and the result is the light travels slower through that material.
But if the light is the right frequency, an electron can absorb it and change its state. It moves to a higher-energy stable state, and it absorbs just the right amount of energy to move to that state. Later something can destabilize it, and then it will radiate just precisely that same amount of energy to return to its previous state.
If the electron absorbs enough energy to get knocked out of its atom completely, it doesn't need precisely the right frequency. It just needs enough to knock it away, and the more energy it absorbs the faster it travels. But light with too low frequency won't knock it out, it just gets re-radiated.
If an electron is in its quasi-stable high energy state and it gets radiation that's the right frequency, it gets destabilized and emits a quantum of light. Does it have to absorb precisely one quantum of light and then radiate away two quanta? Probably not.
So we can describe a whole lot without requiring all light to be photons. Atoms are quantized, and sometimes they absorb a quantum of light or emit a quantum of light. The light doesn't need to be quantized then it's traveling through space. An atom which resonates to a weak electric field can absorb light without having to get the same photon that some other atom emitted. How long it takes for it to absorb enough for a visible result, depends on how much it had before.
These descriptions make sense, but they are not statistical and we can't measure real atoms precisely enough to tell whether they precisely fit individual cases. That makes them inferior to quantum methods which precisely fit statistical distributions of atoms.
Classical physics didn't know about some things, like electron spin. Would it fit better if it was revised to include those things? That would take work that is not needed because quantum methods work, and there's no reason to think that neoclassical methods would work any better.
Photon theory has various paradoxical elements. The double-slit experiment. Bell's Theorem. Etc. If we were thinking in terms of classical theory, these would say we should discard it. But quantum theory assumes photons, and quantum theory makes predictions which hold up. Therefore quantum theory is right. Therefore light is always quantized and is made of individual photons, and electromagnetic fields are an approximation which averages out over many photons. If photon theory gives us parodoxical results, the paradox must be in reality and not in photon theory.
Do you believe that?
