# When hydrogen's energy level is changed I don't know where photon has gone

Before answering my question, I am student of KSA of KAIST (Korea Science Academy). Therefore I have little bit wrong conception or supposition.

If we want to change only one hydrogen's energy level, we must give a photon that has the correct energy. (For example, if we want to change hydrogen's energy level 1 to 2, we must provide a photon that has 10.2eV energy). Then the photon gives 10.2eV energy to the hydrogen atom. Then my teacher says it is the hydrogen that absorbs the photon. But we know a particle isn't made of empty space and it does not disappear. Therefore I think the photon gives energy to the hydrogen atom and the photon looses little energy, but it doesn't make sense. First, if photon looses little energy then some photon's energy will change from 10.2eV to 9.2eV for example. However, photon's energy is determined by frequency. But I know the light's frequency is never changed. Therefore I think this hypothesis is wrong. Second, when one hydrogen atom absorbs energy of a photon then the hydrogen must absorb correctly 10.2eV. Therefore the hydrogen absorbs energy by a lot of photons, it is not happening in one time. Therefore I think my hypothesis is wrong. But I don't have any other hypothesis of this problem. Please give me correct answer and if I had wrong conception, please correct it. (Please write answer in photon's position.) Thank you.

• Sometimes the water is calm and still but sometimes there is a wave. A photon is a wave in the EM field .... electrons put energy in the EM field and electrons take energy out of the field .... we never see the EM field directly ..... only electrons see the EM field. Commented Feb 24, 2023 at 16:25
• The photon is not a particle for your question. In the "standard model" which tries to explain everything there are only fields and excitations of fields ..... the latter being considered particles. The standard model does not assign mass or volume to the photon ..... the photon is localized energy. Commented Feb 25, 2023 at 4:45

A photon and its absorption is a quantum effect. Although propagation of light is wave-like, the interaction with matter, i.e. in particular emission and absorption the energy of light is quantized.

If energy gap $$E_2-E_1$$ between 2 energy levels of the hydrogen's electron is exactly matched by the photon energy $$E_{photon}$$ (with $$h$$ standing for Planck's constant, $$f_{photon}$$ the frequency of the photon)

$$E_2 -E_1 = E_{photon} = hf_{photon}$$

the photon is completely absorbed. Neither partial energy delivery is possible, nor the involvement of many photons (well theoretically it could happen, but their probability (cross section) to happen is very very low.) This is the same mechanism as the photo effect described by A. Einstein. Absorption is a very common process: In a room the light is switched off and it gets immediately dark. All visible light photons in the room are absorbed on the spot by the walls.

You are confusing photons with light. Photons are elementary particles with zero mass and have energy $$hν$$ where $$h$$ is plancks constant and $$ν$$ the frequency of the classical electromagnetic radiation that many photons build up. See how this happens in my answer here,.

As an elementary particle the photon can interact with the hydrogen atom, in various ways that can be studied. If its energy is within the width of the energy level in the hydrogen atom , it can be completely absorbed. Othewise part of its energy is taken up in the interaction and a lower energy photon goes away. This needs relativistic algebra of four vectors.

The photon is not a particle for your question. In the "standard model" which tries to explain everything there are only fields and excitations of fields ..... the latter being considered particles. The standard model does not assign mass or volume to the photon ..... the photon is localized energy.

A photon is an oscillating electromagnetic field with a small amplitude. When the photon comes to the atom the electric field of the photon causes an oscillation of the electron. It increases the energy of the electron. But, at the same time, the amplitude of the electric field decreases so that the electromagnetic field has less energy.

This is a classical description of the absorption of light by a non-linear oscillator. To understand why the energy levels are quantized you need an understanding of quantum mechanics.

We can say the photons energy now lies in the increased sum of the kinetic and potential energy of the electron. But if the photon is a particle where it has disappeared to is for me a very good question. Is it clinging onto the electron waiting to be released when the hydrogen goes back to its ground state.

The electron consists of a magnetic field, an electric field and a centre, of which we know nothing more than that it is point-like. The existence of the intrinsic magnetic field is often overlooked.

Photons are packets of energy that remain unchanging between the emission of excited subatomic particles and absorption. Photons consist of a magnetic field component and an electric field component (and both components oscillate transverse to the direction of travel).

The absorption of a photon by an electron is always temporary because the electron is then an excited particle and sooner or later tries to assume a lower energy state by emitting one or more photons. In addition, a photon that has more energy than the - only gradually excitable - level change of the electron is re-emitted as photon(s) of lower energy.

Interesting may be the view that the emission of a photon (with its field components) is fed from the electric and magnetic field of the electron. Or, in the case of absorption, the field strengths of the electron are increased again. At least this view is more concrete than the mere statement of an energy transfer.