Guest

sir my teacher told me that wave structure of light cannot show photoelectric effect but why it can`t because for photoelectric effect energy must be transferred to electron so that it may rush out and that a wave can do because it has energy.

sir my teacher told me that wave structure of light cannot show photoelectric effect but why it can`t because for photoelectric effect energy must be transferred to electron so that it may rush out and that a wave can do because it has energy.

Grade:11

1 Answers

jyoti bhatia
202 Points
7 years ago
There are two main aspects of experimental results which cannot be explained by wave theory. If light is wave then its energy increases as one increases intensity of the wave but this increases only the number of electrons emitted but does not increase energy of the electron. Secondly, if electrons absorb energy of the wave, emission of electron must be delayed with respect to instance light falls on substance but it is immediate.
Photon theory of light is just not what one states and a ‘photon’ is simply a measure of energy of an electromagnetic wave with a certain wavelength or frequency.
In a wave model, electron has to oscillate as per oscillating electric field and gain enough energy and then get down.
In a photon model which is not just another particle model but is a quantum model emphasizing duality at fundamental level. According to it, light consists of particle each with energy ‘hf’. (Energy is particle aspect and frequency is wave aspect). This particle meets the electron and itself gets vanished, giving all its energy in one-shot process to the electron.
In the photoelectric effect, electrons are emitted from matter (metals & non-metallic solids, liquids or gases) as a consequence of their absorption of energy from electromagnetic radiation of high frequency (short wavelength) like ultraviolet radiation. Electrons emitted in this manner may be referred to as photoelectrons. This phenomenon was first observed by Heinrich Hertz in 1887.
Photoelectric effect is the propensity of high-energy electromagnetic radiation to eject electrons from a given material.
In photoelectric effect, electrons are emitted from matter (mainly metals and non-metallic solids) as a consequence of their absorption of energy from electromagnetic radiation of high frequency (short wavelength) like ultraviolet light).
When electromagnetic radiation interacts with an atom, it either excites electrons to a higher energy level known as an excited state or if energy of light is sufficiently high, it can ionize the atom by removing the electron.
For a given metal, there exists a certain minimum frequency of incident radiation below which no photoelectrons are emitted. This frequency is called the ‘threshold frequency’.
Work function: The minimum energy needed to remove an electron from surface of a material.
Stopping voltage: The voltage required to fully balance the kinetic energy of electrons ejected from a material’ surface.
In the photoelectric effect, electrons are emitted from matter (metals & non-metallic solids, liquids or gases) as            a consequence of their absorption of energy from electromagnetic radiation of high radiation. Electrons emitted in this manner may be referred to as photoelectrons.
Photoelectric Effect:
Electrons are emitted from matter by absorbed light.
The photoelectric effect has been demonstrated using light with energies from a few electron-volts (eV) to over 1 MeV in high atomic number elements. Study of photoelectric effect led to an improved understanding of quantum mechanics and an appreciation of wave-particle duality of light. It also led to Max Planck’s discovery of quanta (E=h), which links frequency with photon energy (E).
Planck’s constant, h, is also known as ‘quantum of action’. It is a subatomic-scale constant and is one of the smallest constants used in physics. Other phenomena where light affects movement of electric charges include photoconductive effect, photovoltaic effect and photo-electrochemical effect).
Emission Mechanism:
All atoms have their electrons in orbital’s with well-defined energy levels. When electromagnetic radiation interacts with an atom, it can excite the electron to a higher energy level, which can then fall back down, returning to ground state. However, if energy of light is such that electron is excited above energy above energy levels associated with the atom. Electron can break free from atom leading to ionization of the atom. This, in essence, is the photoelectric effect.
Photons of a beam of light have a characteristic energy proportional to frequency of the light. In photo-emission process, if an electron within some material absorbs the energy of one photon and acquires more energy than work function of material (electron binding energy), it is ejected. If photon energy is too low, electron is unable to escape the material. Increasing intensity of light increases number of photons in beam of light and thus increases number of electrons excited but does not increase the energy that each electron possesses. Energy of emitted electrons does not depend on intensity of incoming light (number of photons), only on energy or frequency of individual photons. It is strictly an interaction between incident photon and outermost electron.
Electrons can absorb energy from photons when irradiated but they follow an all-or-nothing principle. One photon is either energetic enough to cause emission of an electron or energy is lost as atom returns back to ground state. If excess photon energy is absorbed, some of the energy liberates the electron from the atom and rest contributes to electron’s kinetic energy as a free particle.
Experimental Observations of Photoelectric Emission:
For a given metal, there exists a certain minimum frequency of incident radiation below which no photoelectrons are emitted. This is called ‘threshold frequency’. Increasing the frequency of incident beam and keeping number of incident photons fixed (resulting in a proportionate increase in energy) increase maximum kinetic energy of photoelectrons emitted. The number of electrons emitted also changes because probability that each impacting photon results in an emitted electron is a function of photon energy. But, if just the intensity of incident radiation is increased, there is no effect on kinetic energies of photoelectrons.
For a given metal a are ejected is directly proportional to intensity of incident light and frequency of incident radiation, rate at which photoelectrons are ejected is directly proportional to intensity of incident light. An increase in intensity of incident beam (keeping frequency fixed) increases magnitude of photoelectric current, though stopping voltage remains the same. Time lag between incidence of radiation and emission of a photoelectron is very small, less than 109 second and is unaffected by intensity changes.
Mathematical Description:
Maximum kinetic energy of an ejected electron is given by:
K.E.max = hf - phi
where h is Planck constant (6.626 x 10-34 m2 kg/sec) and f is the  frequency of incident photon. The term is the work function, which gives minimum energy required to remove a delocalized electron from surface of the metal.
Work function satisfies phi=hf0
where  f0  is the threshold frequency for the metal. Maximum kinetic energy of an ejected electron is then
K.E.max=h(f-f0)
Kinetic energy must be positive for ejection to take place so one must have f > f0 for photoelectric effect to occur.
Photomultipliers:
Photomultipliers are extremely light-sensitive vacuum tubes with a photocathode coated onto part (an end or side) of the inside of envelope. Photocathode contains combinations of materials like caesium, rubidium and antimony, especially selected to provide a low work function, so that when illuminated by even very low levels of light, photocathode readily releases electrons. By means of a series of electrodes (dynodes) at ever-higher potentials, electrons are accelerated & substantially increased in number through secondary emission to provide a readily detectable output current. Photomultipliers are still commonly used wherever low levels of light must be detected.
Classically, one cannot explain ‘threshold frequency’. In classical case, energy of emitted electrons would depend on intensity of light, which is not the case in experiments.
The true explanation of the photoelectric effect does not involve ‘photons’, in fact quantum calculations show that light is behaving as a classical wave during this effect. Treating the atom quantum dynamically, one can derive existence of resonant frequencies which are responsible for ‘thresholds’ which characterize this effect. Ironically, Einstein’s Nobel prize winning work was bogus, the photoelectric effect does not require photons to be explained. Beginning in the 1980’s experiments done by Aspect et al detected photons for first (non-bogus) time, for which the team won a Nobel prize.
Photoelectric effect posed a significant challenge to study of optics in the 1800s. it challenged the ‘classical wave theory of light’, which was prevailing theory of the time. It was the solution to this physics dilemma that catapulted Einstein into prominence in physics community, earning him the 1921 Nobel Prize.
Though originally observed in 1839, photoelectric effect was documented by Heinrich Hertz in 1887. It was originally called the Hertz effect.
When a light source (or more generally electromagnetic radiation) is incident upon a metallic surface, the surface can emit electrons. Electrons emitted in this fashion are called ‘photoelectrons’.
To observe the photoelectric effect, one should create a vacuum toward the collector. This creates a current in wires connecting the two ends, which can be measured with an ammeter. By administering a negative voltage potential to collector, it takes more energy for electrons to complete the journey and initiate the current.
Point at which no electrons make it to the collector is called ‘stopping potential’ Vs and can be used to determine maximum kinetic energy Kmax of electrons (which have electron charge e) by using following equation:
 
 Kmax = eVs
 
Not all of the electrons will have this energy but will be emitted with a range of energies based on properties of the metal being used. Above equation allows us to calculate maximum kinetic energy or energy of particles knocked free of the metal surface with greatest speed, which will be the trait that is most useful in this analysis.
Classical Wave Explanation:
In classical wave theory, energy of electromagnetic radiation is carried within the wave itself. As electromagnetic wave (of intensity l) from wave until it exceeds binding energy, releasing the electron from metal. Minimum energy needed to remove the electron is work function phi of the material. Three main predictions come from this classical explanation:
Intensity of radiation should have a proportional relationship with resulting maximum kinetic energy.
Photoelectric effect should occur for nay light, regardless of frequency or wavelength.
There should be a delay on the order of seconds between radiation’s contact with the metal and initial release of photoelectrons.
Result:
By 1902, properties of the photoelectric effect were well documented. Experiment showed that:
1. Intensity of the light source had no effect on maximum kinetic energy of photoelectrons.
2. Below a certain frequency, photoelectric effect does not occur at all.
3. There is no significant delay (less than 10^-9 seconds) between light source activation and emission of the first photoelectrons.
These three results are exact opposite of wave theory predictions. Not only that but all three are completely counter-intuitive. 
 

Think You Can Provide A Better Answer ?

ASK QUESTION

Get your questions answered by the expert for free