The photoelectric effect can be defined as an ejection or emission of electrons a metallic surface as a response to a light incident. Energy within an incident light is taken in by electrons inside the metallic surface. This provides the electrons with the necessary to be emitted from the metal’s surface. There have been many discoveries by many scientists such as Einstein and Maxwell who have identified different experiments of justifying the photoelectric effect. This experiment is the simplest and most convincing and direct proof of the photons existence as well as electromagnetic radiation and light’s ‘corpuscular’ nature. Therefore, this provides adequate evidence that electromagnetic fields can be quantified. Furthermore, it is a revelation of the limitations of Maxwell’s classical field equations.
In normal circumstances, light can be utilized in pushing electrons and setting them free from a solid’s surface. The process of electron emission is what is called photoelectric effect. Solid materials that can successfully exhibit the electron emission phenomena are called photo emissive materials. On the other hand, the emitted electrons are referred to as photoelectrons. Photoelectrons are identical to all other electrons. This is because the two types of electrons have the exactly the same spin, mass, magnetic moments and charge.
The first time the photoelectric effect was noticed was in1887. This was by a scientist, Heinrich Hertz when he was carrying out experiments using a spark-gap generator. During that period, spark-gap generators were the radio receivers of that age. Sparks were generated within small metal spheres in a transmitter. This will induce an identical spark that will skip between two different spheres of metals in the receiver. Spark-gap generators were extremely difficult to operate with compared to other radio devices that came up later on. They often had tiny air gaps, less than one millimeter in size. This made it difficult for the receiver to produce sparks consistently to the transmitter. Later on, Hertz discovered that the sensitivity of the spark-gap generator through further illumination using ultraviolet light or visible light. Illumination increased sensitivity as a result of electrons being pushed by light.
Maxwell’s theory of waves
In 1865, Maxwell came up with the theory of electromagnetism. The scientist suggested that there existed electromagnetic waves that move at the speed of light. At that time, it was thought that light itself was also a wave. Scientists, including Hertz carried out many experiments to prove whether this theory was right. The theory was proved right when Hertz successfully did an experiment that emitted electromagnetic radiations that had similar wavelength as the conductors of the radiation. In 1888, Hallwach devised a simpler approach of this experiment. He used a clean plate of zinc that was circular in shape. The plate was placed on an insulating stand then attached to a gold leaf electroscope by a wire then negatively charged. The observation was that the electroscope gradually lost its charge. On the other hand, if the ultraviolet light fell on the zinc plate from a lamp of arc or burning magnesium, the charge quickly leaked away. When a positively charged plate was used instead, the charge did not leak away fast.
Albert Einstein’s discovery
In 1905, another scientist, Albert Einstein made a discovery about whether the energy the electrons had when they were emitted was not related to the incident radiation’s intensity. He ended the paradox by a proposal which stated that incident light was composed of individual quanta referred to as photons. Photons interacted with electrons in metal like particles that were discreet instead of the continuous wave form. In all the frequencies of incident radiation, every photon had the energy that was a product of a constant and the frequency. In Einstein’s model, an increase in the intensity of light corresponded with an increase in the quantity of incident photons for every unit time. Mean while, the energy level of every photon was constant if the radiation’s frequency was maintained at the same level.
Based on Einstein’s theory, an increase in the intensity level of incident radiation causes greater quantities of electrons to be emitted. However, every electron carries same amount energy since every incident photon carries an identical energy level. All this is on the assumption that dominant processes involve individual photons being taken in and resulting in emission of one single electron. Furthermore, the Einstein’s model suggests that an increase in the frequency instead of the level of intensity of incident radiation increases the mean energy level of the ejected electrons. This was confirmed through experiments and the rate at which energy increase of the emitted electrons with an increase of frequency. This would be measured to enable the determination of the constant, Plack’s constant.
Process of the experiment
The freeing of electrons from a photo emissive plate gives the plate a slightly positive charge. The other plate is connected to the first plate by a wired circuit hence it will also have a positive charge. The positive charge attracts photoelectrons that float freely in vacuum. The electrons land onto the surface of the metal where they began. Usually, the experiment does not involve creating electrons from light; it actually involves using light energy to push electrons which exist in the circuit. The generated photoelectric current through this experiment is very minimal but it can be measured using a microameter. This serves as a way of measuring the rate of photo electrons leaving the surface of a photo emissive object.
The power supply is connected into the circuit in a manner where the negative tip is wired to the plate which is not illuminated. This causes a difference that pushes photoelectrons to the surface of the photo emissive object. If the supply of the power is low voltage, it holds little energetic electrons hence causing a reduction in the current passing through the microameter. An increase in the voltage increases the number of energetic electrons until none of them leaves the metallic surface. At this point, the reading on the microameter will be zero. This level is referred to as the stopping potential because it is a measure of the possible maximum kinetic electron energy ejected due to the photoelectric effect. The intensity of incident light has no impact on the maximum possible kinetic photoelectron energy. The electrons emitted from the exposing of extremely bright light have the same level of energy because those emitted from exposing to dim light at that same frequency level.
Factors of photo electronic energy
There are two factors that affect the maximum possible level of photoelectron energy. These are incident radiation frequency and the surface of the material. The electrons’ energy levels increase as the frequency increases in a linear manner. The relationship between the frequency and energy is constant in all types of materials. Just below the threshold, there is no chance that photoemission can occur. This is because the threshold frequency is dependent on the material. Normally, most elements have their threshold frequencies when using ultraviolet light.
The formula that was proposed by Albert Einstein relates maximum kinetic energy of photoelectrons to frequency levels of the threshold frequency and absorbed photons of a photo ejecting surface. Usually, it is convenient to measure the stopping potential rather than the current itself. Stopping potential is expected to reduce the current to zero and is related to the maximum possible kinetic energy of ejected electrons. Einstein’s explanation of photoelectric effect is built on Planck’s photon hypothesis. He assumed photons have equal energy as the energy difference of two adjacent blackbody levels. E=h.f, where f is the frequency while h is the Planck’s constant.
As these photons hit the metallic object, they give up all or a section of their energy to a single electron. An amount of energy would be needed to set free electrons from the bonds they are to the metals. The energy is referred to as a work function of the metal. The rest of the energy will be in kinetic form of the electron that is released. The most electrons with kinetic energy should contain energy when they leave the cathode. This equation can be summarized as eVstop = h.f-W.
Therefore, light can be used to eject electrons from a surface through a photoelectric effect. Energy within an incident light is taken in by electrons inside the metallic surface. This provides the electrons with the necessary to be emitted from the metal’s surface. This has been proven by many scientific experiments and theories. The electrons are emitted from the object by the use of light unlike the earlier suggestions where scientists thought light contained electrons which ended up on the surface of the surface on an object. The factors affecting photoelectric effects are incident radiation frequency and the surface of the material. The electrons’ energy levels increase as the frequency increases in a linear manner. The relationship between the frequency and energy is constant in all types of materials.
AMACOM Div American Mgmt Assn. Absolutely Small Chapter 4: The Photoelectric Effect and Einstein’s Explanation. New York: AMACOM Div American Mgmt Assn, 2011.
Kotz, John C, Paul M Treichel and John Townsend. Chemistry & Chemical Reactivity. 8. London: Cengage Learning, 2011.
Willett, Edward. The Basics Of Quantum Physics: Understanding The Photoelectric Effect And Line Spectra. illustrated. New York: The Rosen Publishing Group, 2004.