Table of Contents
Photoelectric Effect Definition
The photoelectric effect is referred to as a phenomenon in which electrons are ejected from the surface of a metal when light is incident on it. The electrons ejected are termed as photoelectrons. The emission of photoelectrons and the kinetic energy of the ejected photoelectrons is generally dependent on the frequency of the light that falls on the metal’s surface. Photoemission is referred to the process through which photoelectrons are ejected from the surface of the metal.
The photoelectric effect occurs because the electrons present at the surface of the metal tend to absorb energy when light that is incident on the surface of metal carries enough energy to overcome the attractive forces that usually bind the electrons to the atomic nuclei of the metals.
Photoelectric Effect Equation
According to Albert Einstein, the photoelectric effect is explained as follows:
hν = W + E
• h represents Planck’s constant.
• ν represents the frequency of the incident photon.
• W represents a work function.
• E represents the maximum kinetic energy of ejected electrons: 1/2 mv².
History of Photoelectric Effect
The photoelectric effect was initially observed by Wilhelm Ludwig Franz Hallwachs in the year 1887 and the experimental confirmation was done by the scientist Heinrich Rudolf Hertz. They detected that when electromagnetic radiation is incident on the surface of the metal at a higher threshold frequency, then the radiation is absorbed and the electrons are thus emitted.
Einstein’s Contributions towards the Photoelectric Effect
After constant research in this field, the description for the photoelectric effect was effectively explained by Albert Einstein. Einstein gave the equation as follows;
E = hν
E represents the Energy of a photon ( joules)
h represents planks constant (that is 6.626 × 10-34 J.s)
ν represents the frequency of photon in Hz or hertz
The Concept of Photons
The phenomena of the photoelectric effect cannot be described by considering light as a wave. Though, this effect can be described by the particle nature of light, which defines that light can be imagined as a stream of particles of electromagnetic energy. These ‘particles of light are thus termed as photons. The energy held by a photon is given below;
E = h𝜈 = hc/λ
• E represents the energy of the photon
• h represents Planck’s constant
• 𝜈 represents the frequency of the light
• c represents the speed of light (in a vacuum)
• λ represents the wavelength of the light
Therefore, it can be said that different frequencies of light generally carry photons of varying energies. For instance, the frequency of blue light is greater as compared to that of red light. Thus, the energy that is held by a photon of blue light will be greater as compared to the energy held by a photon of red light.
Properties of the Photon
• A photon is not reflected in a magnetic and electric field and it does not have any mass or charge.
• The momentum and energy of the photons are as follows;
E = pc
P represents the magnitude of the momentum
c represents the speed of light
Threshold Energy for the Photoelectric Effect
Threshold energy (represented by the symbol Φ)is referred to as the minimum amount of energy that is required to remove an electron from the metal. For a photon to hold energy equal to the threshold energy, then its frequency should be equal to the threshold frequency. The threshold frequency is expressed by the symbol 𝜈th and the linked wavelength ( also termed as the threshold wavelength) is expressed by the symbol λth. This can be represented (relation between threshold frequency and threshold wavelength) is as follows;
Φ = h𝜈th = hc/λth
The relationship between the energy of the photon and the kinetic energy of the emitted photoelectron is written below;
Ephoton = Φ + Eelectron
⇒ h𝜈 = h𝜈th + ½mev2
• Ephoton represents the energy of the incident photon, that is always equal to h𝜈
• Φ represents the threshold energy of the metal surface, which is always equal to h𝜈th
• Eelectron represents the kinetic energy of the photoelectron, which is always equal to ½mev2 (that is me = mass of electron = 9.1 x 10-31 kg).
If the energy of the photon is less than the threshold energy, then there will be no emission or production of photoelectrons. Therefore, the photoelectric effect will not occur if the energy of photon
th). When the frequency of the photon is equal to the threshold frequency that is 𝜈 = 𝜈th, then there will be an emission or release of photoelectrons, though their kinetic energy will be zero.
Condition Required for Photoelectric Effect
Threshold Frequency (γth) It is defined as the minimum frequency of the incident radiation that will create a photoelectric effect (that is referred to as expulsion of photoelectrons from a metal surface) is termed as the threshold frequency.
If γ represents the frequency of incident photon and γth represents threshold frequency, then;
• If γ < γTh, then this represents that no ejection of photoelectron will occur and, hence, no photoelectric effect.
• If γ = γTh, then this represents that photoelectrons are just ejected from the surface, yet the kinetic energy of the electron is equal to zero
• If γ > γTh, then this represents that the photoelectrons are ejected from the metal surface. Photoelectrons ejected have kinetic energy.
Threshold Wavelength (γTh)
γTh = c/γTh
Where, γTh represents the threshold wavelength or wavelength of the incident photon.
γTh represents threshold frequency
• If λ < γTh, then the photoelectric effect will occur and electron ejected will also have kinetic energy.
• If λ = γTh, then just the photoelectric effect will occur but the kinetic energy of the photoelectron that is ejected will be equal to zero.
• If λ > γTh, there no photoelectric effect will occur.
Applications of Photoelectric Effect
• It is used to produce or generate electricity in Solar Panels. These panels comprises of metal combinations that allow them to generate electricity from a varied range of wavelengths.
• Lighting sensors for example the ones which are used in smartphones enable automatic modification of screen brightness according to the lighting. This is because the amount of current that is produced through the photoelectric effect is usually dependent on the intensity of light that hits the sensor.
• Digital cameras can spot and thus record light because they also have photoelectric sensors that respond to diverse colors of light.
Photoelectric Effect Citations