8. Write two important observed during photoelectric current
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The photoelectric effect is a phenomenon in which electrons are ejected from the surface of a metal when light is incident on it. These ejected electrons are called photoelectrons. It is important to note that the emission of photoelectrons and the kinetic energy of the ejected photoelectrons is dependent on the frequency of the light that is incident on the metal’s surface. The process through which photoelectrons are ejected from the surface of the metal due to the action of light is commonly referred to as photoemission.
The photoelectric effect occurs because the electrons at the surface of the metal tend to absorb energy from the incident light and use it to overcome the attractive forces that bind them to the metallic nuclei. An illustration detailing the emission of photoelectrons as a result of the photoelectric effect is provided below.
Photoelectric Effect
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Explaining the Photoelectric Effect: The Concept of Photons
The photoelectric effect cannot be explained by considering light as a wave. However, this phenomenon can be explained by the particle nature of light, in which light can be visualized as a stream of particles of electromagnetic energy. These ‘particles’ of light are called photons. The energy held by a photon is related to the frequency of the light via Planck’s equation:
E = h = hc/λ
Where,
E denotes the energy of the photon
h is Planck’s constant
denotes the frequency of the light
c is the speed of light (in a vacuum)
λ is the wavelength of the light
Thus, it can be understood that different frequencies of light carry photons of varying energies. For example, the frequency of blue light is greater than that of red light (the wavelength of blue light is much shorter than the wavelength of red light). Therefore, the energy held by a photon of blue light will be greater than the energy held by a photon of red light.
Threshold Energy for the Photoelectric Effect
For the photoelectric effect to occur, the photons that are incident on the surface of the metal must carry sufficient energy to overcome the attractive forces that bind the electrons to the nuclei of the metals. The minimum amount of energy required to remove an electron from the metal is called the threshold energy (denoted by the symbol Φ). For a photon to possess energy equal to the threshold energy, its frequency must be equal to the threshold frequency (which is the minimum frequency of light required for the photoelectric effect to occur). The threshold frequency is usually denoted by the symbol th and the associated wavelength (called the threshold wavelength) is denoted by the symbol λth. The relationship between the threshold energy and the threshold frequency can be expressed as follows.
Φ = hth = hc/λth
Relationship between the Frequency of the Incident Photon and the Kinetic Energy of the Emitted Photoelectron
Therefore, the relationship between the energy of the photon and the kinetic energy of the emitted photoelectron can be written as follows.
Ephoton = Φ + Eelectron
⇒ h = hth + ½mev2
Where,
Ephoton denotes the energy of the incident photon, which is equal to h
Φ denotes the threshold energy of the metal surface, which is equal to hth
Eelectron denotes the kinetic energy of the photoelectron, which is equal to ½mev2 (me = mass of electron = 9.1*10-31 kg)
If the energy of the photon is less than the threshold energy, there will be no emission of photoelectrons (since the attractive forces between the nuclei and the electrons cannot be overcome). Thus, the photoelectric effect will not occur if < th. If the frequency of the photon is exactly equal to the threshold frequency ( = th), there will be an emission of photoelectrons, but their kinetic energy will be equal to zero. An illustration detailing the effect of the frequency of the incident light on the kinetic energy of the photoelectron is provided below.