Electrons in an atom are present in specific energy levels that are at a different distance from the nucleus. The electrons in the highest energy levels are called valence electrons and they are farthest from the nucleus.
Within these energy levels, there are spaces called orbitals where specific electrons are likely to be located. These orbitals are denoted by the letters(s,p.d,f,g). In most cases, only the electrons in the orbitals s and p are referred to as valence electrons. In quantum mechanics, orbitals are considered as a mathematical function which is used in finding the electrons of the atom at any specific region around the nucleus.
All the atoms have the electrons in orbitals with well-defined energy levels. These electrons in a specific energy level of an atom can be excited to a higher energy level by electromagnetic radiation. These excited electrons later return to the ground state. However, if the energy of the incident electromagnetic radiation is such that it causes the electrons to be excited above the energy levels associated with the atom leading to ionization of the atom then it is called a photoelectric effect.
The photoelectric effect is a phenomenon in which electrons are emitted from metals and non-metallic substances as a result of absorption of energy from the incident light. These ejected electrons are called photoelectrons. The emission of electrons depends on the frequency of the incident light. The minimum frequency of the incident light above which photoelectrons are emitted is called threshold frequency. The process of emission of photoelectrons is called photoemission.
The minimum energy required to just liberate an electron from the surface of the material is called work function.
Discovery of Photoelectric effect
Heinrich Hertz discovered the phenomenon of the photoelectric effect in the year 1887. In his experiment on electromagnetic waves, he observed that electric discharge or high voltage spark in a cathode ray tube was facilitated when the cathode was illuminated by ultraviolet light. Hertz did not investigate this observation in detail. In 1888, Hallwachs followed up the discovery of Hertz. The following observation was made by him when a leaf electroscope is exposed to ultraviolet light,
- A negatively charged zinc plate lost its charge.
- A positively charged zinc plate became more positively charged.
- A neutral charged plate acquired more positive charge.
From his experiment, he concluded that negatively charged electrons are released from the zinc plate during the process.
A photoelectric cell is a technological device based on the photoelectric effect which converts light energy into electrical energy. There are three different types of photoelectric cells namely photoemissive cells, photovoltaic cells and photoconductive cells.
Application of photocells
- Vacuum type photoemissive cells are used in photometry for the measurement of light intensities.
- Gas-filled type photoemissive cells are used in the reproduction of sound in cinema halls.
- Photovoltaic cells are used in converting solar radiation into electrical energy in solar batteries.
- Photocells are used to switch on and switch off the street lights automatically.
- Photocells along with switching circuits are used to control the temperature of the furnace.
- They are used in fire alarms and burglar alarms.
- Photocells are used to estimate the temperature of the stars in astronomy.