Photoelectric Emission
- The work function Φ, or threshold energy, of a material is defined as:
The minimum energy required to release a photoelectron from the surface of a material
- Consider the electrons in a metal as trapped inside an ‘energy well’ where the energy between the surface and the top of the well is equal to the work function Φ
- A single electron absorbs one photon
- Therefore, an electron can only escape the surface of the metal if it absorbs a photon which has an energy equal to Φ or higher
- Different metals have different threshold frequencies, and hence different work functions
- Using the well analogy:
- A more tightly bound electron requires more energy to reach the top of the well
- A less tightly bound electron requires less energy to reach the top of the well
In the photoelectric effect, a single photon may cause a surface electron to be released if it has sufficient energy
- Alkali metals, such as sodium and potassium, have threshold frequencies in the visible light region
- This is because the attractive forces between the surface electrons and positive metal ions are relatively weak
- Transition metals, such as manganese and iron, have threshold frequencies in the ultraviolet region
- This is because the attractive forces between the surface electrons and positive metal ions are much stronger
Laws of Photoelectric Emission
- Observation:
- Placing the UV light source closer to the metal plate causes the gold leaf to fall more quickly
- Explanation:
- Placing the UV source closer to the plate increases the intensity incident on the surface of the metal
- Increasing the intensity, or brightness, of the incident radiation increases the number of photoelectrons emitted per second
- Therefore, the gold leaf loses negative charge more rapidly
- Observation:
- Using a higher frequency light source does not change how quickly the gold leaf falls
- Explanation:
- The maximum kinetic energy of the emitted electrons increases with the frequency of the incident radiation
- In the case of the photoelectric effect, energy and frequency are independent of the intensity of the radiation
- So, the intensity of the incident radiation affects how quickly the gold leaf falls, not the frequency
- Observation:
- Using a filament light source causes no change in the gold leaf’s position
- Explanation:
- If the incident frequency is below a certain threshold frequency, no electrons are emitted, no matter the intensity of the radiation
- A filament light source has a frequency below the threshold frequency of the metal, so, no photoelectrons are released
- Observation:
- Using a positively charged plate causes no change in the gold leaf’s position
- Explanation:
- If the plate is positively charged, that means there is an excess of positive charge on the surface of the metal plate
- Electrons are negatively charged, so they will not be emitted unless they are on the surface of the metal
- Any electrons emitted will be attracted back by positive charges on the surface of the metal
- Observation:
- Emission of photoelectrons happens as soon as the radiation is incident on the surface of the metal
- Explanation:
- A single photon interacts with a single electron
- If the energy of the photon is equal to the work function of the metal, photoelectrons will be released instantaneously
