The photoelectric effect Flashcards
What is the photoelectric effect?
The emission of electrons from a metal surface when the surface is illuminated by light of frequency greater than the threshold frequency.
Photoelectric emissions of electrons from a metal surface does not take place if:
The frequency of the incident electromagnetic radiation is below the threshold frequency. This minimum frequency depends on the type of metal. This means that the wavelengths of the incident light must be less than the lmax = c/fmin
The number of electrons emitted per second is proportional to:
The intensity of the incident radiation, provided the frequency is greater than the threshold frequency. However, if the frequency of the incident radiation is less than the threshold frequency, no photoelectric emission from that metal surface can take place, no matter how intense the incidence radiation is.
Photoelectric emissions occur without delay as soon as:
The incident radiation is directed at the surface, provided the frequency of the radiation exceeds the threshold frequency, and regardless of intense.
What can’t the wave model of light explain?
The wave theory of light cannot explain either the existence of a threshold frequency or why photoelectric emission occurs without delay. According to wave thoery, each conduction electron at the surface of a metal should gain some energy from the incoming waves, regardless of how many waves arriving second.
The photon theory of light was put forward by Einstein in 1905 explain the photoelectric effect.
Einstein assumed that light is composed of wavepackets of photons, each with energy equal to hf.
E = hf = hc/f
To explain the photoelectric effect, Einstein said that:
- When light is incident on a metal surface, an electron at the surface absorbs a single photon from the incident light and therefore gains energy equal to hf, where hf is the energy of a light photon.
- An electron can leave the metal surface if the energy gained from a single photon exceeds the work function Φ of the metal. This is the minimum energy needed by an electron to escape from the metal surface. Excess energy gained by the photoelectron becomes its kinetic energy.
- ∴ the maximum Kineitic energy is : EKmax = hf - Φ.
Electrons that escape from the metal plate:
- Can be attracted back to it by giving the plate a sufficient positive charge.
- The minimum potential needed to stop photoelectric emission is called the stopping potential Vs.
- At this potential, the maximum kinetic energy of the emitted electron is reduced to zero because emitted electrons must do extra work to e x VS to leave the metal surface. Hence its maximum kinetic energy is equal to e x VS.
Validation of the particle theory of light
- Conclusive experimental evidence for Einstein’s photon theory was obtained by Robert Millikan.
- Milikan measured the stopping potential for a range of metals using light of different frequencies.
- His results fitted Einstein’s photoelectric equation very closely.
- After these results were checked by other physicists through peer review, the scientific community accepted that light consists of protons.
Emission occurs if:
EKmax > 0 or hf > Φ. So threshold frequency = Φ/h
At the end of the 19th century:
- Max Plank suggested that the energy of each vibrating atom is quantized
- Only certain levels of energy are allowed.
- Energy could only be in multiples of a basic amount, or quantum, hf, where f is the frequency of vibration of the atom and h is a constant, known as the Planck constant.
How do conduction electrons behave?
The conduction electrons in a metal move around at random, like the molecules of a gas. The average kinetic energy of a conduction electron depends on the temperature of the metal.
What is the work function of a metal?
The work function of a metal is the minimum energy needed by a conduction electron to escape from a metal surface when the metal is at zero potential.
For a photoelectric current:
The number of photoelectrons per second that transfer from the cathode to the anode = I/e.
What is the photoeletric current proportional to?
- The photoelectric current is proportional to the light intensity of the light incident on the cathode.
- Because each photoelectron must have absorbed one photon to escape from the metal surface, the number of photoelectrons emitted per second (i.e. the photoelectric current) is therefore proportional to the intensity of the incident light.
What is a vacuum photocell?
- A vacuum photocell is a glass tube that contains a metal plate, referred to as the photocathode, and a smaller metal electrode referred to as the anode.
- When light of a frequency greater than the threshold frequency for the metal is directed at the photocathode, electrons are emitted from the cathode and attracted to the anode.
- The microammeter in the circuit can be used to measure the photoelectric current. This is proportional to the number of electrons per second that transfer from the cathode to the anode.
