Physics A-Level (AQA) : 2.2 Electromagnetic radiation and quantum phenomena

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What is the photoelectric effect?

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Emission of electrons from the surface of a metal when electromagnetic radiation above a certain frequency is directed at the metal

Image: What is the photoelectric effect?

What is threshold frequency?

The minimum frequency of the incident electromagnetic radiation for photoelectric emission of electrons to take place

Photon explanation of threshold frequency

Light is composed of discrete wavepackets called photons, each of energy equal to hf (i.e. proportional to frequency). When light is incident on a metal surface, an electron at the surface absorbs a single photon and gains energy hf. If the frequency of light is above a certain value (threshold frequency), the photon energy would exceed the work function of the metal, the electron would gain enough energy to overcome attractive forces, so it can escape from the metal surface.

What is work function?

The minimum energy needed by an electron to escape from the metal surface

What is stopping potential?

The minimum potential difference needed to give the metal plate a sufficient positive charge that it can attract the fastest photoelectrons back

Effect of light intensity on photoelectric effect

The light intensity is directly proportional to the number of photons per second incident on the metal surface. Because each photoelectron must have absorbed one photon to escape from the metal surface, the number of photoelectrons emitted per second is therefore directly proportional to the intensity of the incident light.

The photoelectric equation

hf = φ + EK(max)
hf - photon energy, J or eV
φ - work function, J or eV
EK(max) - maximum kinetic energy of the photoelectrons, J or eV

Why does the kinetic energies of the photoelectrons vary up to a maximum?

Electrons deeper within the metal need to do more work to reach the surface of the metal, so when they are emitted they have less kinetic energy than those already at the surface.

Graph for photoelectric effect

What is ionisation?

Ionisation is any process of changing uncharged atoms into charged ions.
When a free electron with enough kinetic energy collides with an electron within an atom, the free electron transfers energy to the electron within the atom. If the electron within the atom gains enough energy (above ionisation energy), it can leave the atom and become free. The free electron knocks an electron out of the atom so the atom becomes a positive ion.

What is excitation?Excitation is when an electron inside an atom move from a lower energy level to a higher energy level and gains energy exactly equal to the difference between the two energy levels. Excitation can take place when an free electron with specific kinetic energy collides with an electron within an atom. The free electron transfers all its kinetic energy to the electron within the atom. Excitation can also take place when an electron within an atom absorbs a photon. The kinetic energy of the free electron or the photon energy must be exactly equal to the difference between the two energy levels of the atom.The electron volt (eV)A unit of energy equal to the work done when an electron is moved through a potential difference of 1V 1 eV = 1.60x10^-19 JHow does the fluorescent tube emit visible lightThe fluorescent tube has a fluorescent coating on its inner surface and contains mercury vapour at low pressure. There is a high voltage across the fluorescent tube which accelerate free electrons to high kinetic energy. The electrons collide with mercury atoms and cause excitation, as well as ionisation which produces more free electrons. The mercury atoms de-excite and emit photons in the ultraviolet range. The ultraviolet photons are absorbed by the atoms of the fluorescent coating, causing excitation of these atoms. The coating atoms de-excite in steps via several intermediate energy levels and emit photons with lower energies in the form of visible light.Line spectra and energy levelsA line spectrum is a spectrum of discrete lines of different colours. Electrons in an atom move about the nucleus in discrete energy levels with fixed energy values. When the atom de-excite (an electron moves from a higher energy level to a vacancy in a lower energy level), it emits a photon with energy exactly equal to the difference between the two energy levels. This means only photons with specific energy values and therefore specific wavelengths (E=hc/λ) are emitted, which correspond to discrete lines on a line spectrum.Why are the lines of a line spectrum of an element characteristic of that element?Atoms of a certain element have unique energy levels with characteristic energy values. So photons that they emit also have characteristic energy values and wavelengths, and their line spectra are characteristic to that element.What is the energy of the emitted photon when an atom de-excite?hf = E1 - E2 hf - energy of the emitted photon, J or eV E1 - energy value of the higher energy level, J or eV E2 - energy value of the lower energy level, J or eVWhat does electron diffraction suggest?Particles possess wave properties. Evidence that electrons have a particle-like nature is that electrons in a beam can be deflected by a magnetic field.What does the photoelectric effect suggest?Electromagnetic waves have a particulate nature. Diffraction of light shows the wave nature of light.What is de Broglie wavelength?The wavelength of matter particles when they behave like waves λ = h / mv λ - de Broglie wavelength, m h - Planck constant, 6.63x10^-34 J s m - mass, kg v - velocity, m s-1 mv - momentum, kg m s -1How and why the amount of diffraction changes when the momentum of the particle is changedWhen the momentum of the particle is changed, its de Broglie wavelength changes. The greater the de Broglie wavelength, the greater the amount of diffraction.