Quantum Phenomena

The Photoelectric effect

If you shine light of a specific frequency onto the surface of a metal, the metal will emit electrons. For most metals, this frequency falls in the UV range.

  • Free electrons on the surface of the metal absorb energy from the light.
  • If an electron absorbs enough energy, the bonds holding it to the metal break. 
  • The electons that are emitted are known as photelectrons. 

Threshold frequency: frequency needed to to emit photoelctrons.

The photoelectrons that are emitted have different KEs. KE increases with the frequency and is unaffected by the intensity of the radiation. 

Photons of light have a one-on-one, particle like interaction with an electron in a metal surface. If a photon collides with an electron when light hits the surface of the metal, the electron will gain energy equal to hf.

Before an electron can leave the surface of a metal, i tneeds enough energy to break the bonds holding it there. This energy is known as the work function and its value dpends on the metal.

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The Photoelectric effect 2

The KE that an electron will be carrying when it leaves the metal is hf minus any energy its lost on the way out. Electrons deeper down in the metal lose more energy than the electrons on the surface, which explains the range of energies. 

KE is independant of the intensity of light. Increasing the intesnity only increases the number of photons emitted. 

Max KE can be measured by using stopping potential. The stopping potential is the p.d needed to stop the fastest electrons. 

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Energy Levels

Electrons in atoms can only exist in certain well-defined levels, where each level is given a number. n=1 represents the ground state. 

When electrons emit a photon, they move down energy levels - which gives the energy of these photons a specific value.

The eV value of energy levels are negative because work must be done to move electrons up levels. 

When an electron absorbs a photon with the exact energy of the energy difference between two levels, the electron is excited. If the electron is completely removed from an atom, it is ionised. 

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Long answer question Mark Scheme

energy is needed to remove an electron from the surface

work function φ (of the metal) is the minimum energy needed by an electron to escape from the surface

light consists of photons , each of energy

E = hf one photon is absorbed by one electron

an electron can escape (from the surface) if hf > φ

kinetic energy of an emitted electron cannot be greater than hf – φ

an electron below the surface needs to do work/uses energy to reach the surface

kinetic energy of such an electron will be less than hf – φ

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Fluorescent Tube

  • Contains mercury vapour across which a high voltage is applied
  • The high voltage accelerates fast moving free-electrons that ionise some of the mercury atoms and produce more free electrons.
  • This flow of free electrons collides with electrons in other mercury atoms, the electrons in the mercury atoms are excited to higher energy levels.
  • When these electrons return to their ground states, they emit photons in the UV range.
  • A phosporus coating absorbs these photons, exciting its electrons to much higher orbits and these electrons then cascade down energy levels, emitting many lower energy photons in the form of visible light. 
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Line Absorption Spectrum

You get a line absorption spectrum when light with a continuous spectrum of energy passes through a cool gas. 

The electrons in this gas absorb photons and are excited. 

These wavelengths are then missing from the continuous spectrum.

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