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Physik-Institut

Wave or particle?

A prominent, albeit not so intuitive, concept in quantum mechanics is the wave-particle duality.
According to the wave-particle duality, small objects behave like waves or particles depending on the context.

Small objects behave more like waves or particles depending on the context. Wave and particle are concepts we are familiar with from everyday life. The concepts of waves and particles are familiar to us from everyday life. However, it is usually obvious whether an object should be described as a wave (e.g. water) or as a particle (e.g. a ball).

Sunlight
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Radiation, eg light from the sun, can be described as a particle or a wave. Illustration: Ruth Bründler, UZH.

Waves propagate through space and can overlap. The vibrations of musical instruments are a good example of this. Waves often propagate through a medium, such as water or air, but there are also waves that do not require a medium, such as electromagnetic and gravitational waves.
Particles, on the other hand, have clear paths; for example, a billiard ball can move in a medium or in a vacuum.

Quantum mechanical objects are a new type of object that exhibit properties of both waves and particles. This is clearly demonstrated by the photoelectric effect.

Photoeffect

The photoelectric effect is the emission of electrons from a material caused by electromagnetic radiation. The energy of the emitted electrons is independent of the light intensity; the latter only determines the number of electrons released. Albert Einstein explained the effect by considering light to be made up of particles (photons).

Photoeffect
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Schematic illustration of the photoelectric effect. Illustration: Ruth Bründler, UZH

 

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More on: Photoeffect

The photoelectric effect is the emission of electrons from a material when it is exposed to electromagnetic radiation, such as light. These emitted electrons are known as photoelectrons. Experiments have shown that electrons are only released when the frequency of the light exceeds a certain threshold, regardless of its intensity or the duration of the irradiation.

These results contradict classical physics. Classical physics predicts that continuous light waves transfer energy to electrons. The electrons are then emitted when they have accumulated enough energy. A change in light intensity would alter the kinetic energy of the emitted electrons, and lead to delayed emission in weak light.

Albert Einstein concluded that a beam of light consists of discrete energy packets, known as light quanta (now called photons). He was awarded the Nobel Prize for this discovery in 1921.
He described the effect using the following equation:

EPhoton = h ⋅ f = Ekin + WA

where
    h: Planck's quantum of action
    f: frequency of the incident light
    EPhoton = hf: Energy of a single photon
    WA: Work Function — the minimum energy required to release an electron from a metal.
    E_(kin): the kinetic energy of the released electron.
An electron will only be released if hf > WA.

Applications:
The photoelectric effect is used in many modern devices for light detection and precisely timed electron emission, and is also responsible for charging spaceships.

Examples:

  • Solar cells: They convert light energy directly into electrical energy by releasing electrons in the semiconductor material when photons hit it.
  • Light sensors (e.g. camera sensors): They detect light particles and convert them into electrical signals for image generation.
  • Photomultiplier: A photomultiplier can detect weak light signals (down to individual photons) by generating an electrical signal. This electrical signal can then be amplified to enable extremely light-sensitive measurements.
  • Light barriers/door openers: These register when light is interrupted by an object and then trigger a reaction (e.g. opening the door).

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