Particles versus Waves

 Light, is it a particle or is it a wave? This is a question scientists have been asking for centuries.

 Newton (1675) was on team particle. Newton thought that light was made up of fast moving particles, a light source was like a gun shooting out light in all directions.

 Grimaldi (1665) was the first to note the diffraction of light.  He coined the term diffraction.

 Huygens was the first to propose that light was made of waves in  With the wave theory, Huygens was able to explain how light changes speed in different media.

 Young, an English physician, became interested in waves while studying the human voice.  While looking in to Newton’s light experiments, he found they could be explained by waves too.  He was even able to measure the wave lengths of visible light.

 Young, sometime from , performed the famous “double slit” experiment.  Light was shown through 2 slits close to each other and on to a wall.  The particle theory would say 2 bands of light would result when light is passed through a double slit.

 The wave theory predicts the light will diffract through the 2 slits creating a light and dark pattern from interfering light waves.  Young did this and a light and dark pattern resulted from wave-interference.

Young’s results were explained by noting that as waves pass through each other, they experience constructive interference where two wave crests or troughs meet which causes brighter fringes. Likewise, where wave troughs and crests meet, destructive interference occurs producing dark fringes.

 In 1819 Augustin Fresnel presented the wave theory to the French Academy.  Fresnel showed that the wave theory explains interference and diffraction.

 Poisson, a mathematician, used Fresnel’s theory of wave diffraction to deduce (1818) that as light diffracted around a solid disk, it would interfere to produce a bright spot in the middle. Poisson thought he had disproved Fresnel’s theory.  This effect was proven true when Francois Arago (1818) tested it.

 Maxwell, a Scottish physicist, predicted the existence of electromagnetic waves (1865), waves with both electric and magnetic components.  He noted that a vibrating electric charge (like an electron) will generate these transverse electromagnetic waves.  His theory predicted that in a vacuum these waves would travel at the speed of light. He proposed that light consisted of these electromagnetic waves.

 Hertz (1888) discovered that electromagnetic waves acted just like light, encountering refraction, reflection, diffraction and interference.  The difference between visible light and the electromagnetic waves he was experimenting with was their wavelengths, visible light having shorter wavelengths.  The EM waves Hertz was studying turned out to be radio waves, EM waves with the longest wavelength.

Max Planck  In the late 1800s, Max Plank had been studying the light radiation given off by a black body as it is heated to various temperatures.  A black body was chosen because it is the simplest and most efficient absorber and emitter of radiation.  Planck found that the experimental data fit with the equation E = hv, the energy of the light emitted (E) = a constant (h) times the frequency of the light emitted (v).

Max Planck  The value of Planck’s constant (h) in his equation E= hv was computed to be x Js.  Planck found that his experimental results could be explained if he assumed that the vibrating molecules in the black body could only give off specific energy values, not continuously variable energy values. Planck called the specific energy values emitted by a hot black body by the name of quanta of light.

Max Planck  To understand quanta, imagine a hot black body with its vibrating molecules to be like a bee’s nest with buzzing bees inside.  As the black body is heated further with its molecules vibrating enough so that they give off light, its like a bee’s nest being shaken and the bees (light quanta) beginning to exit.

Max Planck  Whole bees (light quanta), not fractions of them, exit the hive (black body), each having its own specific size (energy: E=hv) directly related to its wing frequency (frequency : v)  Planck’s explanation for the way light is given off by hot, glowing objects was that it was given off in particles (quanta), each with its own specific energy and vibration frequency.

Planck (Continued)  Planck had really revived the particle theory of light. This theory, the Quantum Theory, has become a more general theory for all of energy and matter.  Planck himself thought of his theory as just a hypothesis to explain his specific experimental results. He never proposed that light should be thought of as a stream of particles. For his black body experiment, the wave theory of light could not explain the results because waves are continuously variable with fractional energy values, not specific values.

The Quantum Theory  The Quantum Theory today states that all energy is quantized, is absorbed and given off in discrete amounts (particulate-like) called quanta. Light energy is composed of photons (light quanta of specific energy values dependent on the wavelength of the particles), gravity is composed of gravitons, etc.

 Einstein (1905) proposed that light was made up of particles that he called photons.  Einstein suggested an experiment with the photoelectric effect to test his idea.  The photoelectric effect is when a substance (metal) emits electrons as light of the right frequency (colour) is shown on it.

 Einstein noted that the predictions the wave and particle theory made for the photoelectric effect were very different.  The wave theory predicts that if the light’s brightness increases, the number of electrons emitted and their speed will increase because brightness is associated with a wave’s amplitude and energy. More energy (brightness) yields more electrons with higher velocities.  The wave theory predicts that wave frequency has no effect.

Albert Einstein Continued.  To measure the amount (number) of photoelectrons being emitted, an ammeter is used to detect the current flow. To determine the maximum kinetic energy of the photoelectrons, a variable voltage is applied against the electron flow and is increased until the current flow stops. Since voltage is the amount of energy per coulomb of electrons, this counter voltage measures the maximum kinetic energy of the photoelectrons.

 The particle theory predicts an increase in brightness will increase the number of electrons emitted, but not their maximum kinetic energy. This is because the kinetic energy of light particles is directly related to their frequency, as Planck determined (E = hv).  In the particle theory of light, photons (light particles) with greater frequency have higher energies.

 By measuring the voltage required to stop a current flow, the maximum kinetic energy of the ejected electrons could be measured.

 R. A. Millikan did the experiment that Einstein suggested in  He found that the kinetic energy of ejected electrons increased as the frequency of light increased.  The intensity or brightness of the light had no effect on the maximum kinetic energy of the photoelectrons.

 Since the frequency of a light (its colour) changed the kinetic energy of the photoelectrons, light must be particles, the quanta of energy following Planck’s equation, E = hv.  The small value of Planck’s constant (6.626 x Js) is due to the fact that light particles (photons/quanta) are extremely small

 Bohr 1927) made the statement that we must be aware of both the wave and particle nature of light to understand it.  In particular experiments, light reveals either its wave nature or its particle nature but often not both. Some experiments are best explained by viewing light as waves while others are best explained by viewing light as particles.  He concluded that the 2 theories complement each other. This is known as the principle of complementarity.

 The conclusion was that light ( a form of energy) is more complex than a beam of particles or a wave, it is now known as a wave-particle duality.

Conclusion.  Interestingly, the wave-particle duality for energy was also applied to matter. In 1924, Louis deBroglie suggested that particles of matter could be thought of as incorporated into or transmitted as waves. de Broglie provided an equation by which to determine the wavelength of the moving particle:  The idea of electrons being considered as waves led to the Quantum Mechanical Model of the Atom.