Could anyone please help my nephew? He is unsure of how to answer this question.

"Is light a stream of particles or a wave?

He has to supply written proof with his answer.

Thank you.

This may be difficult. The accepted answer is: it's both.

There's a long history of debate back and forth. Skip to the bold at the end for a summary.

Around 1675, Isaac Newton propsed that light was made of particles, which he called corpuscles. I won't say more about it, or the objections to it at the time, because it doesn't really contribute to our modern understanding. For the next century, there was considered to be no definitive conclusion on the subject.

Around 1805, Thomas Young performed what is known as the double slit experiment. When waves interact with each other, they can cause positive and negative interference depending on the phase (If you need more information on that part, post again and I will be glad to explain). When a wave passes through two slits, it is effectively split into two waves. These waves end up interfering with each other, giving a distinct pattern of light and dark bands on photographic film placed in their path. Since particles should not exhibit this, light was concluded to be composed of waves. This conclusion stood for about a hundred years.

In the mid 1800s, James Clerk Maxwell developped a theory of light based on its being a travelling wave of electricity and magnetism (again, I can provide more on that if you'd like).

In the 1880s, Heinrich Hertz discovered the photoelectric effect. This is light hitting a metal and causing the release of a spark.

In the 1890s, J.J. Thompson deduced that the spark was the result of negatively charge particles he called corpuscles (we know them now as electrons). Nikola Tesla further developped this.

It was Albert Einstein that related this to the particle nature of light. The energy of an expelled electron is proportional to the frequency of the light, but the number of electrons emitted is proportional to the brightness of the light. Einstein explained this as follows: The brightness of the light is proportional to the number of light particles (which he called light quanta and are now known as photons), but the energy of an individual photon is proportional to it's frequency. The electrons are bound to the metal with some energy. A single photon hitting the surface will eject a single electron. This electron will have the energy of the photon (proportional to it's frequency) minus the energy that was holding the electron to the metal. Since a single photon ejects a single electron, more photons (brighter light) yeild more electrons.

In this same period, other evidence for the particle nature of light was emerging. It had long been known that ojects emit a light that is specific to their temperature (i.e. relatively hot things, such as a stove, glow red). This is known as blackbody radiation. People had stuggled to come up with physical explanation to mathematically predict the observed light. None of the theories worked well until Max Plank, somewhat by accident, created an equation that worked in about 1901. This equation treats light as discrete quanta (photons) with energies proportional to their frequency.

Around 1915, Neils Bohr proposed a structure of atoms (specifically Hydrogen, because things get more complicated when you throw in more than one electron) that explained the nature of light emitted and absorbed by them. The light is emitted and absorbed only at specific frequencies. This structure involved a nucleus containing protons (an neutrons, we now know) with electrons orbiting like planets. we now know it to be a bit more complicated, but that doesn't affect this discussion. In this model, the electrons can only have particular (quantized) energies. To move between energies, the electrons must absorb or emit energy. If this energy is in the form of light, only the light with the right energy (and therefore the right frequency) will be absorbed or emitted.

This was all part of the development of quantum mechanics. As we understand it now, light exhibits something called wave-particle duality, meaning that it behaves both as a wave and as a particle.

What's more, it turns out that this applies to everything. In 1924, Louis-Victor de Broglie proposed that all matter has a wave-like nature. Strangely enough, George Thompson, son of JJ Thompson (who demonstrated electrons as particles), demonstrated experimentally that this was true for electrons in experiments that were more complicated than, but similar in theory to, Young's double slit experiment (they had to do with diffraction thoguh crystals resulting in interference patterns instead of diffraction through slits doing this). As objects become larger, the wave nature becomes insignificant compared to the particle nature, which explains why we don't notice our own wave-like behavior. In 1961, Young's double-slit experiment successfully showed the wave-like nature of electrons, and in the late 1990s was (or perhaps the early 2000s) performed with buckyballs, which are soccer-ball like molecules containing 60 carbon atoms. To make things even stranger, it was performed in 1974 with the electrons passed through the slits one at a time, and the wave nature of electrons was still demonstrated.

In recent years, we have begun performing experiments involving single photons, and have measured these photons.

So, briefly, there was quite a bit of debate about this until Young's double-slit experiment demonstrated the wave nature of light in about 1805. Maxwell developped a theory (which we still use in most circumstances) explaining light as a wave in the mid-1800s. Einstein's explanation of the photo-electric effect and Plank's explanation of blackbody radiation in the first decade of the 20th century provided evidence for the particle nature of light. Bohr's model of the atom and explanation of atomic light emission and absorption added to this evidence. In 1924, de Broglie concluded that all particles as have a wave nature as well. All of this early 20th century work contributed to the development of quantum mechanics, which concludes that light and matter exhibit wave-particle duality, meaning they exhibit both wave-like and particle-like characteristics. Shortly after this, G. Thompson demonstrated this experimentally with something resembling Young's double slit experiment, and in the latter half of the 20th century, the double slit experiment was used to demonstrate the wave nature of various particles. In recent years, we've used and detected single photons

The case for waves:
Young's double-slit experiment.
Maxwell's theories of electromagnetic waves (which govern our understanding of optics)

The case for particles:
Einstein's explanation of the photoelectric effect
Plank's explanation of blackbody radiation
Bohr's model of the atom and atomic emission and absorption
Modern-day single-photon experiments

Quantum mechanics successful merges these with wave-particle duality.

This message has been edited. Last edited by: methos,

I should point out two things regarding blackbody radiation.

First, I mispelled Planck's name both times I typed it above.

Second, Planck did not actually conclude that light was made up of particles from his formula.

Lord Rayleigh created a formula to attempt to predict blackbody radiation based on the theory that it was caused by a collection of oscillatorts that could absorb or emit light at any frequency. He created a formula that didn't quite work based on this. Jeans noticed a flaw in it. The improved formula, which predicted the radiation correctly at low frequencies but failed completely at high frequencies was the Rayleigh-Jeans law.

The essential change from the Rayleigh-Jeans to Planck's law is that Planck proposed that the oscillators were quantized, that is, the energy change could only equal to the frequency times a constant times an integer. This means that the light emitted or absorbed must come in these units, you cannot have 1.5 of them, but you can have 1 or 2 or 3 or 100, or any other integer of them. Planck didn't go so far as to say that light could only exist in integers of these packets (he had over 100 years of institutional conviction that light was not particles holding him back from making that next step), he ascribed the property only to the oscillator. Now, largely thanks to Einstein's photoelectric effect work that followed, we realize that this is also a property of the light, and that these packets are individual photons.