What is light?.. a Wave?.. a Particle?.. or both?


Light, considered to be the fastest thing in the universe. Let’s illuminate ourselves with it’s fundamentals!

The image above, is a snapshot of light behaving simultaneously as both a wave and a stream of particles captured by a team of researchers from École Polytechnique Fédérale de Lausanne, a research institute in Lausanne, Switzerland, that specialises in physical sciences and engineering.

Light behaves both as a particle and as a wave, interesting…isn’t it?

The animation below shows how one series of waves hits a double slit and produces two series of waves that interfere with each other.


Does this ring a bell?

Yes, we know this as the Double Slit or Young’s experiment.

The experiment is very simple. Generally a coherent light source is preferred but the interference phenomena will still happen for an incoherent one. A laser beam would be a good coherent light source. The experiment is demonstrated with beam of light that illuminates a plate which is pierced by two parallel slits and the light passing through those slits is observed on a screen behind the plate. As discussed above, from among the two natures of light, the wave nature causes the light waves passing through the slits to interfere with each other, resulting into bright and dark bands on the screen.

Now, going to the quantum level!

An electron is a tiny bit of matter, when fired through a single slit, it creates a single band on the screen. So if we fire these tiny bits from two slits we should get two bands on the screen. But that’s not the case. The resultant on the screen is a pattern of waves. The interference pattern! The physicists thought that the bits were bouncing off each other and creating that pattern. So they decided to shoot electrons one at a time thinking that there is no way the electrons would interfere with each other. But about an hour or later, the same interference pattern seemed to emerge. The conclusion is inevitable!

The single electron leaves as a particle, becomes a wave of potentials, goes through both slits, and interferes with itself to hit the wall like a particle. But mathematically, it is even more mind boggling… it goes through both slits AND it goes through neither, it goes through just one and it goes through just the other.

Light famously has two natures: it is wave-like, interfering in the same way that water ripples cross each other; it is also particle-like, carrying its energy in discrete bundles known as photons. If the experiment is sufficiently sensitive, the interference pattern appears grainy, where an individual photon appears on the screen, as you can see in the simulated projection pattern shown. In other words, single photons travel as though they are interfering with other photons, but is itself indivisible. Matter also has this dual character; interference of electrons and atoms has been observed experimentally. All of this is backed up by years of work.

The major difficulty with quantum mechanics is its interpretation. The standard Copenhagen interpretation (named in honor of the home city of Niels Bohr, who first formulated it) takes a simple stance: the reason why photons sometimes seem like particles and sometimes like waves is that our experiments dictate what we see. In this view, photons are products of our experiments without independent reality, so if we’re bothered by seemingly contradictory notions of wave and particle properties, it’s because we’re expecting something unreasonable of the universe.

–Source: blog.scientificamerican.com

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