Hi! . . Why does light sometimes behave as a wave, and sometimes as a particle? And what is a "light wave" anyways? I guess photons are the "particle" aspect of light? So many questions! Thanks! :)
Hello there! Excellent question!! Light is a very strange thing, when you really stop and think about it. The wave-particle duality of light (of all matter, really) is really interesting and rather difficult to explain, but here goes. The really short answer is probably, âQuantum Mechanicsâ or something, but that would be a rather boring and unsatisfying answer. DKS wrote a bit about explaining what light is in an earlier post and I wrote another bit explaining what it is that waves when we talk about light waves.
Letâs first look at your first question, âWhy does light sometimes behave as a wave, and sometimes as a particle?â I first have to correct the questionâlight doesnât âsometimes behave as a waveâ nor does it âsometimes [behave] as a particle.â Light always behaves as a wave and as a particle, both at the same time; itâs not as if sometimes weâre running an experiment and light suddenly acts like a particle instead of a wave. Instead what happens is that when we treat light as a wave and run experiments as such, the results of such experiments can best be explained if light were a wave. The same is true if we treat light as a particle. Sounds a little like tautological/circular logic, but let me explain it a bit more.
There are a whole bunch of awesome experiments that show that light is a wave; these experiments are called the Multi-Slit Experiments. They are set up with a light source (nowadays, the light source is usually a L.A.S.E.R.), an obstacle (usually a screen, like a wall, with one or more small openings in a line to allow light to pass through), and then a plain and boring wall where we can see the light of the LASER (you can replace the wall with an array of photodetectors to measure the strength of incident light). Something like this:Â
What happens is that when you shine light at the hole in the obstacle (the thing labeled âslit partitionâ in the top image, and the unlabeled wall in the second image), you get a series of bright and dim spots on your wall (labeled âInterference Light and Dark Fringesâ in the top image) that look like this. These are called interference patterns, and they occur because light behaves like a wave (really, light interferes with itself just like water waves do. Hereâs a cool video). This interference occurs when the slit(s) is/are tiny: usually they are about the size of the wavelength of light youâre using. For red light (λ â 600nm = 0.6ÎŒm) you want the slit to be at around that width for best diffraction. If light only behaved as a particle (photons), then you would expect the photons to simply pass through the opening, hit the wall at that one spot, and create a single point of light. However, this is demonstrably no the case! It doesnât make sense to treat light as a particle when doing the multi-slit experiments because that model is unable to predict interference patterns.
Treating light as a wave means that there are an infinite number of energies that light can carry: it would be a smooth spectrum. There is nothing in those experiments that contradict that possibility. However, Albert Einsteinâs Nobel-prize winning 1905 explanation for the photoelectric effect proved that the energies of light are not infinitely smooth, but are actually quantized energy levels. These âquantized energy levelsâ are called photons.
Light As Both a Wave and Particle, A Quantum Mechanical Approach:
Basically, whatever light is (weâre not really sure exactly WHAT light is, but we have some pretty cool ideas) sometimes itâs best to think of it as a particle, other times itâs more helpful to think about it as a wave. We know that light follows the Electromagnetic wave equation, and the Planck relation, E=h*Æ. The wave equation explains lightâs wave-like properties, the Planck relation explains lightâs particle-like properties. Interestingly enough, applying Einsteinâs famous energy equation, E=m*c^2, for objects with mass we can see thatÂ
E = m*c^2    and    E = h*Æ.
Energy is energy, doesnât matter what form itâs in, so by combining the two, we get,
Now, a thing to note is that the speed of light, c, the wavelength of light, λ, and the frequency of light, Æ, are related by the following equation,
Plugging that into mc^2 = hÆ we get,
Dividing everything by the speed of light, c, we get,
Interestingly enough, the momentum p of a particle of mass m and speed v is simply the product of the two, p = mv. What this means is that the product m*c is the momentum of a photon, which has a value of h/λ; massless object carries momentum!! Likewise, this equation means that the (de Broglie) wavelength of a particle (e.g., an electron, proton, neutron, baseball, etc.) is h/(m*c); a seemingly solid object is also a wave!! This gets us into the realm of quantum mechanics, wave functions, Schrodingerâs equation, and the thought experiment known as Schrodingerâs Cat.
Yea, OK, Nice Story But is Any of That Real?!
Whether a thing is real or not is not really whatâs at debate here. The real question is whether this stuff helps us understand strange phenomena. The math predicts that solid objects will behave like waves under specific conditions; it predicts that a massive object (relatively speaking with respect to massless photons) can experience the same interference patterns from the multi-slit experiment that photons experience. So scientists tested this hypothesis and guess what happened:Â
molecules exhibited the exact same interference patterns!! Crazy!!
If youâre still reading after that wall of text, thank you for time!! Iâm sure you have many more questions so please, please, please feel free to ask them all!! Weâll do our best to answer them!
