Light: Particle or Wave

by: 강서평

        Light is an element that brought a lot of confusion to us several times. Its identity was very hard to determine - whether it was a particle or a type of a wave. Nowadays, it is well known that it has the character of both a particle and a wave. But before this was known, there was a long ‘war’ about whether light is a particle or a wave - ended by the genius scientist : Albert Einstein. This article will tell you about the so-called war - and every single detail that was in it. 


Before the late 18th century, when classical physics were everything, light was defined as electromagnetic waves - meaning that it’s a type of energy that can be transmitted without the need of any sort of medium. At this time - energy meant that it was a wave; for energy couldn’t be changed into mass yet. These waves were conjured by the drop of an electron in the scale of its shell - meaning that in an atom, the electron would change it’s distance from the proton, conjuring light.Of course, this concept is wrong. 


But Newton brought up the theory that light was a particle, using prisms. He showed that light couldn’t be a wave because it got in one prism, and came back out another - being segregated in the process but merged in the end. As a wave, this would rarely happen because of the wave’s interference.[2] 


Netherlands’ scientist Christiaan Huygens strongly opposed this. Having shown light’s movement through light’s wave-characteristics, Huygens just couldn’t grasp the fact that he was wrong. As a particle, light’s diffraction[3] couldn’t be shown. His book named “treatise on light” was published, explaining all sorts of light’s route through this one movement - contradicting Newton's book “opticks”.


Even though Huygens was a talented scientist, he couldn’t overpower Newton. However, Thomas Young simply disproved Newton by this experiment covering light’s diffraction, showing it extremely valid by the ‘double slits experiment’ in the early 19th century. This experiment couldn’t be shown by light’s particle-characteristics but could be explained by light’s wave-characteristics.[4] The experiment used two slits to try and see if light would interfere the way waves would, and it actually did. [3]’s picture shows us the diffraction of light. This couldn’t be shown strictly before this experiment - but was shown through it. The experiment looks like [5]. When light diffracts, it diffracts from the same light source, meaning that if light is wave, the waves would have the same phase, making interference in a more regular way so that it's easier to analyze. So basically, the phase of the two light waves would be the same, making constructive interferences in the same phase and destructive interferences in phases that have 2π difference. This just killed the concept that light was a particle.


Then, Einstein showed up and changed things a bit. He showed interest in the photoelectric effect[7].  This is the phenomenon used to describe electrons sparking up  when light is shone upon metals. Einstein’s idea too perfectly killed the ‘light is a wave’ theory, causing confusion.


This experiment looks like the following. The result is simple. “If a certain light’s frequency is high enough[8], electrons will spark. This minimum amount of frequency[9] required differs for each metal.” But this means a lot of things.


First, it means that light is a particle. Einstein showed that,E=mc^2, which meant whether energy was delivered or not didn’t matter.[10]  Nevertheless, light needed to be a particle because if light was a wave, the frequency wouldn’t have mattered. If we put enough low frequency on light, the lights should make a constructive interference that would make the energy the wave holds higher, successfully sparking the electrons. But this didn’t happen. In fact, no matter how little high-frequency light we shoot, electrons will always spark.[11] Therefore, light is a particle. 


So what is right? Einstein's experiment is right as well as Young’s experiment.  However, Young’s experiment failed to see light from a small perspective. This is because light’s particle( known as a photon now), doesn’t have mass and therefore was too hard to measure. But Einstein’s experiment looked in a very small perspective,  so we could actually know the existence of photons. So light is a particle that has a wave’s personality, which means that it vibrates on its way of moving - functioning as a wave, but actually being a particle.







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