Is it a Particle or a Wave?

 
Is it a Particle or a Wave?

What is a Wave?

Let's try to describe what a wave is. A wave is a moving disturbance in something. It has a pattern of crests and troughs. Think of these as water splashing in your backyard, where there are crests on top of the wave and troughs on the bottom of the wave. As it moves along the pattern, you can see it going up by a certain amount until it reaches its maximum. This is called the amplitude. If you get the distance between two crests or two troughs, that is the wavelength. The number of waves that pass through a fixed point is the frequency.

Particle-wave Duality

You may have heard that light may behave like a wave but also may behave like a particle sometimes. It's a simple fact that both light and matter have particle-like properties and also wave-like properties. This is called particle-wave duality. Light is generally thought of as a wave but in some experiments like the Young’s double slit experiment It behaves like a stream of particles. At first, this can be contradictory. How can it be both? Does it act like a wave or a particle more? Let's go back to the discovery in 1799 when an English physicist named Thomas Young experimented to show the nature of light. He got a beam of light that passes through the two slits and spreads out like waves. These waves interact with each other creating a pattern on the wall. The light may be diffracted in many different directions. For the white spots on the photo, the waves are “in phase”, while the darker spots show that the waves are “out of phase”. After this experiment, everyone believed that light was a wave.  

But hold on a second, what is in phase or out of phase? Interference explains this, when you add two waves that are in phase, meaning that the crests/troughs join. They create a greater amplitude. But if you have a crest and a trough joining, they cancel out.

Using Particles

Unlike particles, which have a definite position,  waves are continuous. If we look at a wave, there's no specific position. In a way, you consider it as a whole(moving wave), unlike particles, where you can count the number. What if instead of light as a wave, we used particles? If we used particles like electrons, it would still show the same pattern as before. The electron that we assumed was a particle was now behaving as a wave. So are they interfering with each other like interference in waves? Not quite, let’s try to slow the experiment by sending one electron at a time. There should be no interference. But somehow the pattern still appears? Even if there isn’t anything to interfere with. So, electrons interfering with themselves, as if they go through both slits at once? This surprising result shows they have wave-like behavior as a particle. From this, we gain even electrons have a wave-like nature, but if observed particle nature appears.

Problem with the Wave Model

The first hint of a problem with the wave model of light came from a German physicist. Max Planck. He was studying how objects emit light based on temperature. The problem was that there was no explanation for the shape of the emitted radiation. Wien’s law only fits short wavelengths and high temperatures, but doesn’t work for longer wavelengths. Explaining how much light of different colors was emitted shunned physicists. He knew that lots of light was emitted at low frequencies and tons of light was emitted at high frequencies, but at the peak of the spectrum, the light emitted was the brightest. This contradicted the wave model.

 Unfortunately, despite creating his formula, he tried to find the justification for this because each method he tried, produced more light at higher frequencies than it was actually seen, this is called the ultra violet catashrophe. So, in desperation, he proposed that the energy of the oscillators is quantized.

E = hf

 His trick was seeing a continuous wave as chunks, like a particle in the specific energy in his oscillators was quanta, but this didn't make sense before Young's experiment. Later on, the first person to take light seriously and who won a Nobel Prize well, was Albert Einstein. He explained it to the photoelectric effect.  

But waves alone couldn't explain everything…

A New Solution: The reintroduction of the Particle

What is the photoelectric effect? It's to describe when you shine light on a piece of metal and the electrons run out. It's like shaking the electrons off the metal. The wave model did not work for this because it should depend on the energy of light rather than the frequency. Theory did not match observations. When you observe them at low frequencies, you will never get the electron to run away. No matter how hard you shake, no matter how high the energy is. But at high frequencies, A gentle shake can fling the electron away because each photon carries more energy at higher frequencies. So, what did Einstein do? He did the same thing, applying Planck's formula to light itself. As we explained earlier. He described a beam of light as particles. If the energy of the single photon is more than needed, then the electron will knock loose. The higher the frequency, the higher the single photon energy, and the more energy the electrons get flung away. Energies lower than the minimum energy for knocking out the electron, nothing happens.

In the end, light and even matter refuse to be boxed into a single category as physicists wanted. It’s both a particle and a wave, depending on how we look, would go a long way.