At the beginning, the atom was suggested to have a spherical shape (Dalton, ca 1700). After that, there have been a lot of researches to describe what happen inside the atom. The early modern structure of atom was suggested by Ernst Rutherford. He suggested an atom is someway similar to the solar system but there is a problem in this model. If the electron orbits the nucleus, so the interaction between electron and nucleus is electrostatic force. By this interaction, the electron will be accelerated along its orbit and it causes the electron will form have spiral orbit, and it will collapse to the nucleus with lifetime almost 0. If we refer to this model, the result in spectroscopy would show a continuous spectrum, but the result is a discrete or line spectrum. Spectroscopy itself shows interaction between matter and radiation. The theory behind spectroscopy cannot apply the classical mechanics, but another mechanics theory should be applied for object that very small such as atomic particle, and it is called the quantum mechanics.
Spectroscopy basically operates as shown in schematic diagram below.
A sample is prepared and it is shined by a light source. Then, as the sample is radiated, there are interaction between sample and the light that will be detected by the detector. If the detector measures the intensity of the light that is absorbed by sample, the result will be absorption spectrum. In the other sides, if the detector measures the emission from the sample it will be a emission spectrum.
From this experiment it is shown that an atom has wave properties, besides its particle properties. This phenomenon is called wave-particle duality. As shown in H atom spectrum, it is emitted electromagnetic radiation, and it travels in waves and it carries energy. To begin the discussion about wave-particle duality, let’s see the wave properties first.
1. Wavelength (λ)
Wavelength can be described as the shortest distance between two equivalent points in the wave and it is measured in unit of length. Mostly, for practical reason it is commonly measured in micrometre (μm) or nanometre (nm). Distance from A to B is a wavelength.
2. Frequency (ν)
Frequency is the number of oscillation every unit of time. Since number of oscillation is dimensionless, so the unit is s-1 or Hz.
3. Wavenumber ()
Wavenumber is the number of oscillation per unit of length and it is commonly measured in cm-1 or m-1. The relationship between wavenumber and wavelength is:
There is no definite description of amplitude, because it depends on the type of wave. Generally, it is the maximum or the minimum point at the wave, or mostly called the height of the crest or the depth of the trough of the wave.
In this case, velocity can be defined as how far the wave travel in unit of time and it is measured in m s-1. Velocity is a vector quantity, so it has direction and magnitude. Electromagnetic wave travels at the speed of light (c) in vacuum (in vacuo). Therefore, the relationship between c and the other wave properties is:
From the description above, all the wave properties are not independent with each other.
As electromagnetic wave, Maxwell describes the propagation of electromagnetic wave as shown below:
The wave is synchronously oscillating which means it is mutually perpendicular varying sinusoidal wave with the position of time. Moreover, a monochromatic radiation is a single frequency.
The wave properties of electromagnetic radiation are also experimentally proven from the diffraction experiment. A beam of electromagnetic wave shine through a small gap which has the separation of λ and it produces a pattern bright and dark.
The electromagnetic wave also has particle properties. This was proven by an experiment that a plate of alkali metal (e.g. Cs) in vacuum was shot by light. In a light there are numbers of wavelength which carries different energy in a package that called photon. Then, a detector was placed and it confirmed that there are electrons were knocked out from the metal plate. This phenomenon is called Einstein’s photoelectric effect. Moreover, not all wavelengths can knock out the electron from an atom. Only the precise wavelength (so it implies the precise energy) can knock out the electron from an atom, it will not work if the energy is too low or too high. The energy that is required can be calculated as: