In a previous entry I talked about the nature of time. In that post I mentioned the speed of light casually. Today I want to talk about what makes light move. We all know that light travels in a straight line but why it travels at all is a mystery to most of us. Why is it that when we light a torch or a light bulb in dark room, light travels and fills the room?
How light is formed.
Light is produced when an electron is excited and then ‘loses’ the excitation. Atoms have a nucleus containing protons, neutrons and other subatomic particles this nucleus has electrons around it. The electrons are arranged into levels that are called energy levels. The electrons closer to the nucleus have lower energy than the ones farthest.
So, when enough energy is added to an electron, it is able to absorb this energy. When sufficient energy is absorbed, the electron moves from its energy level (ground state) to a higher energy level (excited state).
There is always an electric field that is created between the proton (positively charged) and the electron (negatively charged)
It has also been shown that whenever an electric field is created, a magnetic field is also induced that is perpendicular to the electric field.
However, the energy of elementary particles are quantized i.e. the electron can only carry a specific amount of energy (quanta). This is analogous to a bottle with a hole at the side, if you add water into the bottle from a fast flowing water tap, chances are the bottle will be filled with water. When the tap is closed then water oozes from the hole until the water level in the bottle coincides with the hole. This is what we observe when the energy source of the electron is removed. The electron losses this excess energy.
This excess energy is produced in the form of light. This is because interaction between electric and magnetic fields at right angles produces electro-magnetic waves; light is a form of electromagnetic waves (other forms include gravitational waves).
Light as a wave-particle
Before the birth of quantum mechanics, light was largely described as a wave because it had the characteristics of a wave; it had an amplitude, frequency and wavelength. However the discovery that light also had characteristics of particles such as having quantized energy and photoelectric effect.
Depending on the experimental setup, we can find light being both a wave and a particle. If we set our experiment to test light as a wave, then a wave we get and if out experiment want to investigate its particle nature, then a particle we get.
Therefore light becomes a wave and a particle.
What pushes light?
When light travels it does not travel as a particle or a wave. It does not travel as a particle because to do this it will mean that it will need momentum, uniform acceleration or uniform velocity. It also does not travel as a wave because mechanical waves need a medium for propagation and light does not.
During the formation of light, the energy is converted to a particle (mass) using E=mc2. But we know that photons do not have mass. The particle has a momentum (p) defined by Planck’s constant (h) and wavelength ƛ as;
After some mathematical calculations, we prove that the particles are massless and are ‘born’ with the speed of light of 3X108 m/s. if you are interested in the derivation of the math click here.
Bottom line is that light moves because when a photon is formed, the interaction of the electric and the magnetic fields gives it that momentum. Light is born running.
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