Humans have been dependent on alternate light sources in absence of natural ones, this tendency has pushed them to discover innovative ways to illuminate their surroundings. In this article, you will find how light is emitted from different artificial light sources humans have resorted to.
List Of Artificial Light Sources:
- Incandescent lamp
- Fluorescent lamp
- Halogen lamp
- LED Light
It is very important to know some basic terminologies and concepts before understanding the light-emitting mechanism of all the above-mentioned light sources.
The ultimate physical source of all light is; movement of electrons.
Simply speaking, in an atom, there are several electrons revolving in orbits around its nucleus. The number of orbits and electrons depends on the element they belong to. These electrons are at different energy levels. As they jump from one orbit to the other they change their energy level. When they jump from higher energy to lower energy levels they emit a particle known as a photon (a well-known light particle). In the same way, when an electron absorbs a photon it jumps from lower to higher energy levels. Photons give an extensive electromagnetic spectrum. Visible light is a part of it.
Visible light is actually a very small section of the broad 'electromagnetic spectrum' even if you include all the shades of different colors. It means; there is a lot more radiation around us than what our eyes can perceive. As a matter of fact, the radiation with lower frequencies, which our eyes can't detect is infra-red, radio, and microwaves. While higher frequency waves are ultra-violet, x-ray, and gamma rays which are invisible as well. Generally, electromagnetic radiation is absorbed and emitted by every object (atom) continuously.
As mentioned above when an electron jumps from a lower energy level to a higher energy level then it needs to absorb a photon (light), vice-versa when it jumps from a higher energy level to a lower energy level, energy is released by the atom as a photon (light).
The complexity lies in the fact, that when an electron jumps from higher to lower orbital the difference between orbital energy decides the frequency of the radiation. This simultaneously decides the wavelength i.e. color in case of visible radiation. Ex. when it jumps from 2nd to 1st orbital, its frequency will be lower, but if it jumps from 3rd to 1st orbital or from 5th to 3rd orbital the frequency would be greater.
Heat forces electrons to jump to higher orbitals, more heat pushes them to even higher orbitals. In such situations, electrons may instantly jump back to their original orbital emitting a very high frequency of radiation. In normal heat, radiation may fall in micro-waves or infra-red but when it becomes very hot; radiation may fall in the visible light spectrum. This is the reason why very hot objects emit visible light and this phenomenon is known as incandescence.
Whenever a light source is illuminated because of heat, incandescence is taking place. Many artificial light sources are using this property of matter.
NOTE: 'High-potential' the terminology used in 'electrics' is a different phenomenon. It concerns the number of electrons in a particular electrical conduction system.
Possibility Of Radiation In a Simple Atom With One Electron
Electricity passes through many different mediums; either solid or liquid. When we try to pass electricity through a gas, a special phenomenon occurs, called 'electric glow discharge'. Two carbon electrodes with a very high potential difference (voltage) are kept too close to each other and because of this: air or any other gas present in-between electrodes feel the force. Outermost electrons from the molecules or atoms of this air get separated and air between electrodes is ionized. As electrons are negative they move towards the anode and left-over positive ions move towards the cathode. In a fraction of a second electricity pass-through this plasma of ions and generate a spark. If there is enough thermal energy(heat) in the air that can sustain the availability of electrons and ions then this spark doesn't vanish and becomes constant, then it is known as an arc. Let us understand from where does light comes from in the spark; the aligned movement of electrons and ions discussed above is not so smooth, they randomly collide with each other in their path towards electrodes. Photon (light) emission is achieved in the following manner:
- When a free electron collides with an ion and forms a bond, the ion achieves a lower energy state again, resulting in photon emission.
- When a free electron collides with an ion and it excites another electron present in the ion to a higher energy level, the excited electron again falls back to its lower energy state resulting in light (photon) emission.
Wait! as far as we know, carbon arc lamps give light as bright as daylight! is this all illumination comes out from this spark/arc only? What is the role of the carbon rod here? If it is all about electrical potential difference then any metal or conductor can do, why only carbon?
When electrons and ions physically move towards electrodes, lots of vibrational energy is produced due to collisions, which results in heat. Temperature becomes so high that carbon rods start to vaporize at their tip, this vapor is so hot that it instantly burns and emits light. This burning is the main source of light in carbon arc lamp, its easy to burn carbon compared to other elements which emit white light.
When electricity is passed through a gas, a spark/arc is produced. Every particular gas has a specific emission spectrum at low pressure. When pressure increases, electrons in the atoms become more diverse & varied in their ground-state because of too much surrounding force. Hence electric discharge in high-pressure gas produces more varied wavelengths of radiation compared to low-pressure.
This property of high-pressure gas discharge is exploited in the 'Mercury high-pressure lamp'. When voltage is applied to electrodes argon gas is ionized and spark/arc is produced which heats the mercury present in the same capsule. Initially, the mercury starts to vaporize and adds up into plasma which emits blue light. Gradually when the mercury vaporizes, pressure increases, this also increases the flow of electric current. Discharge from high-pressure gas emits more intense white light because of more atoms involved and more variation of 'electron energy levels.
Total illumination in the lamp is due to the ionization of the gases. When electricity passes through mercury vapor it produces a stationary spark(arc). The arc here is very powerful, compared to carbon arc it is more efficient because no replacement is required.
Mercury Vapor Lamp Working
The mechanism of producing light in the metal-halide lamp is similar to a 'carbon arc lamp'. The surrounding in which arc is formed here is made up of very specific gases. As the name suggests, the fundamental gas in which the arc is formed is metal-halides like sodium iodide or scandium iodide, mixed along with argon/xenon and mercury.
Initially when voltage is given, argon gas is ionized and spark/arc is produced which heats the mercury and metal-halides present in the same capsule, mercury slowly starts to vaporize and adds-up into plasma which emits blue light, and the temperature further increases which vaporizes the halides, the arc becomes more intense and adds-on more colors to the spectrum.
Total illumination in the lamp is due to the ionization of the gases and when electricity passes through these gases it produces a stationary spark(arc). The arc here is very powerful and efficient.
Working of Metal-Halide Lamps
Fluoresence is a scientific terminology, which refers to a phenomenon; where an electron of a molecule absorbs a photon(radiation) and jumps to a higher energy orbital, instantly it jumps back to its initial energy state, emitting a photon. Question arises; what is the meaning in absorbing a radiation and emitting again a radiation instantly? Fascinating fact is that in 'fluorescence'; emitted radiation is always of lower energy (long wavelength) than absorbed higher energy radiation(short wavelength). So when radiation of invisible short wavelength (ultraviolet and x-rays) is absorbed by some specific materials, longer visible wavelengths (light) is radiated back and rest of the energy goes in molecular vibrations. Same thing is happening in fluorescent tube light and lamps: with the help of electricity short invisible radiation (x-ray) is produced and it is absorbed by flourescent material present in the tube, which inreturn emit visible radiation(light).
Candle / Combustion Based Light Source
Before invention of electricity all light sources were combustion type. Light is produced by the flame when the fuel burns. So here we just need to see what it is in the flame which emits light.
In a candle, or any other combustion type light-source, flame is initiated by means of friction or heat, and fuel is burned in a controlled manner which produces a flame which is ultimate light source.
Combustion is a chemical reaction in which fuel is changed into carbon dioxide, nitrogen oxide and water. When this reaction takes place it releases lots of heat energy turning surrounding air very hot, this hot air is actually visible to us as flame.
Here, the electricity is used to raise the temperature of 'tungsten metal' to such a limit that it produces light. Basically electricity is: movement of electrons towards low potential within the conductor, when electrons move they randomly collide and transfer vibrational energy to the atoms of the metal, this is actually heat energy, it forces electrons in atom orbitals to jump-up higher orbitals and when they fall back, light is emitted.
Electrons in Tungsten, and all other metals occupy too many different levels of orbitals compared to lighter elements, so when they get excited and fall back many different combinations of energy differences are achieved, these diffefent colors of light makes-up to white light. Tungsten is a metal which has very high melting point and tensile strength, that's why it is preferred upon other metals.
Halogen lamp is advanced version of tungsten/incandescent lamp. In simple incandescent lamp tungsten coil is suspended in vacuum glass bulb to protect it from atmosphere. Although when it lights-up it gets hot and gradually evaporates, this vapor gets stuck to the glass surface, eventually when the coil gets too thin, it breaks down and becomes useless.
Instead of keeping tungsten coil in vacuum, when an inert gas is filled into the bulb with presence of halogen and iodine a chemical process 'halogen cycle' takes place. This reaction deposit evaporated tungsten particles again to the coil, hence the life of coil is increased. It also allows to raise temperature of tungsten hence more light is emitted from it.