Lightbulbs
The lightbulb is a necessity we use everyday. How has the chemistry changed since Thomas Edison’s famed invention?
One does not usually associate the phrase “life changing” with “lightbulbs,” but in the early 19th century when the lightbulb was first discovered, it was life changing. Artificial light allowed people to extend their hours of productivity by completing tasks and picking up hobbies after sunset. Today, lightbulbs are so commonplace that you probably don’t notice just how often you interact with them. Let’s explore how lightbulbs have evolved since its invention.
Thomas Edison is credited with the discovery of the lightbulb in 1879, however it is worth mentioning scientists Joseph Swan and Sir Humphry David for their contributions. The first lightbulb invented was an incandescent lightbulb. These lightbulbs use a Tungsten (W) filament – a long, thin strip of Tungsten – connected to an electric circuit. The Tungsten filament has a high electrical resistivity. This means that as the current reaches the filament, the electrons have a hard time passing through the Tungsten. This leads us to one of the most important concepts in science: The Conservation of Energy. Energy cannot be created nor destroyed; it can only be converted. Because of the high resistivity in the Tungsten, most of the current is converted to heat and causes the Tungsten filament to reach temperatures of 2500 °C. At these high temperatures, the electrons in Tungsten have an excess of energy and emit the familiar yellow light.
To prevent the lightbulb from lighting on fire, the filament must be shielded from the oxygen in the atmosphere. To achieve this, incandescent lightbulbs are often filled with a noble gas like Argon (Ar). Noble gases have a full count of electrons making them inert; they do not react. Argon prevents the Tungsten filament from degrading which prolongs the life of the lightbulb.
The chemistry of the lightbulb has evolved since the incandescent lightbulb, and for good reason. An incandescent bulb transforms most of the current into heat, which then creates light. This process is purposefully inefficient and draws a lot of energy. The next invention in the lightbulb saga is the fluorescent lightbulb. These lightbulbs are filled with two gases: Mercury (Hg) vapour and an inert, noble gas. As current reaches the gas mixture, the energy excites the gas molecules and their kinetic energy increase. The gas molecules will collide with each other and consequently excite Mercury’s valence electrons. Using the same principle as incandescent lightbulbs, the electrons will return to their ground state and emit light. However, Mercury will emit UV light which is invisible to the human eye. To transform the UV light a phosphor coating on the inside of the bulb is used. The UV light excites the electrons in the Zinc (Zn) sulphide phosphor coating, which re-emits a photon in the visible light spectrum that we can see!
So far we have incandescent lightbulbs (which are fire hazards) and fluorescent lightbulbs (which have the toxic element Mercury), so today, the most common type of lightbulb is the light emitting diode (LED). LEDs use two semiconducting layers composed of Gallium (Ga), each respective layer mixed with other elements. The two Gallium based layers have an energy gap between them. As current reaches the semiconductor layers, electrons are excited and then release light proportional to the energy gap. By fine tuning the elements and proportions in the two semiconductor layers, chemists can change the energy gap and consequently change the light that is emitted. That allows LEDs to emit any colour of light! LEDs are very energy efficient and their tunability makes them very popular.
As the lightbulb has evolved over hundreds of years from tungsten filaments and mercury vapour to semiconductors in LEDs, the chemical principle of electrons transforming their energy into light has remained the same. And with that, I hope this article has enlightened your chemistry knowledge about lightbulbs!
