occurs when an electron in an atomic orbital absorbs energy from some
source (like an interaction with a photon, or a collision with another
atom) is promoted to a higher energy level and then, on 'falling back',
releases some of that energy in the form of visible light.The missing
energy is usually converted to thermal energy (infra red transitions),
making the tube slightly hot.
A fluorescent light is
- a type of gas discharge tube,
- it is made of glass and is narrow, with two electrical
connections on each of the metal caps that seal the ends of the tube.
- it contains an inert gas (such as argon, neon,
or krypton) and mercury vapour.
- The gases inside the tube have a pressure of about
- The mixture of mercury and gas is not conductive
when the tube is off.
How it works:
- A high voltage discharge is needed to start
the flow of current. The initial high voltage burst is necessary to produce electrons with enough kinetic eergy to ionise the mercury atoms - the atoms in the gas need to be sparse enough (low pressure) so that KE can be built up to a level that will allow ionisation of the mercury atoms.
- The high speed electrons cause the mercury atoms to ionize.
After this has taken place, the voltage can lowered - as the initial ionisations produce more of the same. The voltage required ranges from 100
volts for tubes under 30 watts and 100 to 175 volts for tubes of 30
watts or more.
- A pair of electrode filaments are located at the
metal ends of the tube. They produce electrons that ionise the mercury atoms. They do this by thermionic emission. The filaments remain hot when the tube is lit producing
a continuous electrical discharge.
- The flow of electrons through the gases excites
the electrons in the mercury atoms, which then emit ultraviolet
- There is a good reason why the lamp contains only a small amount of mercury, which must be vaporized to support the lamp current and generate light. At low temperatures the mercury is in the form of dispersed liquid droplets. As the lamp warms, more of the mercury is in vapour form - therefore more can be ionised by the electrons passing through it BUT at higher temperatures, self-absorption in the vapour reduces the yield of UV and visible light - so you cannot have too much mercury in the vapour form otherwise it will not produce light. Since mercury condenses at the coolest spot in the lamp, careful design is required to maintain that spot at the optimum temperature, around 40 °C.
- The inside of the tube is coated with a phosphorous
material that emits visible light when excited with UV and the tube gives off light. By carefully selecting the fluorescent
powders (called "phosphors"), the manufacturer of the light
can tailor the light's coloration. Phosphors in CF bulbs are a mix
to produce excellent color rendering and warmth similar to incandescent
bulbs. Other phosphor mixtures are warm white, cool white, deluxe
warm white, and deluxe cool white.
over the filament light bulb
- Fluorescent bulbs are four to six times more efficient
than incandescent bulbs.
- The lamp allows brightly lit workplaces to remain
at a cool temperature due to its greatly increased efficiency.
More detailed background reading
fluorescent tube (or bulb) consists of a partially evacuated glass
tube filled with mercury vapour. Only a small fraction of the gas
within the bulb is mercury vapour; argon gas atoms outnumber the mercury
atoms by about 300 to 1. Both kinds of atoms combined are only at a total
of about 0.3% of atmospheric pressure.
Fluorescent tubes and electroluminescent panels typically
require 200 to 600 V for starting and running illumination, so a voltage
of about 500 V is applied across the tube. This causes a small
percentage of the mercury atoms to ionize (release electrons).
These free electrons then accelerate towards the positive electrode and
collide with the mercury atoms on the way. If an electron's energy is
high enough, it can strip an electron off the target atom and create an
additional free electron. This collection of free electrons and residual
mercury ions classifies the argon-mercury combination as a plasma.
If the impact electron is of lower energy, excitation of electrons
in the orbitals around the mercury atom can occur. The mercury gives emission
lines at 254 nm and 367 nm (in the ultraviolet region of the electromagnetic
spectrum). See emission spectrum.
The light from fluorescent light bulbs looks white in
most cases, and that white colour is a combination (as it is with sunlight)
of all of the colours of the visible spectrum.
The inside of the fluorescent tube, or bulb, is coated
with a powder such as Sr5(PO4)3F mixed
with small amounts of rare earth elements. This powder (commonly called
a phosphor, although it may not have any phosphorus in it) gives off the
white light we see through a process called fluorescence, which is the
basis of the name fluorescent light bulb.The UV photons emitted by the
excitation of mercury orbital electrons are absorbed by the coating, resulting
in the emission of light in the visible portion of the electromagnetic
Ready for some questions?
Click on the icon - questions and answers at A level standard await you!
For more topics that have practice questions associated with them see the top menu bar - examination preparation