Quantum Physics Tutorial 5 - Fluorescence
If photons strike materials, and they have exactly the right energy, electrons in the atoms will be excited, and be raised to higher energy levels. The photon has to have exactly the right energy for this to work. If it doesn’t, the photon will not be absorbed.
Note that atoms can be ionised by photons that have energies greater than the ionisation energy. The energy value does not have to be exact.
We have seen how the electron goes back to its ground state after being excited. It emits a photon according to the difference in energy levels. It’s a bit like falling down the stairs (do not try this at home):
You can fall down the whole flight of stairs;
You could bounce from stair to stair;
Or any combination…
The bigger the fall, the more it hurts…
There are huge numbers of electrons being excited and falling back to their ground state, so all possible transitions are being made at the same time.
The bigger the transition, the more energy is involved, so the shorter the wavelength.
If some materials are exposed to ultra violet light, they absorb the UV radiation, and will reradiate it. Some electrons will fall straight back to their ground state, giving off UV light, but others will go through several lower energy transitions, giving off visible light. They appear to glow under UV light, a phenomenon called fluorescence. There are a number of materials that can be made to fluoresce.
Both pictures by Pieter Kuiper (Wikimedia Commons)
The fluorescent tube (found in all offices) uses this effect. It is full of mercury vapour. It is set up like this:
With a fluorescent coating, they are very useful (even if rather unpleasant).
Ionisation of mercury atoms occurs because electrons collide with the atoms and knock electrons off.
Electrons then go back to the mercury atoms and return to the ground state. The emit photons as they fall down the energy ladder.
Some photons are in the visible range, but most are in the UV range.
A fluorescent coating absorbs the UV light and electrons in the coating material are excited. They are raised to a higher energy level.
Then they fall back to their ground state. They emit photons of visible light.
The way the fluorescent tube works is shown in the picture:
The white light is made from a mixture of red, green, and blue photons. These arise from different transitions of the electron as it falls through different energy levels, as shown on this simplified representation.
Fluorescent tubes give a slightly green colour cast on photographic film that is designed for daylight. This indicates the presence of more photons in the green region.
Advantages and disadvantages
You could well be asked a question about the advantages and disadvantages of fluorescent lighting.
They use a lot less electricity than an ordinary filament lamp (incandescent lamp). An 11-watt fluorescent lamp gives out as much light as a 60-watt incandescent lamp. Therefore it makes sense to use fluorescent bulbs as less energy is wasted as heat.
Also the bulbs last a lot longer than the incandescent lamps.
Soon incandescent lamps will not be available.
Fluorescent lamps are NOT environment-friendly. They have mercury in them, although it is in small amounts. Mercury is a highly toxic heavy metal, and breathing in mercury vapour is dangerous. If a fluorescent lamp is broken, then there are risks to people in the room.
Disposal has to be carried out with care, as mercury could get into ground water.
They are expensive to buy.
They may last longer, but they do not last for ever. A flickering tube is unpleasant and distracting.
A normal fluorescent tube flickers at 50 Hz. Stroboscopic effects can be seen. Nowadays there are high frequency fluorescent lamps that avoid this, but are a lot more expensive. If they fail, the whole fitting has to be replaced.
While it is easy to replace an incandescent bulb with a brighter one, the same is not true for fluorescent lamps. New fittings have to be bought.
Although a lot of effort has been put in to make the designs more homely, they still look unpleasant.
The light is harsh and can make people feel unwell.
They cannot be dimmed easily.
Photographs taken with daylight film would show that the fluorescent lamps would give a slightly green tinge to the pictures. A filament lamp gives an orange tinge. The effect is less marked with digital cameras, as most cameras will compensate for any colour cast, but you can still see the orange tinge to the lamp behind the door.
Many people still prefer the warmth of a filament lamp, and will continue to buy them in preference to the fluorescent fittings.
In the exam, fluorescence is quite a likely subject for an essay question (worth 6 marks in the AQA papers). To do well in these questions, you must not only get all the points, but also present them in good English. You will be expected to write about three quarters of a side of A4, about the upper limit for many physics students doing the AQA syllabus.
The question below gets you to consider the points you would raise when attempting one of these questions. The key thing is to keep your answer short and to the point. Do not be tempted to add unconsidered supplementary material (waffle) as it not only will not give you any extra marks, but also you could lose marks due to contradictions. Use good English and well constructed sentences. Keep the sentence structure simple. Avoid figures of speech like metaphors; you are not writing a great literary masterpiece.
Explain in as much detail as you can how the electrical energy that is put into a fluorescent tube in a classroom (or office) is converted into light energy that is useful in lighting a room.
The most modern form of lighting is based on LEDs. Their light is much more pleasant than fluorescent lamps. They use less electricity. This one takes 3 watt (compared to 33 W for the filament equivalent). They are much more expensive, though.
They are meant to last for tens of thousands
of hours. Except they don't.