Quantum Physics Tutorial 5 - Fluorescence

Fluorescent Tubes

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):

 

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).

 

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.

 

 

Advantages:

 

Disadvantages

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 a "six-pointer".  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 half a side of A4, about the upper limit for many physics students.

 

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.

 

Question 1 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.

Answer