Illuminating stuff! Luminescence: Light without heat

Shedding some light

To understand this impressive sounding example of light we’re going to need to understand what light is.

Light is a form of energy transfer, with different amounts of energy giving different colours.  The light we see is often released by atoms as a way of getting rid of energy.

In an old-style incandescent light bulb electrical energy from your mains supply is transferred to the bulb filament producing light and some heat too.  The electricity heats up the filament and gives it almost enough energy to pull the outer electrons out of an atom. We call this an excited state. When the electrons ping back into the atom (to the ground state) they release energy as light.

Luminescence

The big difference between luminescence and the light you get from a light-bulb is that luminescence is an example of light without heat. There are many types of luminescence; here we focus on chemiluminescence, photoluminescence, and triboluminescence.

Chemiluminescence

Blue and green glow sticks -an example of chemiluminescence. (Photo: David Mulheims, Public Domain)

The luminescence in a glow stick gives us light without heat (so they don’t burn our hands!) As the glow stick is cracked and the chemicals inside combine they form a new substance.  This new substance though, has an electron in an excited state.  As the electron moves back to the ground state the substance releases that extra energy as light.

 

Photoluminescence

In photoluminescence we have the same thing happening: an excited electron in an atom or molecule releasing energy as light as it moves to the ground state.  This difference is just in how the electron got excited in the first place.  With photoluminescence shining light on the molecule gives the electron the energy to jump into the excited state.  Sometimes this energy is from visible light, a great example of this is photoluminescent paper.

Blue light shining on photoluminescent paper

Science made simple staff shine blue light on photoluminescent paper
(Photo: science made simple CC-BY-NC)

tryityourselfplpaper

 

Sometimes the light that does the exciting is invisible light and that can be even more impressive. If you have access to a Ultraviolet (UV) lamp you can try this out.

photoluminescencetodoMany washing powders make use of this photoluminescence: chemicals which do just this are added to detergent so that they will absorb UV and emit blue light when on our clothes, making our whites look whiter than white! We look at UV along with other types of invisible light in our show Beyond the Rainbow.

Triboluminescence

This is luminescence caused by snapping!  In some substances if you pull apart the molecules quickly you get a flash of light. The positive and negative charges in some of the molecules are separated.  You do need to be somewhere really dark to see this but the equipment is very simple.

Photo: Andrew Mason (CC-BY)

Photo: Andrew Mason (CC-BY)

Take a polo mint and snap it in half.  You should see a flash of light as it breaks.  You can also try pulling some sticky tape quickly off the roll or crushing those posh sugar crystals. The effect has been used by Uncompahgre Ute Indians in night time ceremonies to produce rattles that glow when shaken.  For more on why dark skies can be great for doing science (and not only star-gazing) take a look at our Earth Hour blogpost.

 

Have a look at this clip via the Ri channel to see a source of luminescence that has been around for millions of years and how it’s helping in the search for new medicines!

Curriculum Links

England

KS3 Physics Waves

Light transferring energy from source to absorber, leading to chemical and electrical effects; photosensitive material in the retina and in cameras.

Colours and the different frequencies of light, white light and prisms (qualitative only); differential colour effects in absorption and diffuse reflection.

Wales

KS3 How things work

How familiar devices/machines work by using electricity, light, sound and other energy transfers.

KS4 energy electricity and radiations

Radiations, including ionising radiations, can transfer energy.

Scotland

Vibrations and waves

By exploring radiations beyond the visible, I can describe a selected application, discussing the advantages and limitations. SCN 3-11b

Northern Ireland

KS3

Light
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Posted in Activity, Physics