Dark Flames

28 September 2009


If you put metal salts into a flame they can glow bright colours, copper salts glow blue or green, strontium red, barium green, and one of the brightest and most common is sodium which glows bright orange.

Sodium Chloride in a flame

Sodium is so good at emitting light that it is used in streetlights to emit the same orange light. They work by essentially passing a spark through low pressure sodium gas, which gives the atoms energy and causes them to glow very brightly.

If you put a sodium flame in front of a streetlight, something rather fascinating happens.

It will also work in an area illuminated by a streetlight (though it does have to be an old fashioned yellow low pressure sodium streetlight).

What is going on?

Normally, if something can emit light of a certain colour, it can also absorb light of this colour. Sodium gas is very very good at emitting a certain orange light, which means that it is also very very good at absorbing it. So if you put a sodium flame in front of a streetlight the sodium atoms will absorb light coming from the lamp. This blocks most of the light from the lamp making the flame look dark.

Most of the absorbed light is immediately re-emitted, but in a random direction so the flame will actually look brighter than normal, but it is still less bright than the lamp so the flame looks dark.

Sodium flame in front of a streetlight Sodium flame in front of a streetlight
The sodium in the flame absorbs the light from the streetlight, and then re-emits it in random directions This means that the flame looks dark when it is put in front of the lamp.

This is how astronomers can discover the make up of a star millions of light years away. Each element in its atmosphere will absorb certain colours of light, leaving a characteristic fingerprint in the starlight. In fact Helium was discovered on the sun before it was found on earth, hence why it is named after Helios the Greek god of the sun.

Solar spectrum showing where various wavelengths have been absorbed by various elements in the sun's atmosphere including Hydrogen (H), Calcium (Ca), Iron (Fe) and Magnesium (Mg)

Why is sodium so good at emitting and absorbing orange light?

Atoms are very complex quantum mechanical objects made up of  a positively charged nucleus and negatively charged electrons. These electrons can only exist at certain energy levels and not at energies in between.

Atomic energy levels Heat can give electrons energy
The electrons in an atom can only have certain amounts of energy The electrons can move up energy levels by absorbing energy from for example heat.

The electrons can move between energy levels. so the atom can absorb or emit certain energies. If an electron if moved into a higher energy level it is excited, and can release this energy in the form of light. So the light the atom given out will be of only certain energies, as the light can only be emitted by electrons ,oving between the energy levels.

Atomic emission Dropping multiple levels
An electron can emit a photon of light by dropping its energy level. Different energies (colours) of photon can be emitted by moving between different levels

As the energy of a light particle (photon) is related to its wavelength and therefore its colour, atoms can only emit certain colours of light.

If an atom can emit a certain energy (colour) then it can also absorb the same energy (colour). Sodium has one (technically 2 very close together) very strong transitions between energy levels which corresponds to yellow light, and very few transitions of a lower energy. So if a sodium atom is excited and has energy, most is emitted as yellow light.

Atomic emission Absorbing a photon
If an atom has the energy levels to emit a colour of light. It also has the right energy levels to absorb it.

Do all things have energy levels?

Yes but some have so many energy levels that you can't see the gaps. This is particularly the case with solids, where the surrounding atoms and bonds can distort an atom's energy so as a whole the solid looks like it has a smooth spectrum.

Why do atoms have energy levels?

This is a question about quantum mechanics which is very complex, and virtually any answer is a simplification, but one way of thinking of it that electrons are in some ways like a wave, and they are orbiting the positive nucleus.

This means that the electrons' waves overlap with themselves. If the wavelength doesn't correspond to the circumference of the the orbit then the wave will cancel itself out. However if it is 1, 2 or more whole wavelengths then the wave will reinforce itself and get stronger forming a standing wave. These are the states the electron is stable in, and these have specific energies, as different wavelengths are associated with different kinetic and potential energy.

Rotational mode 1.5 Cirular wave 2 nodes
If a wave is traveling in a circle and it doesn't meet itself it will cancel itself out. If the circumference is a whole number of wavelengths around it will reinforce itself.
Cirular wave 3 nodes Circular waves 4 nodes
Other stable solutions are also possible with more waves around the circle.

Real life is of course 3D, the waves are not constrained to a ring, and you also get radial standing waves with different potential energies, but it works on the same general principle.

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