Gr 12 Physical Science Optical Properties – Emission and absorption spectra
Types of spectra and their origins.
In the 1900s it was found that a liquid or a solid heated to high temperatures would give off white light,
which could be dispersed using a glass prism into a wide range of colours, but a gas at low pressure
heated to high temperatures would emit light of specific colours or frequencies only.
Spectra can be explained in terms of the electron structure of atoms. Each atom has its own characteristic
electron structure and hence its own characteristic spectrum.
There are several different types of spectrum. For any one element the spectral types are related.
1. Continuum spectra
White light (from a white hot, incandescent (heated till glowing) solid or liquid) emits all the
possible frequencies of visible light. If a beam of white light is passed through a prism, the light is
dispersed into its component colours and a continuous or continuum spectrum results.
White
Light
Prism
http://www.physast.uga.edu/~rls/1020/ch6/prism.jpg
2. Line emission spectra.
These are spectra emitted by hot incandescent gases at low temperatures, or by glowing discharge
tubes. Line emission spectra of the elements can be studied by using discharge tubes and passing
the light produced by the tube through a spectroscope which causes the light to be dispersed into
its component colours.
1|Page
, Gr 12 Physical Science Optical Properties – Emission and absorption spectra
Below is a photograph of the discharge tubes that produce ‘neon’ lights.
http:/upload.wikimedia.org/wikipedia/commons/d/d3/Edelgase_in_Entladungsroehren.jp g
A discharge tube is a tube which is filled with a gas at low pressure with a high potential
difference applied across the electrodes. The diagram below illustrates the working of a discharge
tube.
Neutral Ionised
High voltage supplied atom atom of
to electrodes of gas gas electron
(positive)
Emitted photon of
Collision
light
resulting in
excited
state atoms
2|Page
Types of spectra and their origins.
In the 1900s it was found that a liquid or a solid heated to high temperatures would give off white light,
which could be dispersed using a glass prism into a wide range of colours, but a gas at low pressure
heated to high temperatures would emit light of specific colours or frequencies only.
Spectra can be explained in terms of the electron structure of atoms. Each atom has its own characteristic
electron structure and hence its own characteristic spectrum.
There are several different types of spectrum. For any one element the spectral types are related.
1. Continuum spectra
White light (from a white hot, incandescent (heated till glowing) solid or liquid) emits all the
possible frequencies of visible light. If a beam of white light is passed through a prism, the light is
dispersed into its component colours and a continuous or continuum spectrum results.
White
Light
Prism
http://www.physast.uga.edu/~rls/1020/ch6/prism.jpg
2. Line emission spectra.
These are spectra emitted by hot incandescent gases at low temperatures, or by glowing discharge
tubes. Line emission spectra of the elements can be studied by using discharge tubes and passing
the light produced by the tube through a spectroscope which causes the light to be dispersed into
its component colours.
1|Page
, Gr 12 Physical Science Optical Properties – Emission and absorption spectra
Below is a photograph of the discharge tubes that produce ‘neon’ lights.
http:/upload.wikimedia.org/wikipedia/commons/d/d3/Edelgase_in_Entladungsroehren.jp g
A discharge tube is a tube which is filled with a gas at low pressure with a high potential
difference applied across the electrodes. The diagram below illustrates the working of a discharge
tube.
Neutral Ionised
High voltage supplied atom atom of
to electrodes of gas gas electron
(positive)
Emitted photon of
Collision
light
resulting in
excited
state atoms
2|Page
