The Focas Institute Working with Spectroscopy
The Focas Institute Workin
The mysteries of light
Light photons and electrons
Light has always intrigued scientists. Interest really exploded
when Newton discovered that visible ‘white’ light is composed
of a series of coloured lights. He used a prism to split white light
into its constituent colours and another prism to recombine
the colours as white light. He published his major work on
light called Opticks in 1704. His theory of ‘particles’ was
generally accepted until the wave theory of light emerged.
Our modern theory tells us that light is emitted and absorbed in photons.
A photon is considered to be a particle having no mass, no electric
charge, and an indefinitely long lifetime. A photon is a packet or quantum
of electromagnetic energy.
The energy of a photon is given by e = hf where h is Planck’s constant
and f is the frequency of the light. The higher the frequency, the greater
the energy so a photon of blue light is has more energy than one of red.
Ultra-violet photons have even more energy; whereas infra-red photons
have less.
The mysteries of light
Light photons and electrons
Many famous experimental and theoretical scientists have contributed to
the process leading to our current level of understanding of light as a form
of electromagnetic radiation. Such scientists include Maxwell, Hertz, Bohr,
Michelson and Einstein. We now know that light behaves like straight line
rays and also like waves. It also behaves like particles, which we now call
photons.
Higher frequency (f) also means shorter wavelength (_) because the very
Ourc modern
well known equation tells us that f_ = c where
is the speedtheory
of light. tells us that light is emitted and absorbed in photons.
Since the speed of light is a constant. If f increases then _ must decrease.
A photon is considered to be a particle having no mass, no electric
In short, if we know the frequency f we know the wavelength _ or vice
charge, and an indefinitely long lifetime. A photon is a packet or quantum
versa.
Light has always intrigued scientists. Interest really exploded
when Newton discovered that visible ‘white’ light is composed
of a series of coloured lights. He used a prism to split white light
of electromagnetic energy.
Fig.2 Example of emission spectrum for Helium
intoisits
colours
and another prism
to
recombine
What
theconstituent
electromagnetic
spectrum?
The energy levels of atoms
The
energy
of
a
photon
is
given
by
e
=
hf where h is Planck’s constant
Lightthe
is notcolours
the only formas
of electromagnetic
radiation.
total range
white light.
HeThepublished
his
work
onexist in particular states or energy levels. In
In an major
atom, electrons
can only
of electromagnetic radiation is called the electromagnetic spectrum. The
useful
for identifying
substances. Forthe
example,
when
and
f is
thelevel,
frequency
ofSpectroscopy
the light. isThe
higher
the frequency,
greater
hydrogen, the electron normally occupies the
lowest
energy
which
lightis usually
called
Opticks
in 1704.
His below
theory of
‘particles’
spectrum
divided
into seven sections.
This is shown
electrons in a helium atom move from one state to another, photons of a
is called
the ground state.was
To make an electron change (or jump) from this
the energy so a photon of
blue
light is
more
energy
one
of red.
along with the constituent colours of visible light.
distinct
frequency
arehas
emitted
or absorbed.
Thethan
presence
of light
of this
ground state to the first excited level requires an ultra-violet photon, as this
generally accepted until the wave theory ofislight
emerged.
is a strong
unique indicator
or fingerprint
of thephotons
presence of
Ultra-violet
photons
even more
energy;
whereas
infra-red
a relatively big step. Each succeeding energy
level requires
a smaller havefrequency
hydrogen.
step of energy. To go from the first excited level to the second, third, fourth
have less.
fifth levels, requires only a photon of visible light. Conversely, when an
Many famous experimental and theoretical scientistsor
have contributed to
One practical example is measuring pollution. The spectral lines for various
electron descends to a lower level it emits energy.
pollutants in the air are known. If light is shone through the atmosphere and
Component Colours
the
process
of light as a form
Higher
frequency
(f)
also
shorter
wavelength
because
the very
Radio
Wavesleading to our current level of understanding
ismeans
found to have
a measured
reduction in (_)
strength
at the frequencies
of a
presence or absence of energy at particular wavelengths can be
of Visible
White
Light includeThe
of electromagnetic radiation.
Such
scientists
Maxwell,
Hertz,
Bohr,
particular
pollutant,
thencthe
existencecand
of that
wellas known
tells
us that
f_ =
where
is concentration
the speed
of pollutant
light.
detected using a spectrometer. The trace appears
visible linesequation
known as
can
be
established.
This
is
the
basis
of
atmospheric
pollution
monitoring,
spectral
lines.
Electrons
descending
to
the
fi
rst
excited
level
produced
the
Michelson
and Einstein. We now know that light behaves like straight line
Since the speed of light isonea ofconstant.
increases
then
Radio Waves
the activities If
of fthe
Focas Institute,
DIT. _ must decrease.
Red
four visible lines that Angstrom first measured in hydrogen and for which
rays
and also like waves. It also behaves like particles,
which we now call
In short,
if we
know
Balmer found the formula. These lines are called
the Balmer
series.
The the frequency f we know the wavelength _ or vice
Microwaves
Orange
The spectra of many molecules and of parts of molecules are well known
reason these lines were found first, rather than those emitted by electrons
photons.
versa.
Infrared
Yellow
Visible Light
Green
Ultraviolet
Blue
Gamma Rays
Violet
descending to the ground state, is that good ultra-violet detectors were
not available. Lines emitted by electrons descending at the higher energy
levels are infra-red and these were also identified later.
and this information can be used to identify known substances or, indeed,
discovering new ones.
Spectroscopy is an amazingly powerful tool. It can tell us what is
happening on the sun. Examination of missing lines in the sun’s spectrum
The energy required for an electron to move from a level with energy E2
X-Rays
What
is the electromagnetic
spectrum?
Indigo
The energy levels
ofpresence
atoms
showed the
of hydrogen and an unknown substance which was
to one with energy E1 is E2 - E1 = hf. This gives exactly the energy and
called helium. The relatively cooler gases in the sun’s atmosphere absorbs
frequency of the photon which is emitted or absorbed when the electron
Light is not the only form of electromagnetic radiation.
The total range
at these
as it leaves
the sun,
making dark
lines in In
the
In an atom, electrons canlight
only
existwavelengths
in particular
states
or energy
levels.
makes the transition.
continuous spectrum. Later, helium was also found on earth.
of electromagnetic radiation is called the electromagnetic spectrum. The
hydrogen, the electron normally
occupies the lowest energy level, which
In molecules, there are many different energy levels that an electron
spectrum is usually divided into
seven sections. This
is shown below
that the
universe(or
is expanding.
starsthis
move
is called
theAccordingly,
ground state.Spectroscopy
To makealso
an revealed
electron
change
jump) As
from
Fig.1 Visible Spectrum
can occupy, so there are many states to move
between.
away from us the wavelengths they emit seem longer than they should
along with the constituent colours of visible light. molecular spectra contain many lines, often clustered
in bands,
ground
stateand
to many
the first excited
level requires an ultra-violet photon, as this
of these may be in the infra-red.
Radio waves have the longest wavelengths (105 _ 10_3 metres) while
gamma rays are the shortest (10_11 _ 10_14 metres).
The term electromagnetic derives from the fact that the wave is a result of
changing electric and magnetic fields. All electromagnetic radiations travel
in a vacuum at the same speed, generally referred to as the speed of light.
Radio Waves
be because of the Doppler effect. Because the wavelengths have moved
toward succeeding
the red end of the energy
spectrum, this
is also
called red shift.
is a relatively big step. Each
level
requires
a smaller
step of energy. To go from the first excited level to the second, third, fourth
Spectroscopy is a powerfulortool
fifth levels, requires only a photon of visible light. Conversely, when an
Spectroscopy is the scientific study of spectra
and descends
spectral lines.to a lower level it emits energy.
electron
A spectroscope is the instrument used to disperse a beam into its
Component
Colours
component
frequencies. This dispersion can be done using a prism, but
instruments generally use a diffraction
grating.
The
presence or absence of energy at particular wavelengths can be
of Visiblemodern
White
Light
Radio Waves
Red
Microwaves
Orange
Infrared
Yellow
Visible Light
Green
Ultraviolet
Blue
X-Rays
Indigo
Gamma Rays
Violet
detected using a spectrometer. The trace appears as visible lines known as
spectral lines. Electrons descending to the first excited level produced the
four visible lines that Angstrom first measured in hydrogen and for which
Balmer found the formula. These lines are called the Balmer series. The
reason these lines were found first, rather than those emitted by electrons
descending to the ground state, is that good ultra-violet detectors were
not available. Lines emitted by electrons descending at the higher energy
levels are infra-red and these were also identified later.
The energy required for an electron to move from a level with energy E2
to one with energy E1 is E2 - E1 = hf. This gives exactly the energy and
frequency of the photon which is emitted or absorbed when the electron
makes the transition.
Fig.1 Visible Spectrum
In molecules, there are many different energy levels that an electron
can occupy, so there are many states to move between. Accordingly,
molecular spectra contain many lines, often clustered in bands, and many
of these may be in the infra-red.
Radio waves have the longest wavelengths (105 _ 10_3 metres) while
gamma rays are the shortest (10_11 _ 10_14 metres).
Spectroscopy is a powerful tool
The term electromagnetic derives from the fact that the wave is a result of
changing electric and magnetic fields. All electromagnetic radiations travel
in a vacuum at the same speed, generally referred to as the speed of light.
Spectroscopy is the scientific study of spectra and spectral lines.
A spectroscope is the instrument used to disperse a beam into its
component frequencies. This dispersion can be done using a prism, but
modern instruments generally use a diffraction grating.
The Focas Institute Working with Spectroscopy
AND
SCIENCE TECHNOLOGY
ACTION
IN
king with Spectroscopy
The mysteries of light
Light photons and electrons
Light has always intrigued scientists. Interest really exploded
when Newton discovered that visible ‘white’ light is composed
of a series of coloured lights. He used a prism to split white light
into its constituent colours and another prism to recombine
the colours as white light. He published his major work on
light called Opticks in 1704. His theory of ‘particles’ was
generally accepted until the wave theory of light emerged.
Our modern theory tells us that light is emitted and absorbed in photons.
A photon is considered to be a particle having no mass, no electric
charge, and an indefinitely long lifetime. A photon is a packet or quantum
of electromagnetic energy.
Many famous experimental and theoretical scientists have contributed to
the process leading to our current level of understanding of light as a form
of electromagnetic radiation. Such scientists include Maxwell, Hertz, Bohr,
Michelson and Einstein. We now know that light behaves like straight line
rays and also like waves. It also behaves like particles, which we now call
photons.
The energy of a photon is given by e = hf where h is Planck’s constant
and f is the frequency of the light. The higher the frequency, the greater
the energy so a photon of blue light is has more energy than one of red.
Ultra-violet photons have even more energy; whereas infra-red photons
have less.
Higher frequency (f) also means shorter wavelength (_) because the very
well known equation tells us that f_ = c where c is the speed of light.
Since the speed of light is a constant. If f increases then _ must decrease.
In short, if we know the frequency f we know the wavelength _ or vice
versa.
Fig.2 Example of emission spectrum for Helium
What is the electromagnetic spectrum?
The energy levels of atoms
Light is not the only form of electromagnetic radiation. The total range
of electromagnetic radiation is called the electromagnetic spectrum. The
spectrum is usually divided into seven sections. This is shown below
along with the constituent colours of visible light.
In an atom, electrons can only exist in particular states or energy levels. In
hydrogen, the electron normally occupies the lowest energy level, which
is called the ground state. To make an electron change (or jump) from this
ground state to the first excited level requires an ultra-violet photon, as this
is a relatively big step. Each succeeding energy level requires a smaller
step of energy. To go from the first excited level to the second, third, fourth
or fifth levels, requires only a photon of visible light. Conversely, when an
electron descends to a lower level it emits energy.
Component Colours
of Visible White Light
Radio Waves
Radio Waves
Red
Microwaves
Orange
Infrared
Yellow
Visible Light
Green
Ultraviolet
Fig.2Blue
Example of
X-Rays
Indigo
Gamma Rays
Violet
Spectroscopy is useful for identifying substances. For example, when
electrons in a helium atom move from one state to another, photons of a
distinct frequency are emitted or absorbed. The presence of light of this
frequency is a strong unique indicator or fingerprint of the presence of
hydrogen.
The Higher Education Authority and
One practical example is measuring pollution. The spectral lines for various
pollutants in the air are known. If light is shone through the atmosphere and
the PRTLI
is found to have a measured reduction in strength at the frequencies of a
The presence or absence of energy at particular wavelengths can be
particular pollutant, then the existence and concentration of that pollutant
detected using a spectrometer. The trace appears
as visible linesfuture
known as prosperity
Ireland’s
requires that a knowledge-based
can be established. This is the basis of atmospheric pollution monitoring,
spectral lines. Electrons descending to the first excited level produced the
onegoal
of the is
activities
of the FocasaInstitute,
DIT. where social
four visible lines that Angstrom first measured in
hydrogen be
and created.
for which
society
The
to develop
society
Balmer found the formula. These lines are called the Balmer series. The
quality of
and
personal
enhanced
The life
spectra
of many
molecules growth
and of parts are
of molecules
are well known
reason these lines were found first, rather than development,
those emitted by electrons
this information can be used to identify known substances or, indeed,
descending to the ground state, is that good through
ultra-violet detectors
were in aand
innovation
thriving
economy.
discovering
new
ones.
not available. Lines emitted by electrons descending at the higher energy
levels are infra-red and these were also identified later.
Spectroscopy
is an amazingly
powerful
tool. role
It canintellthis.
us what is
The HEA (Higher Education
Authority)
is playing
a key
emission spectrum for Helium
happening on the sun. Examination of missing lines in the sun’s spectrum
The energy required for an electron to move from a level with energy E2
The HEA advises the government
and
it provides
muchsubstance
funding
showed the presence
of hydrogen
and an unknown
which was
to one with energy E1 is E2 - E1 = hf. This gives exactly the energy and
called helium.
The relatively
in the sun’s atmosphere
frequency of the photon which is emitted or absorbed
when the
electron
in higher
education
through
the
PRTLI,cooler
angases
investment
vehicleabsorbs
light
at
these
wavelengths
as
it
leaves
the
sun,
making
dark
lines
in the
makes the transition.
intended to transform continuous
Irish research.
The
PRTLI
(Programme
spectrum. Later,
helium
was also
found on earth. for
substances. For example, when
Spectroscopy is useful for identifying
In molecules, there are many different energy levels that an electron
Research in Third Level
Institutions)
allows
third-level
institutions
electrons in a helium atom moveFig.1
from
one
state to another,
photons of a
Spectroscopy
also revealed
that the
universe is expanding.
As stars move
Visible
Spectrum
can occupy, so there are many states to move between. Accordingly,
away
fromprovide
us the wavelengths
they emit seemfor
longer
than they should
molecular
many lines, often clustered
in bands,
and many
distinct frequency are emitted or absorbed. The presence
ofspectra
lightcontain
of this
to build
infrastructure
and
a career-path
talented
be
because
of
the
Doppler
effect.
Because
the
wavelengths
have moved
be in the infra-red.
frequency is a strong unique indicator or fingerprint ofofthese
themaypresence
of
researchers. Its funds,toward
combined
those this
from
other
funding
the red endwith
of the spectrum,
is also
called red
shift.
hydrogen.
bodies, are creating a first class research ecosystem in Ireland.
Radio waves have the longest wavelengths (105 _ 10_ metres) while
Spectroscopy is a powerful You
toolcan find out more about the HEA at www.hea.ie.
gamma rays are the shortest (10_ _ 10_ metres).
Spectroscopy
is the
scientific study of spectra and spectral lines.
One practical example is measuring pollution. The spectral
lines for
various
The term electromagnetic derives from the fact that the wave is a result of
A spectroscope is the instrument used to disperse a beam into its
The scale of the investment is significant. To date, PRTLI funding
pollutants
the
air are
known.
If light isradiations
shonetravel
through the
atmosphere and
changing
electricin
and
magnetic
fields.
All electromagnetic
component frequencies. This dispersion can be done using a prism, but
inis
a vacuum
thehave
same speed,
generally referred
to as the speed
of light.
modern
instruments generally
grating.exceeded €600 million in some years. The PRTLI invests in
foundatto
a measured
reduction
in strength
at the
frequencies
of a use a diffraction has
particular pollutant, then the existence and concentration of that pollutant
infrastructure (buildings and equipment) and the researchers who
can be established. This is the basis of atmospheric pollution monitoring,
use it. Thirty-three research centres will be provided when the
one of the activities of the Focas Institute, DIT.
programme is complete.
3
11
14
The spectra of many molecules and of parts of molecules are well known
and this information can be used to identify known substances or, indeed,
discovering new ones.
Spectroscopy is an amazingly powerful tool. It can tell us what is
happening on the sun. Examination of missing lines in the sun’s spectrum
showed the presence of hydrogen and an unknown substance which was
called helium. The relatively cooler gases in the sun’s atmosphere absorbs
light at these wavelengths as it leaves the sun, making dark lines in the
continuous spectrum. Later, helium was also found on earth.
Spectroscopy also revealed that the universe is expanding. As stars move
away from us the wavelengths they emit seem longer than they should
be because of the Doppler effect. Because the wavelengths have moved
toward the red end of the spectrum, this is also called red shift.
One of these centres is the Focas Institute in the DIT (Dublin
Institute of Technology), situated on Camden Row, in the centre of
Dublin. Researchers are engaged in a range of projects involving
physics, chemistry, materials science, health, environment and
new technology. These researchers use a variety of sophisticated
instruments for examining light. How it is emitted from, reflected
from, absorbed or transformed by materials particles or gases.
You can out more about FOCAS and these research
projects at www.focas.dit.ie and at
www.sciencetechnologyaction.com
The Focas Institute Working with Spectroscopy
The Focas Institute Workin
The mysteries of light
Light photons and electrons
Light has always intrigued scientists. Interest really exploded
when Newton discovered that visible ‘white’ light is composed
of a series of coloured lights. He used a prism to split white light
into its constituent colours and another prism to recombine
the colours as white light. He published his major work on
light called Opticks in 1704. His theory of ‘particles’ was
generally accepted until the wave theory of light emerged.
Our modern theory tells us that light is emitted and absorbed in photons.
A photon is considered to be a particle having no mass, no electric
charge, and an indefinitely long lifetime. A photon is a packet or quantum
of electromagnetic energy.
Syllabus Reference
Many famous experimental and theoretical scientists have contributed to
the process leading to our current level of understanding of light as a form
of electromagnetic radiation. Such scientists include Maxwell, Hertz, Bohr,
Michelson and Einstein. We now know that light behaves like straight line
rays and also like waves. It also behaves like particles, which we now call
photons.
The energy of a photon is given by e = hf where h is Planck’s constant
and f is the frequency of the light. The higher the frequency, the greater
the energy so a photon of blue light is has more energy than one of red.
Ultra-violet photons have even more energy; whereas infra-red photons
have less.
Activities
Higher frequency (f) also means shorter wavelength (_) because the very
well known equation tells us that f_ = c where c is the speed of light.
Since the speed of light is a constant. If f increases then _ must decrease.
In short, if we know the frequency f we know
the wavelength
_ or vice
Examine
the components
versa.
Leaving Certificate Physics: Higher Level and Ordinary level
Mandatory Demonstration
Wave Nature of Light –Dispersion; Electromagnetic
Spectrum; The spectrometer
What is the electromagnetic spectrum?
Light is not the only form of electromagnetic radiation. The total range
of electromagnetic radiation is called the electromagnetic spectrum. The
spectrum is usually divided into seven sections. This is shown below
along with the constituent colours of visible light.
Radio Waves
Component Colours
of Visible White Light
Learning
Objectives
Radio Waves
Red
Microwaves
Orange
On completing this section,
the
Infrared
•
Visible
Light
Identify
Ultraviolet
of light
•
X-Rays
Yellow
some famous scientists
Green
Blue
Fig.2 Example of emission spectrum for Helium
Mandatory Experiment
The energy levels of atoms
In an atom, electrons can only exist in particular
states or the
energy
levels. In
Measure
wavelength
of
monochromatic light
Spectroscopy is useful for identifying
hydrogen, the electron normally occupies the lowest energy level, which
is called the ground state. To make an electron change (or jump) from this
ground state to the first excited level requires an ultra-violet photon, as this
is a relatively big step. Each succeeding energy level requires a smaller
step of energy. To go from the first excited level to the second, third, fourth
or fifth levels, requires only a photon of visible
light. Conversely,
when an
Research
the lives
and
electron descends to a lower level it emits energy.
Investigative
The presence or absence of energy at particular wavelengths can be
detected using a spectrometer. The trace appears as visible lines known as
spectral lines. Electrons descending to the first excited level produced the
four visible lines that Angstrom first measured in hydrogen and for which
Balmer found the formula. These lines are called the Balmer series. The
student should reason
be able
to: were found first, rather than those emitted by electrons
these lines
descending to the ground state, is that good ultra-violet detectors were
notour
available.
Lines emitted by electrons descending at the higher energy
who contributed to
understanding
levels are infra-red and these were also identified later.
Indigo
of the spectrometer
True or False
substances. For example, when
electrons in a helium atom move from one state to another, photons of a
distinct frequency are emitted or absorbed. The presence of light of this
frequency is a strong unique indicator or fingerprint of the presence of
hydrogen.
work of Newton, Einstein and Bohr
One practical example is measuring pollution. The spectral lines for various
pollutants in the air are known. If light is shone through the atmosphere and
is found to have a measured reduction in strength at the frequencies of a
particular pollutant, then the existence and concentration of that pollutant
can be established. This is the basis of atmospheric pollution monitoring,
one of the activities of the Focas Institute, DIT.
The spectra of many molecules and of parts of molecules are well known
and this information can be used to identify known substances or, indeed,
Indicate whether the
following
discovering
new ones. are true (T) or false
(F) by drawing a circle
around
T or F.
Spectroscopy is an amazingly powerful tool. It can tell us
The energy required for an electron to move from a level with energy E2
what is
happening on the sun. Examination of missing lines in the sun’s spectrum
showed the presence of hydrogen and an unknown substance which was
to one with
energy E1 is E2 - E1 = hf. This gives exactly the energy and
Describe the main components of the electromagnetic
spectrum
Gamma Rays
Violet
light
has a longer
wavelength
thancooler
yellow
light
Violet
T absorbs
F
called
helium. The relatively
gases
in the sun’s atmosphere
frequency of the photon which is emitted or(a)
absorbed
when
the electron
light at these wavelengths as it leaves the sun, making dark lines in the
makes the transition.
• Name the colours in the visible spectrum
continuous spectrum. Later, helium was also found on earth.
(b) The electromagnetic spectrum
is broadly divided into seven T
F
In molecules, there are many different energy levels that an electron
• Describe the relationship between
energy
for
a
photon
sections
Spectroscopy also revealed that the universe is expanding. As stars move
Fig.1 Visible
Spectrumand frequency
can occupy, so there are many states to move between. Accordingly,
•
molecular spectra contain many lines, often clustered in bands, and many
of these may be in the infra-red.
State the relationship between frequency and wavelength
away from us the wavelengths they emit seem longer than they should
be because of the Doppler effect. Because the wavelengths have moved
(c) Visible light is the onlytoward
formtheofredelectromagnetic
radiation
T
end of the spectrum, this
is also called red shift.
• waves
Describe
the
function
of the
Radio
have the
longest
wavelengths
(105spectrometer
_ 10_3 metres) while
gamma rays are the shortest (10_11 _ 10_14 metres).
(d)tool
X-rays have a higher frequency than microwaves
Spectroscopy is a powerful
Identify some practical applications of spectroscopy
Spectroscopy is the scientific study of spectra and spectral lines.
The term electromagnetic derives from the fact that the wave is a result of
The components
A spectroscope is the instrument used (e)
to disperse
a beam into its of white light in order of appearance in the
changing electric and magnetic fields. All electromagnetic radiations travel
component frequencies. This dispersion can be spectrum
done using a prism,
but orange, blue, yellow, green, indigo, violet
are red,
T
F
T
F
(f) All electromagnetic radiation travels at the speed off light
T
F
(g) A photon is a particle that weighs less than a nanogram
T
F
(h) In the formula e=hf, f is called Planck’s constant
T
F
(i) You can derive the wavelength if you know the frequency
and the speed of light
T
F
(j) The lowest level of energy that an electron can occupy is
called the ground state
T
F
(k) The instrument used to measure detect spectra is usually
called a voltmeter
T
F
(l) Spectroscopy can be used to measure atmospheric
pollution
T
F
•
in a vacuum at the same speed, generally referred to as the speed of light.
F
modern instruments generally use a diffraction grating.
General Learning Points
•
Spectroscopy is the scientific study of spectral lines
•
In effect, spectral lines are the signatures of materials.
•
The development of spectroscopy still depends on better ways of
making diffraction gratings and detectors. This requires precision
engineering and manufacturing. Science today still creates a constant
need for better research instruments.
•
Early spectroscopy was not easy because it was often difficult to get
pure samples of materials. The spectra from impurities would spoil
the results.
•
Scientists need to be alert and open-minded. Herschel discovered
infra-red (see Did You Know). A less observant person might have
moved the control thermometer elsewhere and carried on.
•
We can learn much about the composition of distant objects just from
their spectra.
•
When Fraunhofer found the dark lines in the sun’s spectrum, he then
looked for them in the spectra of the moon and Venus and Mars. His
observations confirmed the sun as the source of light in the solar
system.
•
The concept of the quantum gave rise to a revolutionary branch of
physics called quantum mechanics.
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The Focas Institute Working with Spectroscopy
AND
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IN
king with Spectroscopy
The mysteries of light
Light photons and electrons
Light has always intrigued scientists. Interest really exploded
when Newton discovered that visible ‘white’ light is composed
of a series of coloured lights. He used a prism to split white light
into its constituent colours and another prism to recombine
the colours as white light. He published his major work on
light called Opticks in 1704. His theory of ‘particles’ was
generally accepted until the wave theory of light emerged.
Our modern theory tells us that light is emitted and absorbed in photons.
A photon is considered to be a particle having no mass, no electric
charge, and an indefinitely long lifetime. A photon is a packet or quantum
of electromagnetic energy.
The energy of a photon is given by e = hf where h is Planck’s constant
and f is the frequency of the light. The higher the frequency, the greater
the energy so a photon of blue light is has more energy than one of red.
Ultra-violet photons have even more energy; whereas infra-red photons
have less.
Biographical Notes
Did you know?
Many famous experimental and theoretical scientists have contributed to
the process leading to our current level of understanding of light as a form
of electromagnetic radiation. Such scientists include Maxwell, Hertz, Bohr,
• Theand
production
of aknow
spectrum
with alike
prism
was
Michelson
Einstein. We now
that light behaves
straight
line
rays and also like waves. It also behaves like particles, which we now call
photons.
Higher frequency (f) also means shorter wavelength (_) because the very
well known equation tells us that f_ = c where c is the speed of light.
known
in antiquity.
Since the speed of light is a constant. If f increases then _ must decrease.
In short, if we know the frequency f we know the wavelength _ or vice
versa.
was born in Germany.
Albert Abraham Michelson (1852-1931)
He was the first American scientist to win a Nobel
In 1800 William Herschel studied the heating effects of different colours.
prize. Michelson invented the interferometer,
an instrument that measures
He put thermometers at different colours of a spectrum and a control
Fig.2 Example of emission spectrum for Helium
What
is the electromagnetic
the interference of waves. It is said that his work inspired Einstein’s work
energy
levels
thermometer
in the dark area spectrum?
beyond the red. He wasThe
amazed
to find
the of atoms
Light is
not the only
form
of electromagnetic
radiation.
total
range
on relativity.
In
an atom, electrons can only exist in particular
states or energy levels. In
control
to be
hotter
than the rest.
HeThe
had
discovered
infra-red.
•
of electromagnetic radiation is called the electromagnetic spectrum. The
spectrum is usually divided into seven sections. This is shown below
along
the researchers
constituent colours
of visible light. “lines” in spectra.
• with
Later
discovered
hydrogen, the electron normally occupies the lowest energy level, which
is called the ground state. To make an electron change (or jump) from this
Spectroscopy is useful for identifying substances. For example, when
electrons in a helium atom move from one state to another, photons of a
to one with energy E1 is E2 - E1 = hf. This gives exactly the energy and
showed the presence of hydrogen and an unknown substance which was
distinct frequency are emitted or absorbed. The presence of light of this
ground
state to the
firstlight
excited level requires an ultra-violet photon, as this
(Lines,
because
the
frequency is a strong unique indicator or fingerprint of the presence of
Einstein
is a relatively big step. Each succeeding Albert
energy level requires
a smaller (1879-1955)
came through a narrow slit.). In 1814 Joseph von Fraunhofer,
saw
and
hydrogen.
step of energy. To go from the first excited level to the second, third, fourth
or fifth levels,
requires only
light. Conversely,
an famous scientist of all time. He was a professor
mapped the dark lines in the sun’s spectrum. He noticed
one strong
linea photon of visible
is probably
thewhen
most
One practical example is measuring pollution. The spectral lines for various
electron descends to a lower level it emits energy.
missing in the sun’s spectrum
coincided
Zurich, Prague and Berlin.pollutants
Although
became
German
citizen
in and
in the airhe
are known.
If light isashone
through the
atmosphere
Component
Colourswith a bright line emitted from
Radio Waves
is found to have
measured
reduction
in strength
the frequencies
The presence
absence
particularhe
wavelengths
can behis citizenship
a flame. This “line” (weofnow
know
to be
from sodium)
turned or
out
to beof energy at 1914,
renounced
in a1933
and
emigrated
toatthe
USA as of a
Visible
White
Light
particular
pollutant,
then
the
existence
and
concentration
of
that
pollutant
detected using a spectrometer. The trace appears as visible lines known as
two very close lines. Later, he was able to make and
use a diffraction
of produced
Professor
Physics
at basis
Princeton.
Hispollution
early monitoring,
work
can be established.
This is the
of atmospheric
spectral lines. Electrons descending to theProfessor
first excited level
the of Theoretical
Radio Waves
onephoton
of the activities
of the Focas
Institute,
DIT.published many
Red and measure the wavelength
four visible
that Angstrom
and for which of the
grating, instead of a prism,
oflines
these
lines first measured
led intohydrogen
the foundation
theory
of light.
He
Balmer found the formula. These lines are called the Balmer series. The
Microwaves
Orange
more accurately.
including
Special Theory
Relativity
(1905),
General
Theory
The spectraof
of many
molecules
and of parts
of molecules
are well of
known
reason these lines were found first, rather papers
than those emitted
by electrons
Infrared
and this information
cannon-scientifi
be used to identifycknown
substances
or, indeed,
Yellow
descending to the ground state, is that good
ultra-violet
detectors He
werealso wrote
Relativity
(1916).
many his
books
including
discovering
new
ones.
not available. Lines emitted by electrons descending at the higher energy
Visible Light
• Hydrogen’s
spectrum wasGreen
especially interesting because
there were four
WhyedWar?
levels are infra-red and these were also identifi
later. (1933), and My Philosophy (1934).
Ultraviolet
Blue wavelengths of these were measured very
Spectroscopy is an amazingly powerful tool. It can tell us what is
bright
visible-light lines. The
happening
on the sun. Examination of missing lines in the sun’s spectrum
Hefrom
received
the
Nobel
in 1921.
The energy required for an electron to move
a level with
energy
E2 Prize
X-Rays
Indigo
accurately
in 1871 by Anders
Angstrom. A Swiss maths
teacher, Johannes
Gamma Rays
Violet finding as a number puzzle
called helium. The relatively cooler gases in the sun’s atmosphere absorbs
frequencyand
of thecame
photon which
Balmer
decided to treat this
up is emitted or absorbed when the electron
light at these wavelengths as it leaves the sun, making dark lines in the
makes the transition.
with a simple formula that gave the four wavelengths
and predicted
continuous spectrum. Later, helium was also found on earth.
Sir Isaac Newton (1643-1727)
molecules,
there series
are many different energy levels that an electron
more. His formula was soon modified to successfullyInpredict
other
Spectroscopy also revealed that the universe is expanding. As stars move
Fig.1 Visible Spectrum
can occupy, so there are many states to
moveone
between.
Accordingly,
was
of the
greatest scientists
of all time. He worked in astronomy,
of lines in the hydrogen spectrum.
away from us the wavelengths they emit seem longer than they should
molecular spectra contain many lines, often clustered in bands, and many
because
of the Doppler
the wavelengths
have moved
physics and mathematics. beHe
described
the effect.
lawsBecause
of motion
and gravity
of these may be in the infra-red.
toward the red end of the spectrum, this is also called red shift.
1687 in a famous book called
Principia. He was Professor of Mathematics
• This was very exciting but the formula was empirical. That is, it worked but
Radio there
waves was
have the
wavelengths
(105to_support
10_ metres)
while
at Cambridge
when he was 26. Almost everyone knows the story of the
no longest
underlying
theory
it. Attempts
to
explain the formula
Spectroscopy
is a powerful
tool
gamma rays are the shortest (10_ _ 10_ metres).
apple falling on his head. I fact he seems to have been looking out of a
led initially to Bohr’s model of the atom (1913) and quantum
mechanics
Spectroscopy is the scientific study of spectra and spectral lines.
The term
electromagnetic
derives from the
that the wave
is anow
result be
of fully Aexplained.
atahome
when
spectroscope is the instrument usedwindow
to disperse
beam into
its he saw the apple fall.
(1920s).
The spectrum
of fact
hydrogen
can
3
11
14
changing electric and magnetic fields. All electromagnetic radiations travel
in a vacuum at the same speed, generally referred to as the speed of light.
component frequencies. This dispersion can be done using a prism, but
modern instruments generally use a diffraction grating.
•
The unit used for measuring small wavelengths is named after Angstrom.
An angstrom is 10-10 metres.
•
Neils Bohr invented the term quantum.
•
The relationship between frequency and energy: e = hf, was derived in
1905 by Einstein when explaining the photoelectric effect.
•
Helium is named after Helios, the Greek sun-god.
•
Newton had nearly completed his Opticks manuscript when he went out
for a walk one day. He left a candle burning in his house. When he returned
the book had been completely burnt. He was extremely frustrated and
gave up optics for a long time.
•
Opticks proposed the theory that light is composed of particles. Newton
is quoted as saying ‘Light is composed of tiny particles, or corpuscles....’
•
Newton is also quoted as saying ‘If I have seen further than other men, it
is because I stood upon the shoulders of giants.’
•
Einstein is quoted as saying ‘One has been endowed with just enough
intelligence to be able to see clearly how utterly inadequate that intelligence
is when confronted with what exists’.
•
Bohr is quoted as saying ‘The opposite of a correct statement is a false
statement. But the opposite of a profound truth may well be another
profound truth’
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Review the Terms
Can you recall the meaning of these terms? Revising
terminology is a powerful aid to recall and retention.
Electromagnetic; photons.;Maxwell; Hertz; Bohr; Michelson;
Einstein.; speed of light; quantum; Planck’s constant; ground
state; excited level; spectrometer; spectral lines; Angstrom;
Balmer series; Spectroscopy; spectroscope; diffraction grating;
Doppler effect; red shift; electromagnetic; electromagnetic
spectrum; photons; speed of light; Planck’s constant.
Check the Glossary of Terms for this lesson at
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