SPECTRAL ANALYSIS
Spectral Analysis
Spectrometry is a field of research in physics investigating the intensity and distribution of
electromagnetic spectra produced by different types of matter. Spectrometry originated in the 1660s
when Isaac Newton, performing experiments with prisms, split white light into the colours of a
rainbow. He showed that, by using prisms, white light can be broken up into colours and
reassembled into white light.
Joseph von Fraunhofer (1787-1826) examined the spectra produced by ‘broken down’ white
light using diffraction gratings. He discovered dark lines in the solar spectrum, the so-called
Fraunhofer lines, although he was unable to explain their appearance. Robert Wilhelm Bunsen and
Gustav Robert Kirchhoff studied the spectra of flame-tests from different elements, finding clear
sharp spectral lines. Experimenting further, they found that on passing white light through the same
flames the spectra displayed dark lines at the corresponding wavelengths. This appeared to be the
result of the same phenomenon discovered in the solar spectrum by Fraunhofer.
The explanation of these spectral lines was finally found in the early 1900s, when Niels
Bohr introduced the new atomic model based on Max Planck's quantum hypothesis.
The splitting of white light into coloured light
Explore the colours produced when white light passes through a prism and through a grating.
• How does each case differ?
• What is the basis of the phenomena?
• What quantitative laws relate to the phenomena?
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SPECTRAL ANALYSIS
White light is composed of coloured light. When light hits the surface of a material reflection, and
sometimes refraction, occurs. For white light incident on a prism the blue component of the light is
refracted the most, while the red part the least. The behavior of light incident on a diffraction
grating can be modeled according to Huygen’s Principle; whereby each point of every wave-front
is a new elementary wave source. This effectively produces new light sources at the diffraction
grating and these diffracted waves interfere to create the interference pattern seen on the screen.
Refraction and diffraction are familiar properties of wave motion. The size of the change in
direction of wave propagation in each case depends on the wavelength, the velocity and frequency.
Since light of different colours has the same speed, and because neither refraction nor diffraction
changes the wave frequency, the change of beam direction depends on the wavelength. Light
colour therefore corresponds to wavelength.
The ‘Pocket’ Diffraction Grating Spectroscope
The ‘Pocket’ Diffraction Grating Spectroscope scatters light into its component colours through a
grating. Observe different light sources through this spectroscope.
• What types of spectra do you see?
The ‘Pocket’ Diffraction Grating Spectroscope splits emitted light into the colours of a spectrum.
Some sources emit only a few colours, while other sources emit light that appears to be composed
of countless different colors. This is because some of the sources emit only particular wavelengths
of radiation while others contain all the wavelengths of light.
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SPECTRAL ANALYSIS
Vernier ESRT-VIS Spectrometer
The Vernier Spectrometer diffracts light into its component colours through a grating (number 5
shown in Figure 1). This coloured diffracted light then travels to the detector (number 8) which
measures the intensity of individual colours. The spectrometer converts the measured data into
digital form which can then be analyzed using Logger Pro software.
Figure 1: Components of The Red Tide Spectrometer (Ocean Optics).
• Use this spectrometer to investigate the light sources previously examined with the ‘Pocket’
Diffraction Grating Spectroscope
• How do the spectra differ from each other?
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SPECTRAL ANALYSIS
When examining a light source, the ‘Pocket’ Diffraction Grating Spectroscope detected light
emission of either separate colours or a smooth continuum of colours. With the Vernier
Spectrometer the light emitted by a source can be examined more carefully and in particular the
wavelengths and their relative intensities can be measured. It was again found that some sources
emit only certain wavelengths of radiation, while others contain all the wavelengths of light. In
addition the Vernier Spectrometer shows that in cases where spectral lines are detected some lines
have intensity peaks higher than those of other wavelengths.
The generation of line spectra and continuous spectra
Based on observations the spectra of the light sources can be classified according to one of two
categories: line spectra and continuous spectra.
• What types of sources produce continuous spectra?
Do you observe differences between these continuous spectra?
• What types of sources produce line spectra?
Compare the spectra of light emitted from gas-filled tubes of helium, argon, krypton and water
vapor. Which types of gases have many spectral lines and which types have few?
Compare the spectra of the hydrogen and water vapor. What do you notice?
Continuous spectra arise when the particles that constitute the matter in question vibrate through
a whole range of different degrees of freedom. The number of degrees of freedom depends on
the temperature of the material. As the temperature increases, the degrees of freedom that
require higher energy are activated. This is the reason that light emitted by a hot object
producing a continuous spectrum has a peak located further towards the blue wavelengths than
the light spectrum produced by a cold object. Remember that, according to the equation E = hf,
producing higher frequency photons requires greater energy.
A line spectrum arises when an electron moves from one electron level to another. This produces
a photon whose energy corresponds to the difference between the energies of the electron levels.
The frequency of this photon is determined by the equation E = hf. Since each chemical element
has distinct energy differences between its electron levels, studying the line spectra of different
materials makes it possible to determine which elements the object contains.
Examine the spectrum of an energy saving lamp. What can you deduce about the lamp?
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