Worksheet
No.____a
Lower atmosphere (troposphere)
class
substances, light and spectroscopy
name
date
The interrelation of light, absorption and colour
red
yellow
green
blue violet
Fig. 1: spectrum of white light
Visible light is a certain part of the spectrum of electromagnetic radiation. It consists of tiny energy units
called light quanta or photons. The energy of a photon is determined by its wavelength.
If the same amount of photons of all wavelengths of the visible spectrum meets our eyes, we perceive the
light as white light. An object also appears white if it reflects photons of white light in a regular way.
With a prism or a diffraction grid (a great number of thin, parallel lines, e.g. a CD) wavelengths can pass or
be reflected at different angles. We then see a spectrum with all colours arranged according to their
energy content (see Fig.1).
If a part of the light of a certain energy is absorbed, we perceive the complementary colour of the
absorbed light.
T1
Which visible colour belongs to each of the spectra below?
spectra
red
yellow green blue violet
resulting colour
white
yellow
orange
red
purple
blue
green
In nature many things appear coloured. Most of them absorb in several
areas of the visible spectrum. Usually the absorption in these areas is not
of the same quantity (see Fig. 2). This results in characteristic absorption
spectra that can be used for the identification of substances. Nevertheless,
two substances that are made up of the same components can sometimes
be of different brightnesses.
T2
Suggest a hypothesis why two samples of the same substance
appear differently bright and give reasons for your hypothesis.
Draw the spectrum of a “darker” chlorophyll b into Fig 2.
Samples of the same substance can appear differently
Fig.2: absorption spectrum of
bright because of a difference in the concentrations of
chlorophyll b
the dissolved absorbing substances. The sample with
the higher concentration absorbs a greater amount light quanta of the same
wavelength. Therefore the sample appears darker.
© 2004 ESPERE-ENC / Seesing, Tausch, Universität-Duisburg-Essen; Duisburg
Worksheet
No.____a
Lower atmosphere (troposphere)
class
substances, light and spectroscopy
name
date
Basic spectrometry
Spectroscopy is used for the identification of substances. In the laboratory you can examine different
substances with a photometer (also called spectrometer). These measuring results could for example be
used to find them again in satellite measurements of the atmosphere of the earth. (c.f. the spectrum of the
atmosphere made by the GOME satellite at the ESPERE-webpages: section on “lower atmosphere”
/more/1.Oxidants &.../observation spectroscopy".)
A photometer consists of the elements shown in Fig.3:
Figure 3: Set-up of a photometer
A: light source; B: monochromator
(prism or diffraction grid);
C: aperture; D: cuvet with sample;
F: photosensor; G: amplifier with
calibrated reading showing the
EXTINCTION
Io: light intensity in front of the
sample
I: light intensity behind the sample
The measured value EXTINCTION gives the relation of the light intensity that has passed (I) and the
original light intensity (Io). Depending on the apparatus and the tuning the measured values are given
either in percent or logarithmically.
There are many parameters that can be adjusted in a photometer. One can choose between different
lamps, especially at apparatuses that also work in the UV-region of the electromagnetic spectrum, or the
width of the aperture, the position of the monochromator (and therefore the measured wavelength) the
temperature of the sample, and the amplification. One can also change the thickness of the sample by
taking cuvets of different geometries. It remains the same though if the same cuvets are used.
T3
What do you have to change exactly and continuously in order to get a spectrum as shown in Fig.
2? Give reasons for your answer.
The position of the monochromator has to be changed very exactly and
continuously in order to be able to measure the absorption of all wavelengths
of the visible spectrum of light.
The scientists Johann Heinrich Lambert (1728-1777) and August Beer (1825-1863) set up a law that was
named after them, the law of Lambert and Beer:
E: extinction; ε: molar extinction coefficient (constant that is dependent on the wavelengths and if
used also on the solvent; d: layer thickness of the sample; c: concentration of the absorbed
T
substance
Give an interpretation of your hypothesis in Ex. 2 and keep in mind the law of Lambert and Beer.
4
Interpret your hypothesis from Ex 2. Which proportionality in relation to the extinction can be
experimentally produced and used?
Actually the hypothesis is prooved. But it has to be extended concerning the
layer thickness d of the sample. If d is constant (determined by the thickness
of the cuvet) the extinction ("darkness") is proportional to the concentration.
(ε is a constant.) This proportionality can easily be experimentally produced
and used.
T5
Discuss your results from Ex 3 and 4!
© 2004 ESPERE-ENC / Seesing, Tausch, Universität-Duisburg-Essen; Duisburg