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Transmission of light The Beer-Lambert law Absorption spectra Scattering of light Additional questions |
Emission and absorption spectra The photoelectric effect |
 Data Glossary |
| Learning Outcomes | ||
| After studying this section, you will (a) be familiar with the phenomenon of transmission of light, (b) know and be able to apply the Beer-Lambert Law, (c) know what is meant by an absorption spectrum, and (d) understand the phenomenon of light scattering. | ||
Some of the material in this section is at a higher level than is expected of South African Grade 12 learners, and is included here for the benefit of educators.
Transparent objects TRANSMIT light, by which we mean that some of the light that is incident to the surface of the object passes through the object. Generally speaking, not all the light passes through - window glass with a thickness of 2 mm for example, allows most of the light to go through, but if the glass is very much thicker, or if the glass is tinted, we notice that the transmitted light is appreciably altered.
The light that does not get through is said to be ABSORBED by the medium through which it passes.
When a beam monochromatic light passes through a transparent medium, part of the light is absorbed and the transmitted beam has a lower intensity than the intensity of the incident beam. Solutions are placed in containers (called "cuvettes") whose material is transparent: quartz is commonly used, but for visible light, one also uses disposable cuvettes made of polystyrene or polycarbonate. |
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The TRANSMITTANCE, T, of the solution is defined as the ratio of the intensities of the transmitted beam, I to the intensity, Io of the incident beam:
T = I/Io
The ABSORBANCE, A, of a solution is defined as A = -log10T. Since A is a logarithmic function, it is dimensionless.
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Both of the above quantities are readily measured using an instrument called a SPECTROPHOTOMETER, which delivers monochromatic light over the ultraviolet and visible ranges of the electromagnetic spectrum, and have suitable light detection and recording devices. |
The Beer-Lambert law (also known as Beer's law) (as it applies to solutions of light-absorbing substances) states that the absorbance is directly proportional to the path length, l of the sample and its molar concentration, c:
A = εcl
where ε is the MOLAR EXTINCTION COEFFICIENT (with dimensions of dm3.mol-1cm-1) of the solute, c is the molar concentration (in moles.dm-3), and l, the path length, is measured in centimeters. Typically, cuvettes are made so that the path length, that is the thickness of the liquid through which the light passes, is exactly 1.00 cm. This simplifies the calculations!
The molar extinction coefficent is a constant for a particular solute, and varies with the wavelength of the light.
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By using a spectrophotometer, one can determine the molar extinction coefficient at various wavelengths. A plot of ε against the wavelength, l, is called an ABSORPTION SPECTRUM. Such spectra, an example of which is shown on the left, may be in the ultraviolet, visible, or infrared ranges of the electromagnetic spectrum. |
The wavelength(s) at which the spectrum displays a "peak" value is said to be an ABSORPTION MAXIMUM. In the diagram on the left above, the spectrum displays three absoption maxima in the range of wavelengths that was measured.
The absorption spectra in the visible and ultraviolet regions of the electromagnetic spectrum arise from the excitation of electrons in multiple bonds of the absorbing molecules. Infrared absorption spectra arise from energy absorbed by molecules that cause bending and stretching of covalent bonds. Another type of absorption spectra, (discussed in another topic) arise from the excitation of electrons in elements or simple molecules.
If a beam of light falls onto a perfectly smooth surface (as in the diagram above, left), the light will be reflected, and an image of the object will be formed. The angle of incidence of the light will be equal to the angle of reflection.
If however the light falls on a rough surface, the angles of incidence of various part of the beam will vary (above, right). Reflection still occurs, but such a reflection is said to be DIFFUSE, and we say that the reflected light is SCATTERED.
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If one looks through a liquid in which very finely divided liquid droplets or solid particles are suspended, that suspension will appear TURBID ( |
The scattering of light is not the same for all wavelengths. The intensity of the scattered light decreases in inverse proportion to the fourth power of the wave length: I ∝ λ-4. When white light is scattered, the blue component is scattered more than the red component, and scattered light therefore tends to be enriched in the blue region of the spectrum.
This, by the way, is why the sky appears blue, as white light from the sun is scattered in all directions by air molecules. In the absence of air, the sky would look black. This is what the astronauts on the moon saw when they landed! |
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