Science Content Knowledge

Light is a form of energy whose properties can be explained either on the basis of wave and particle instead theory. We talk about the so-called particle-wave dualism. The wave nature of light is indicated in the Planck relation E = hν, which relates the energy of the particle to one of the wave properties by one of the wave properties – frequency. In the following work, we will focus on light as an electromagnetic wave. Electromagnetic wave is characterized by two main parameters: A – the amplitude, and frequency (ν ) and wavelength ((λ). ) that determines the colour of the light[1] ν(λ). Frequency and wavelength are related to each by a constant, the speed of light. 

The colours of items that are perceived by the human are images resulting from the interpretation of light waves reaching the eye. The human eye is sensitive to waves responsible for the following colours: blue, red and green. Other colours are perceived thanks to the analysis of the differences in the intensity of those three waves.

The colour parameters of light-emitting elements could be encoded thanks to the RGB colour scale (from the English words: R – red, G – green, B – blue). The scale gives the intensity ofof the primary colours in the range of 0-255, for example: white (255,255,255), black (0, 0, 0).

The human eye perceives colour thanks to the presence of three types of so-called cones. Each type of the cones is sensitive to 1 colour so that it responds to light of a different wavelength. Colour blindness (daltonism) is a result of the impairment of one or more of the three types of cones.

Camcorders and digital cameras record the electromagnetic waves through silicon detectors. These detectors are sensitive to light with wavelengths shorter than 1.1 μm (of an energy of over 1.1 eV). LED diodes, commonly used in remote controls for operating various devices, emit light of wavelength of 0.9 μm (having an energy of over 1.4 eV). Silicon detectors can thus be used for recording light not visible to the human eye. It is interesting, that bees have the ability to see the ultraviolet light – I think.

Matter may:

• produce the light (emission),
• consume (absorption),
• change the direction of propagation (scattering).
• Transmit the light

Mechanism of the colour creation is a combination of above-mentioned processes.

A continuous spectrum  of light is a characteristic of objects that are luminous because of their high temperature, such as light bulbs and the sun. Coloured substances absorb certain wavelengths, and reflect the remaining. Absorption is connected for example with the excitation of electrons, which are moved from their ground states to the excited states. 

Hydrogen and other elements are able to emit electromagnetic waves not only in case of heating them to high temperature. After excitation (need to apply a current), the light creates a band spectrum, characteristic for each element. Fluorescent lamps are examples of the „cold” light source.

The ability to absorb (and therefore emit) light from the visible spectrum is a characteristic of the coordination compounds of transition metals. The d-type atomic orbitals of the isolated atoms of a given element are of the same energy. In the surroundings of ligands, the energy levels are split into sublevels. For the compounds that have an octahedral structure (coordination number 6), the characteristic splitting is that, when three energetic sublevels are of the lower energy, and two – of the higher energy. 

Difference in energy of the levels corresponds to the energy of the visible light and depends on the geometry of the coordination compound[2], coordination number and the type of ligands. However, it should be remembered, that according to the quantum chemistry theories, the d-d transitions in octahedral complexes with a centre of symmetry are forbidden.

This is the main problem that occurs during the interpretation of the spectra of complex compounds. The Laporte selection rule states: the only transitions allowed are those, which are accompanied by the parity change. Nevertheless, as a result of asymmetric vibrations, the complex centrosymmetry is disturbed. Therefore, due to the lack of the centre of symmetry, d-d transitions are no longer forbidden and gain slight intensity. In such cases, when thanks to asymmetric vibrations in the molecule, the transition gains in intensity, it is called the vibronic transition.

Spectroscopy is a set of techniques involved in studying the interactions of electromagnetic radiation with matter; it is also a broad area of various techniques of chemical analysis. Depending on the energy of the analysed waves, different types of spectroscopy are distinguished e.g, infrared, visible, uv.

Another method based on the interaction of radiation with matter is colorimetry. It allows the determination of the solution concentration on the basis of its colour intensity. In the measurements, a device called a colorimeter is used, and it was invented by a Polish researcher Jan Szczepanik. The colorimeter measures the amount of light that passes through the sample. The dependence of light intensity on the concentration is described by the second absorption law, called the law of Lambert-Beer. It is expressed by the equation ; where: A represents absorbance, I0 – the radiation intensity before passing through the sample, I – the intensity of radiation that passed through the sample, c – the concentration of the solution, b – the cuvette thickness, ε – the molar absorption coefficient. As follows from the above-presented equation, the absorbance (A) – which describes the ability of the absorption of radiation  - is directly proportional to the concentration of substance in the solution and that proportionality is used in the colorimetric measurements.

The absorbed energy of electromagnetic waves may be emitted, as it is case of simple chemical compounds. However, plants possessed the ability of storing and processing the solar energy – in the process called photosynthesis. Photosynthesis can be defined as an anabolic biochemical process, as a result of which, with the use of solar radiation and with the participation of assimilatory pigments and enzymes, monosaccharides (in the form of hexoses) are produced from the carbon dioxide. Before the solar energy will take part in this complicated biochemical process, it has to be "captured" – in order to do this, the plants need chlorophyll. Thanks to this dye, plants are capable of absorbing light and using it in photosynthesis. Chlorophyll is green, because it absorbs blue and red light, whereas green light is passed through it without being absorbed. The energy of light is absorbed by chlorophyll for wavelengths corresponding to the red and blue colour of light and then it is used for the conversion of carbon dioxide into the glucose. If the amount of the glucose produced in this process in greater than a demand for it, the sugar is converted into starch by linking many glucose molecules together to form a long chain (polymer). At night, it is the starch that is transformed back to the grape sugar (glucose) to provide energy for plants, until the sun rises again and the chlorophyll is able to absorb the appropriate amount of light to renew the starch reserve.

The possibility of using solar energy and transforming it into electricity is extremely important for the future of humanity. The solar cells are semiconductor devices, in which occurs the conversion of solar energy into electricity as a result of the photovoltaic phenomena. Solar cells are used e.g. for the artificial satellites, space probes, calculators and watches. However, their disadvantage is that they absorb mainly the ultraviolet light, whereas the highest amount of solar energy that reaches the Earth is from the region of visible light. Hence, the idea of sensitization of semiconductor by organic dyes, which efficiently absorb the visible light was brought up. Such a cell was first built in the Swiss Federal Institute of Technology in Lausanne, Switzerland in 1991 and now attracts considerable attention of researchers - although it is not yet used commercially.

Literature:

• Physics for Scientists and Engineers (6th ed.) by Serway and Jewett,Thomson Brooks/Cole, 2004
• Krzysztof Korona, Materiały do wykładu Fizyka w doświadczeniach, Uniwersytet Warszawski 2010
• John Green, Sadru Damji, Chemistry (3rd ed.), IBID Press, 2007
• Peter Atkins, Physical Chemistry, sixth edition Oxford University Press, Oxford Melbourne Tokyo 1998.
• Stefan Paszyc, Podstawy fotochemii, PWN, Warszawa 1981.
• Łukasz Boda, Materiały dydaktyczne: ‘Nanokrystaliczne ogniwo słoneczne’ Uniwersytet Jagielloński