Colours electromagnetic spectrum

When white light (such as the light from an ordinary light bulb) is passed through a prism, a rainbow effect is seen. This is known as a continuous spectrum and includes all the colours in the visible part of the electromagnetic spectrum (from about 400 to 700 nm). 

Think back to the split d orbitals. Electrons in the lower energy d orbitals can absorb energy and move to the higher energy d orbitals. If the energy absorbed in these so-called d-d transitions is in the visible part of the electromagnetic spectrum, the colour of the transition metal compound will be the complementary colour of the absorbed colour. So the colour we see will be white light minus the colour absorbed. 

The effects of d-d transitions can be studied using spectroscopy. If the absorbed energy is in the visible part of the electromagnetic spectrum, giving a coloured compound, visible spectroscopy is used. If the absorbed energy is in the ultraviolet part of the electromagnetic spectrum, the compound will be colourless and ultraviolet spectroscopy is used. 

Thus, although the colour of sparks is dependent upon flame temperature and may be similar to that of black body radiation, the overall colour effect can include contributions from atomic line emissions, from metals (seen in the UV and visible regions of the electromagnetic spectrum), from band emissions from excited oxide molecules (seen in the UV, visible and IR regions) and from continuum hot body radiation and other luminescence effects. So far as black body radiation is concerned, the colour is known to change from red (500 °C glowing cooker... 

When potassium perchlorate is included in the composition, potassium ions are formed as seen in reactions (8.2) and (8.3). However, potassium emits in the near-IR region of the electromagnetic spectrum and so has little effect on the colour. On the other hand, the ionised form of Ba is undesirable since it emits in the blue region, and potassium salts are often added to Ba stars to suppress ionisation. 

The loss in colour is due to a loss in conjugation of the aromatic molecule. Dyes absorb light in the visible region of the electromagnetic spectrum by virtue of transitions between electronic energy levels. 

The trapped electron provides a classic example of an electron in a box . A series of energy levels are available for the electron, and the energy required to transfer from one level to another falls in the visible part of the electromagnetic spectrum, hence the colour of the F-centre. There is an interesting natural example of this phenomenon The mineral... 

Electronic Transitions In an electronic transition an electron is excited from an occupied to an empty molecular orbital (M.O.). The energy of such transitions normally corresponds to photons in the near IR, visible or UV region of the electromagnetic spectrum. Electronic absorption bands give rise to the colours of compounds, including ones without transition metals. 

Visible light n. The narrow band in the electromagnetic spectrum that the human eye perceives from about 380 nm (violet) to 760nm (red). Johnson SF (2001) History of hght and colour measurement a science in the shadows. Taylor and Francis, UK. 

Describing the relationship between colour, wavelength, frequency and energy across the electromagnetic spectrum... 

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