Doc Brown's Chemistry Advanced Level Pre-University Chemistry Revision Study Notes for UK IB KS5 A/AS GCE advanced A level organic chemistry students US K12 grade 11 grade 12 organic chemistry courses Spectroscopic methods of analysis and molecular structure determination

All my advanced A level organic chemistry notes

(a) The wavelengths of the colours of the visible light spectrum

Table of the colours and wavelengths of the visible light spectrum, every colour has its own range of wavelengths and corresponding frequencies.

E, the energy of a photon in J, is given by Planck's equation: E = hv

h = Planck's constant = 6.63 x 10^-34 Js, v = frequency, Hz.

You need to be familiar with inter-converting wavelengths and frequencies: c = λ

c = speed of light, 3.0 x 10⁸ ms^-1, λ = wavelength, m, v = frequency, Hz

In my uv-visible spectroscopy pages, wavelengths are usually quoted in nanometres (1 nm = 1.0 x 10^-9 m)

See general spectroscopy index page for examples of calculations

Below is a rough table guide to what colour is transmitted if the specified colour is absorbed.

See a colorimetric exercise for a similar table of colour matching for analysis using a colorimeter.

It should be emphasised that the colour you perceive, and the reason for it, is quite complex, as you will find out in my discussions of individual spectra.

The table above, and colour wheels illustrated in (b) and (c) are only an approximation to the predicted colour and what is actually observed!

What is absorbed by, or transmitted through a material, in the uv-visible region of the electromagnetic spectrum, all depends on the energy of the electronic quantum levels of the outer bonding/non-bonding electrons of the molecule.

See The in depth theory of uv-visible absorption and reflectance spectra

A more visual representation of the wavelengths of the colours of visible light in nanometres (1 nm = 1.0 x 10^-9 m)).

(b) Why and how do we see different colours?

The colour of an object depends on which wavelengths of visible light are absorbed, transmitted or reflected.

Complementary colours

To a good approximation you can often predict a colour or vice versa from what colours of visible light are absorbed by use of a complementary colour wheel.

Which ever segment colour is absorbed from white light, the material will display the complementary colour in the opposite segment.

e.g. something absorbing blue-violet will look yellow-green

 

The 'extremes' of 'colour'

A material that is transparent to all visible light wavelengths will appear colourless e.g. water, ethanol, pure sodium chloride crystals, glass with no pigment, perspex plastic.

A material that reflects all visible light wavelengths will appear an opaque white e.g. chalk, titanium dioxide powder or any other 'white' mineral pigment.

A material that absorbs all visible light wavelengths will appear an opaque black e.g. charcoal or soot.

These are the 'extremes' in terms of 'colours', but so-called 'coloured' materials owe their colour to what wavelengths are absorbed, transmitted or reflected.

 

Examples of coloured liquids or solids

The diagrams below illustrate resulting colours from absorption, transmission and reflectance of particular wavelengths of visible light

A simple colour absorption-transmission diagram of coloured liquids or solutions of materials. more labels

1A chlorophyll absorbs in the blue and red, so appears green by transmitted light.

1B absorption in the green-yellow-red region produces, in transmission, a violet-blue range of colour.

1C carotene absorbs in the green and blue and so appears as an orange colour.

For more see absorption spectra on the more advanced theory page.

 

A simple colour absorption-reflectance diagram of coloured solids

2A Absorption of e.g. red and blue on the surface makes the object look green from the reflected light.

2B Absorption in the red-yellow-green on the surface makes the object look a violet-blue colour.

2C Absorption on the surface of the blue-green wavelengths makes the solid look orange-red

For more see absorption reflectance spectra on the more advanced theory page.

The diagram illustrates a comparison of a blue solid pigment that is soluble in a solvent.

Absorption by the pigment in the yellow-red region produces, by transmission, a blue coloured solution e.g. of a dye.

Absorption by the pigment surface in the yellow-red region produces, by reflectance, a blue coloured solid.

See also The uv-visible absorption spectra of the photopigments in the human eye

(c) Colour wheels, complimentary colours and the wavelengths of visible light

I emphasise again that the colour wheels illustrated above are only an approximation to the predicted colour and what is actually observed!

The complexity of electronic excitations from ultraviolet and visible light photons in organic molecules or inorganic positive and negative ions doesn't always produce the colour you expect, but the complementary colour discs work well enough in most situations.

SPECTROSCOPY INDEXES

All Advanced Organic Chemistry Notes

[WEBSITE SEARCH BOX]