# Salter's Advanced Chemistry A2 Unit 5 F335 Chemistry by Design Module 1 Colour b Design

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A substance expresses colour because the light falling on it contains photons of energy that is absorbed by electrons.
The electron shells absorb various wavelengths.

Speed of light

Wavelength

Frequency

Calculating the wavelength/frequency of light.
c=xf
where, c = speed of light, 3x108
x = wavelength in metres
f = frequency in hertz sec-1

Wavelength is the distance between peaks in a wave
Frequency is the number of cycles per second (peaks passing in a period of time)

Red light has the longest wavelength and the lowest frequency.

Wavelength is usually expressed in nanometres, 1x10-9m

Speed of light

Wavelength

Frequency

Red light wavelengths are around 700 nm and blue wavelengths, to work out the frequency of red light:

So,

Whereas the frequency for blue light is so blue light has a higher frequency that red light.

Absorption, reflection & transmission spectra.
In these the wavelength is shown increasing from left to right from blue to red.
The intensity of the absorption peak indicates the colours absorbed.
Opaque objects will also produce a reflectance spectrum which is the opposite of the absorption.
Transparent objects will give a spectrum of colours transmitted.

Coloured organic molecules

Colour is an absorption effect. Ie, electrons in a molecule absorb photons of light energy and jump to higher energy levels.
The energy level difference must be such that the frequency absorbed is in the visible region. If the energy difference is too great then the absorbed frequency may be in the ultra violet region.

∆Ε=hf

∆Ε=hf

UV light

Visible light
Where a molecule has a series of alternating double or triple carbon-carbon bonds called a conjugated system, the energy levels of the electrons are suitable for absorbing visible wavelengths. The system from a chromophore (the part of the molecule that absorbs coloured light)

A common chromophore found in many azo dyes is:

Adding different groups to the rings alters the electron energy levels and hence the colour absorbed.
The above chromophore is found in…