2. What is colour?

From our point of view, colour is what we perceive when we are looking at objects.

Our retina is sensitive to red, green and blue and their various proportions to make up the rest of the colours we experience.

Other animals do not necessarily see the same colours as we do.

Some animals cannot detect the full range of the visible spectrum and some can detect ultraviolet like any other colour, which we cannot.

White light is a mixture of all the different colours.

The actual colours we see, or what an instrument detects, depends on the interaction of the light on the surface of an object.

For more on the structure and function of the eye (school biology notes)

Different colours arise due to the absorption, reflection and transmission of the different wavelengths of the light rays.

Different surfaces absorb, transmit or reflect the different wavelengths of visible light in different ways.

Advanced non-gcse note: Many of the differences in colour are due electronic changes caused by the photons hitting the surface, or being transmitted through the material e.g. the colours of transition metal compounds (GCSE chemistry notes). Even the size of solid particles can affect the 'shade' of a colour and sometimes a material in a very finely divided form can have a completely colour than the bulk material. If you want to forge a Van Gogh you will have trouble getting the correct shade for his famous yellow coloured sunflowers! You need the right particle size of the chrome yellow pigment.

Selected technical data on the colours of the visible spectrum

The figures quoted for wavelength and frequency are 'typical mid-range' for that colour.

Narrow bands of wavelengths/frequencies would look the same colour to us.

Visible colour ir and uv for comparison	wavelength (nm)	frequency in 1014 Hz	What you see via your eye!	Photon energy in kJ/mol	wavenumber in 104 cm-1
infrared (invisible to us)	1000	3.0	invisible to us	120	1.00
RED	700	4.3		171	1.43
ORANGE	620	4.8		193	1.61
YELLOW	580	5.2		206	1.72
GREEN	530	5.7		226	1.89
BLUE	470	6.4		254	2.13
INDIGO	445	6.8		285	2.26
VIOLET	420	7.1		343	2.38
near ultraviolet (invisible)	300	10	invisible to us	400	3.33
far ultraviolet	200	15	invisible to us	600	5.00
				for doc b's reference only - ignore!	for doc b's reference only - ignore!