Although I have written about chromatography, and how it can be used to separate mixtures of coloured compounds, it occurs to me that I have not really talked about why certain chemicals are a particular colour or even what colour really is. I shall try to address this omission here.
Our experience of colour is down to light. The visible spectrum of light (often called white light) contains a mixture of different wavelengths. Our eyes experience these different wavelengths as light of different colours. You might read this recent post for more information on the wavelengths of light. There are three primary colours (that cannot be dispersed with a prism into different colours) and these are red, green and blue. Where these colours intersect you get the complementary colours of magenta, yellow and cyan. This can be shown on a colour wheel.
Colours that are opposite one another on the wheel are complementary. If you mix two complementary colours you get white light again.
Coloured materials reflect or absorb light across different wavelengths. A material that absorbs all wavelengths appears black; one that reflects all wavelengths is white. Other colours are produced by mixtures of reflected light. (Hydrated) copper sulphate strongly absorbs light in the red part of the spectrum, therefore reflecting green and blue so it appears a light blue (or cyan). Chlorophyll absorbs red and blue light (which it uses for photosynthesis) and so reflects the green light.
When you are mixing colours in art, then you are removing wavelengths of light towards black rather than adding wavelengths towards white. The diagrams below show the difference. We don’t need to consider the behaviour of paint at this point. Let’s focus on the behaviour of light.
Ink jet and laser printers use black ink as well as cyan, magenta and yellow (CMYK) to produce all the colours that they print. The designers have found that mixing cyan, magenta and yellow produces a rather muddy black in practice.
Your eyes have two types of light sensitive cells. Rods, which operate in black and white but respond very quickly to any changes in light level, and cones, which are colour sensitive but slower to respond. The cones are concentrated around the centre of the retina whilst the rods are only found around the outer areas of the retina. Your brain combines information from both types of receptor to colour in the image that you see.
Now, if you have coped with all of that, you could watch the two videos on this post here that I posted recently. They are of a lecture at TED about how we perceive the world with our eyes and how all is not always as it seems.
Questions…
- What is the area of the retina where the optic nerve attaches to the eye known as?
- What are the names given to the wavelengths of light (a) just above and (b) below the visible spectrum?
- What property of light is affected by the amplitude of the wave?
- What are (a) the reactants and (b) the products of photosynthesis?
- I have read that dogs only see in black and white. What evidence do you think scientists might use to support that hypothesis?
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