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GCSE Physics notes: Investigating colour and the visible spectrum
The visible spectrum and colour
Doc Brown's Physics Revision Notes
Suitable for GCSE/IGCSE Physics/Science courses or their equivalent
This page will help you answer questions such as ... Why are some colours designated as primary? What are secondary colours? How does a light filter work to produce particular colours? What has the colour of an object got to do with absorption and reflection? Why are stained glass windows a complex mixture of light filters?
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(a) Reminder - using the dispersion of light by a prism to show the visible spectrum
This experiment was described on the Refraction to produce the visible light spectrum notes page.
In the 1660s the famous scientist Isaac Newton proved that white light 'contained' all the colours from violet to red. The experiment was quite simple, having dispersed the colours with one triangular prism (left to right in the above diagram) he then used a second triangular prism to recombine all the colours and reproduced the original white light (right to left on the above diagram). That's genius for you - all you have to do is think up a really simple experiment that nobody else had though of!
If you disperse sunlight with a prism or diffraction grating, this is what the solar spectrum looks like.
The wavelengths of visible light range from ~700 nm down to ~400 nm - remember this is only one small part of the whole of the electromagnetic spectrum (EM radiation).
(b) What is colour?
From our point of view, colour is what we perceive when we are looking at objects.
White light is a mixture of all the different colours.
(c) What do mean by opaque, transparent and translucent materials?
In terms of visible light, materials may be opaque, transparent or translucent.
(d) Primary and secondary colours (refer to the Venn colour diagram)
In this section, imagine you are shining lights onto an object or a screen, including mixing beams of light of different colours.
Although it is possible to mix two colours to make a different colour (e.g. yellow = green + red) it has not been found possible to produce either red, green or blue by mixing other colours.
Therefore red, green and blue are referred to as the primary colours. When these three are mixed together you make white light.
Yellow, cyan and magenta are referred to as secondary colours, because they can be created by mixing two of the primary colours.
yellow = red + green
cyan = green + blue
magenta = red + blue
By mixing a primary colour and a secondary colour you can reproduce white light.
red + cyan = white, green + magenta = white, blue + yellow = white
You can demonstrate all these effects with a suitable projector, screen and coloured light filters (but only certain types of filter work effectively).
Colour sources e.g mineral pigments or organic molecules can be mixed together to make a wide variety of colours.
Cautionary note with reference to the visible spectrum diagram above:
(e) The eye and TV screens (from a physics point of view)
The eye and the brain work together when see light from objects viewed around us. Different cone cells at the back of the eye in the retina can detect red, green and blue light photons - all these visible light photons have sufficient energy to trigger a tiny electrical impulse that goes to the brain. When all three types of cell (RGB cones) are triggered we see white. Combinations of the cell responses create signals to the brain which it interprets into all the colours we see.
Colour television uses the properties of primary colours. In a TV (and computer) screen there are three electron guns that hit a sort of paint called a phosphor. When hit by electrons the phosphors glow red, green or blue. The screen consists of thousands of pixels each containing a set of the three phosphors. By making these three phosphors glow at different intensities (brightness) they create the illusion of all the colours you experience when viewing the screen.
(f) Visible Light Filters and the colour of objects subjected to a variety of light colour
The diagram is presented in terms of primary and secondary colours only, but the principles described below apply to all the 'intermediate' colours.
Transparent materials allow some of visible light through
Colour filters are so called because they only let some light wavelengths through, but the image is not distorted.
A colour filter absorbs some colours from white light but allows your desired colours to be transmitted.
A basic set of six filters is illustrated above.
1 to 3 are primary colour filters because they only allow the transmission of one of the three primary colour band of wavelengths - red, green and blue on passing a beam of white light through them.
4 to 6 are secondary colour filters because they allow two of the three primary colours through to produce a secondary colour AND the colour of the filter itself. Looking at the spectrum diagram above, you can see all the primary and secondary colours somewhere in the spectrum.
By using various coloured mineral pigments or organic molecule pigments you can produce any shade or any colour you desire.
Stained glass windows use mineral pigments that absorb or transmit particular visible light wavelengths so when light streams through you see a selection of bright colours. A stained glass window is essentially a complex arrangement of visible light filters.
St Mary's RC Cathedral, Newcastle - stained glass window of the industrial heritage of north-east England
Many of the pigments are based on coloured transition metal compounds.
Now that's what you call a light filter display from stained glass window panels! (medieval Chester Cathedral)
I love the 'red devil' in this medieval stained glass window, St Martin-le-Grand Church, York, England
Medieval monks were skilled at mixing mineral pigments to colour the glass - very expensive work!
They made good use of all the primary colours of red, green and blue and the secondary colour yellow.
Over a thousand years ago they were producing yellow and green stained glass using iron oxides, but other colours soon came along. Copper minerals could produce red or blue glass and lots of colour appeared in the great cathedrals of Europe from the late 12th century onwards.
Art and chemistry, what a combination! OK it is physics too! So,
including the wonderful appreciative 'eye', lets called it art + science = manufactured beauty!
Light filters are widely used at pop concerts and stage lighting in the theatre.
(g) More on what determines the colour of an object you observe without the screening effect of filters
The colour of an object depends on relative absorption, reflection and transmission of different wavelengths of visible light.
Every colour is formed from a narrow band of wavelengths.
All objects absorb, reflect or transmit particular and often different wavelengths of visible light.
In the daytime you continually illuminated with white light, but every object has its own characteristic colour - very few objects are 'white'.
If a surface reflects all the wavelengths of the visible spectrum, it will appear white.
If a surface absorbs all visible light wavelengths it will appear black.
In reality, there are few perfect surfaces or transparent materials which behave with one of these extremes, but we do 'experience' these phenomena in 'black and white' terms!
In a sense, most colours we experience are somewhere in between these two extremes.
BUT, first - you need to appreciate in colour situations whether you are dealing with:
So, the colour of a material that you experience is usually which wavelengths are reflected of an object's surface or which colours are transmitted if the material is transparent.
Everyday examples of coloured objects viewed in 'white' light - simplified in terms of primary and secondary colours
Some reminders ...
Opaque objects that don't have a primary colour will reflect the actual wavelengths of light of that colour or wavelengths of primary colour light that mixed together give that colour.
White objects reflect or scatter all the wavelengths of visible light without differentiation.
Black objects absorb all wavelengths of visible light, therefore cannot be scattering any light - or you would see some colour.
Transparent means some or all of visible light wavelengths can pass through a material - allowing a clear image be seen on the other side.
Translucent also means partial transmission of light, but some of the light is scattered or absorbed and no clear image can be seen on the other side.
All the objects here are opaque apart from a glass pendant.
Note on leaves and the seasons
The chlorophyll molecule strongly absorbs red and blue light in the visible region - the energy absorbed for photosynthesis. The green wavelengths are not absorbed and are reflected giving leaves their characteristic green colour. In the autumn, when photosynthesis stops, the leaves turn many colours e.g. browns - yellows - reds etc.. This is because the chlorophyll breaks down when photosynthesis stops and the green colour disappears. The leaves no longer absorb in the yellow-red region, so the yellow to orange colours become visible giving the leaves some of their autumn colour, but happens to the blue and red no longer absorbed? At the same time other chemical changes may occur, which emphasize orange-red colours through the development of red anthocyanin pigments - they absorb blue and green! So you are still getting blue absorption, but not by chlorophyll. The overall effect is to give us a wonderful spectrum of autumn leaf colours changes from green ==> yellow ==> orange ==> dark red-brown - all captured in one photograph. The photograph above almost displays the full range of autumn colours.
Carrots contain an organic molecule called carotene. This molecule strongly absorbs visible light in the green-blue wavelength region of the visible spectrum. It does not absorb at all in the yellow and red regions, therefore carrots look red or orange and sometimes yellow.
The transparent 'colourless' pendant on the right, allows the transmission of all visible light wavelengths, but you do get some great spectrum effects! Many diamonds are almost colourless but nobody complains about the lack of colour since you get a great sparkle from the refractions and reflections that they create!
Another example but viewed when illuminated in various coloured lights
The ceramic toadstool in white light looks red with white spots.
What will it look like if it is viewed and illuminated with just one primary colour at a time and one secondary colour one at a time.
1. In red light it will look red all over, both red and white surfaces reflect red.
2. In green light it will look black (no red light to reflect) with green spots. The red surface absorbs green and the white spots reflect green.
3. In blue light it will look black (no red light to reflect) with blue spots. The red surface absorbs blue and the white spots reflect blue.
4. In cyan (green + blue) light it will look black (no red light to reflect) with cyan spots. The red surface absorbs green and blue and the white spots reflect all colours.
5. In magenta (red + blue) light it will look red with magenta spots. The red surface absorbs the blue and reflects the red and the white spots reflect any colour.
6. In yellow (red + green) light it will look red with yellow spots. The red surface absorbs the green and reflects the red and the white spots reflect all colours.
You can analyse in the same logical way any multi-coloured object illuminated by any of these light beams.
The colour theory of pigments can get complicated - I've just described the colour of objects in simple terms of reflection and absorption of particular wavelengths of visible light - in other words I've only deduced colours in terms of the three primary colours and three secondary colours.
(h) More examples of coloured stuff - solutions you may come across in chemistry!
These solutions of ionic salts could be:
R ? example, red colour, so ions absorb in the blue-green region (but ruby stones contain a chromium ion that absorbs in the green and blue)
G chromium(III) sulfate solution, green colour, so ions absorb in red and blue wavelengths
B copper sulfate solution, blue, here the copper ions strongly absorb the red-orange wavelengths, less so the green
C ? example, ions absorb red wavelengths of light
Y potassium chromate(VI) solution, yellow, the ions are absorbing in the blue region
M very dilute potassium manganate(VII) solution, ~purple, the ions absorb ?
Solutions like that of sodium chloride ('common salt') are colourless because they do not absorb any wavelengths of EM radiation in the visible region.
(i) APPENDIX 1. 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.
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