SITEMAP   School Physics Notes: Visible spectrum-colour 7. The natural colours of objects

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Visible spectrum and colour: 7. More on what determines the natural colour of an object you observe without the screening effect of filters and examples of coloured solutions in chemistry

Doc Brown's Physics exam study revision notes

Explaining why objects are observed to have a particular colour

7. More on what determines the colour of an object you observe without the screening effect of filters

What has the colour of an object got to do with absorption and reflection?

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.

Opaque objects do not transmit visible light so certain wavelengths are absorbed and others are reflected - to give you the colour you see.

Transparent materials allow most (e.g. glass) or selective wavelengths (e.g. colour filter) of visible light through.

Apart from colourless materials, the colour you see is due to which wavelengths are absorbed and the transmitted wavelengths make up the colour see.

You can also see a clear image through a transparent material.

Translucent materials allow the same range of colours through, but the surface scatters the light so no clear image can be seen through it e.g. course paper, frosted glass.

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 most of the wavelengths of the visible spectrum, it will appear white.

With transparent materials like water, glass or Perspex the majority of visible light wavelengths pass through, so they are described as colourless (NOT white!).

Materials like chalk or white paint are not transparent, but they do reflect all the coloured wavelengths of visible light.

If a surface absorbs most of visible light wavelengths it will appear black.

In fact all black objects still reflect a tiny amount of light, but we perceive it as 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:

(i) a reflection/absorption surface situation like most opaque objects around you OR

(ii) a transmission/absorption situation with transparent materials, where some light is passing through a material e.g. coloured glass ornaments, coloured solutions in the chemistry laboratory, colour filters in the physics laboratory and 'quality' sweet papers!

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.

In other words what you see is white light minus the colours absorbed by the material's surface or absorbed on transmission if a transparent material.

Everyday examples of coloured objects viewed in 'white' light

Examples 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.

Your eyes perceive black as the absence of light from the object - no colour seen.

Watch out for complications:

e.g. a yellow object might be reflecting both red and green light OR yellow light itself.

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 shown below are opaque apart from a glass pendant.

 (i) most colours absorbed - bin looks dark, (ii) blue reflected and red & green absorbed - bin looks blue red reflected - not absorbed by paint pigment, green and blue wavelengths absorbed - car looks red yellow pigment in car body paint, doesn't absorb green and red - they are reflected (or just yellow light itself?) red reflected - blue & green absorbed by the red pigment, all colours reflected by the white pigmented spots orange - red and yellow reflected or transmitted, green and particularly blue absorbed by the petals road markings use a yellow pigment, yellow (red + green) reflected - artists like Van Gogh used similar pigments! blue mineral, blue reflected, green and red absorbed, not sure if this isn't a copper ore? violet-blue-cyan wavelengths transmitted through the glass pendant, but red-yellow wavelengths absorbed magenta coloured flowers, green absorbed, blue and red reflected off the petals or transmitted through blue pigment in paint doesn't absorb blue - reflected, but red and green absorbed, locomotive looks blue green reflected off leaves, blue and red absorbed by chlorophyll - so leaves look green in spring and summer brown autumn leaves - blue still absorbed, green also absorbed but not yellow-red (see note below)

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 what 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!

The glass pendant is acting like a triangular prism producing a lovely visible spectrum of colours - due to the fact that the different frequencies (or wavelengths) refract at different angles at the glass/air boundary.

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.

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 (red 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, as does green chlorophyll in plant leaves.

B copper sulfate solution, blue, here the copper ions strongly absorb the red-orange wavelengths, less so the green

C ? example of cyan, ions absorb mainly red wavelengths of visible 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 mainly in the green region of the visible spectrum

Solutions like that of sodium chloride ('common salt') are colourless because they do not absorb any wavelengths of EM radiation in the visible region.

Stained glass windows make use of pigments to absorb colours and allow the glass transmission of other colors, giving the most wonderful artistic coloured effects.

Keywords, phrases and learning objectives for visible light and natural colours of objects in white light

Be able to describe why objects are observed to have a particular colour what determines the colour of an object you observe when illuminated with white light.

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