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?



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).

 


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.

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. 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.

In terms of visible light, materials may be opaque, transparent or translucent.

Different objects (materials) may absorb, transmit or reflect different wavelengths of visible light in different ways - this makes explaining 'colour' a bit tricky at times!

Opaque means that visible light cannot be transmitted through the material. If the object does not transmit light then some colour wavelengths are absorbed (you don't see these) and other colours are reflected - the combination of these wavelengths make up the colour you see.

So, the colour of an opaque object depends on which colours in the visible light spectrum are the most strongly reflected and those most strongly absorbed.

Examples are described in the next sections e.g. a blue opaque object looks blue because all the other colours are absorbed and only blue wavelengths are reflected back into your eye.

Mixing colours is discussed in the next section.

Perfectly white objects reflect all the coloured wavelengths of the visible spectrum - white means you are seeing all or almost all the light that shone on the object - very little, if any, is absorbed by a shiny white surface.

Perfectly black objects absorb all the coloured wavelengths of the visible spectrum - black means you see little or none of the light that shone on the object - very little, if any, is reflected by a black matt surface.

Transparent materials will transmit all or a selection of visible light wavelengths through - not all the light that hits the surface is absorbed or reflected.

The observed colour of a transparent material depends on which wavelengths are transmitted.

e.g. 'colourless glass transmits most of the visible spectrum. Blue glass transmits 'blue' wavelengths and absorbs all the 'green to red' wavelengths. More examples are described in the next sections.

Translucent materials only allow a portion of the light through and some wavelengths may be reflected or absorbed.

You cannot see a clear image through a translucent material - tissue paper and frosted glass are good examples, and, can be a variety of colours.

For both transparent and translucent materials, their colour is determined by the colour wavelengths most strongly transmitted.


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:

An opaque object that looks cyan colour might look that for two reasons

(i) the surface reflects the green and blue frequencies of the visible spectrum

(ii) or the 'cyan' coloured wavelengths are reflected off the surface

An opaque object that looks yellow might look that for two reasons

(i) the surface reflects the green and red frequencies of the visible spectrum

(ii) or the 'yellow' coloured wavelengths are reflected off the surface

An opaque object that looks magenta might look that for two reasons

(i) the surface reflects the red and blue frequencies of the visible spectrum

(ii) or the 'magenta' (purple like) coloured wavelengths are reflected off the surface

In each case there are two definite possibilities and you can't tell which is which!


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.

Infrared photons do not have enough energy to trigger a response from the retinal cone cells, so we don't visually detect infrared EM radiation.

Ultraviolet photons have far more energy than visible light photons and can cause damage from chemical changes in the retinal cells.

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.

Digital cameras and colour printing also work on the basis of the three primary colours.


Visible Light Filters and the colour of objects subjected to a variety of colours

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.

Colour filters are designed to filter out different wavelengths of light, so only certain wavelengths are transmitted.

Tissue paper is translucent because the light is scattered by all the fibres of the paper, so no clear image is seen through it, though all light colours might come through if white paper.

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.

If you view a red object illuminated with white light through a red filter, it will look red because red light is reflected off the surface and will pass back through the filter. If you view it through a green or blue filter it will look black because neither of these colours is transmitted back through a red filter, they are absorbed, so no light seen - black.

You can logically analyse other situations by 'theoretically' viewing primary/secondary coloured objects through primary coloured filters.

 

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.

A cyan filter allows green, blue AND cyan through,

a magenta filter allows red, blue and magenta (sort of purple) through

and yellow filter allows green, red and yellow through,

on passing a beam of white light through them.

If you observe a green or blue object illuminated with white light through a cyan filter you see the object in its true colour (of green or blue). This because a cyan filter allows both green and blue light through.

If you observe the same two objects through a magenta filter the blue object will look blue, but the green object will look black because the magenta filter absorbs green light.

The yellow petals of a daffodil will appear black in blue light, but will appear yellow in yellow light. The petals will appear red in red light and green in green light since they reflect both those colours.

You can logically analyse other situations by 'theoretically' viewing primary/secondary coloured objects through secondary coloured filters.

 

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.

 


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.

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 or selective wavelengths 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.

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.

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 all 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.

 


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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.

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

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.

(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 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.


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.

 


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.

Visible colour

ir and uv for comparison

wavelength

(nm)

frequency

in 1014 Hz

What you see

via your eye!

Energy

kJ/mol

wavenumber

104 cm-1

infrared (invisible to us) 1000 3.0   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   400 3.33
far ultraviolet 200 15   600 5.00
        for doc b's reference only - ignore! for doc b's reference only - ignore!

 


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WAVES - electromagnetic radiation, sound, optics-lenses, light and astronomy revision notes index

General introduction to the types and properties of waves, ripple tank expts, how to do wave calculations

Illuminated & self-luminous objects, reflection visible light, ray box experiments, ray diagrams, mirror uses

Refraction and diffraction, the visible light spectrum, prism investigations, ray diagrams explained gcse physics

Electromagnetic spectrum, sources, types, properties, uses (including medical) and dangers gcse physics

The absorption and emission of radiation by materials - temperature & surface factors including global warming

See also Global warming, climate change, reducing our carbon footprint from fossil fuel burning gcse chemistry

Optics - types of lenses (convex, concave, uses), experiments and ray diagrams, correction of eye defects

The visible spectrum of colour, light filters and explaining the colour of objects  gcse physics revision notes

Sound waves, properties explained, speed measure, uses of sound, ultrasound, infrasound, earthquake waves

The Structure of the Earth, crust, mantle, core and earthquake waves (seismic wave analysis) gcse notes

Astronomy - solar system, stars, galaxies and use of telescopes and satellites gcse physics revision notes

The life cycle of stars - mainly worked out from emitted electromagnetic radiation gcse physics revision notes

Cosmology - the Big Bang Theory of the Universe, the red-shift & microwave background radiation gcse physics


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