Colour

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This topic has influences in all three scientific disciplines. Although it can be complicated (it is not necessary for a technician to understand the full complexities of electromagnetic radiation), an understanding of what lays behind some experiments is quite useful.


Biological background

The eye has light sensitive receivers at the back of the eye called rods and cones. The rods are sensitive to low level light and the cones are responsible for colour vision. They are sensitive to light between ~680nm and ~350nm wavelength this will vary between individuals.( See diagram). However- there are imperfections (genetic) that mean some people are not able to tell colours apart. Different imperfections show up as different types of colour blindness. It is unlikely that a sufferer will only see in black and white, (or sepia as this articles’ originator is accused). Tests are available that replace the “ISHIHARA” tests. A web search will show the traditional dot patterns for these charts. If you have these charts they should be stored in the dark as the inks deteriorate with exposure to light. Certain combinations can be seen by colour-blind persons that the normal eye cannot see. A useful investigation is to look at these charts with different coloured filters over each eye (e.g. red and blue) Colour perception can be investigated in various situations and is normally dealt within the physics part of a course.
Cone-response.png Bees and other animals see light differently. Some animals have upto eight colour receptors. It may be worth spending a few minutes searching for UV images of flowers on the web and IR of mice etc. These can then be printed and passed around to show how bees butterflies etc are guided to the pollen.

Experimental suggestions

"Ishihara" websearch
Ishihara test using filters over eyes
"UV" photographic webseatch

External Links

Colourblind test:on line test Note don't take this too seriously as you monitor needs to be set up exactly!
Or...Genetic inheritance, Test and animal vision
The role of the cone cells in vision | Wiki article
Colour perception | Colour perception wiki
UV images of flowers Be warned copyrighted images!

Light adding –additive colour mixing

This is how your TV works. The colour perception theory using three colours was first suggested by Newton. The Primary light colours are RED, GREEN and BLUE. This is not the same as you were taught in your primary school art class. Paint relies on reflective properties and the colour is a trick of the eye and is discussed later.

  • Red and Blue give Magenta.
  • Blue And Green give Cyan
  • Green and Red give Yellow

Mixing these colours in equal proportions gives white.

light mixing

The image left shows what happens when light is projected. If the light is not of equal intensity there will be a colour cast. If three projectors are used this effect can be shown by spreading the beam from one projector (either by a lens or by moving the projector), the white centre will lose some of the colour of the moved projector.

Experiments

Full details: Colour Mixing (additive)

  • 3 coloured slide and mirrors
  • 3 Projectors

Three projectors can be trained on a central spot. A "Variac" mains transformer can be used to vary the power to the projectors and reduce the intensity of light.

  • Dedicated "PAR" spotlamps on dimmers can be set up to show this effect.
  • Magnifying glass and a TV can be used to show the 3 phosphors
  • If available a 3 lamp projector or a "safe" TV can be inspected (well away from any mains!) to show the three guns.
  • "Photoshop" an image to reproduce the first permanent colour picture.


Light subtraction – subtractive colour mixing

Inks and paint pigments are not "pure", that is the one strong colour being reflected often has a less noticeable impact than other wavelengths. This is easily explained by the action of filters (See suggested experiments).

subtractive colour mixing (filters)
  • YMCK system on paints/inks: Ink printing uses combinations of dots of Yellow Y magenta M Cyan C and black K to acheive most colours some metallic colours will require an extra one or two colours but in general YMCK system is good for most printing processes.


Experimental investigations

For suggested investigations Colour Mixing Subtractive


Origin of Light

Light requires a source of energy. In the case of most things around us it is the Sun. This is a Nuclear Reaction called fusion. What may be of interest is that the centre of the Sun is several million Kelvin yet the surface is only several thousand. Other Stars are a different colour. This is due to their surface temperatures. It is possible to predict how hot an object is by using Wien's law. For more information on hot (incandescent) bodies follow this internal link. This should explain why your computer monitor has a temperature setting in the set-up functions.

Light sources

There are several light sources that are of interest. Some are mentioned for reference only.

  • Incandescent (hot objects)
  • Spectral emissions (discharge lamps) including Laser (Stimulated emissions)
  • Bio-luminescent (created by animals)
  • Chemical luminescent (created by chemicals)
  • Fluorescence (*****)
  • Phosporescence (****)
  • Cherenkov radiation (radiation glow)
  • Triboluminescence and Fractoluminescence(Crush lighting)

Bio- and chemical- luminescence are a chemical reaction. (more detailed desciption required). A Quick demo is the light sticks used as emergency lighting. There are (somewhere??? luminol experiments on the web).

Phosphorescence and flourescence require energy to be absorbed and re-emitted. This involves high energy (U.V.) light being captured and converted into lower energy visible light. Flourescent samples can be obtained using Tonic water (brands vary in colour!) Flourescein (sp?) and methyl-salicilicate. Phosphors include Zinc sulphide and are, due to their toxic nature of most best kept enclosed. Try the "Maltese cross" Teltrontube.
A "Youtube" video is included to show how a blue laser can be used for a spectacular demonstration of refraction.
{{#ev:youtube|C02ryI-MRNE}} Rutherford used a Zinc Sulphide screen (a sprithariscope) to observe the path of alpha particles. If you have access to one it may get useful outing here!
Triboluminescence can be shown by crushing a polo mint! Often there are reports of flashes near earthquake zones.

Dispersion and spectra:

When a wave passes through a medium, it doesn't necessarily follow that it speed will be the same for all wavelengths. This is called dispersion. Sometimes this is desirable and other times it is a nuisance. Crown glass is not very dispersive to visible light but flint glass is. The phenomenon is not restricted to light and glass. Some solid materials have a strong dispersive nature with sound waves.
If you want more details behind the refractive index of glass and other materials, measurement definitions and values can be found by following this link:
kaye and laby refractive index and dispersion tables


Experiments

Refractive index
Spectrum -producing spectra in the laboratory
Flame Test

The car and coloured light as described above

External links

[1] Demos using an OHP and spectrum
[2] Neat link showing some good stuff
--D.B.Ferguson 20:32, 14 March 2007 (GMT)
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