3b Colour

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Colour
By Talar Hagopian and Rima Debs
École la Dauversière, Montreal, June 2001
Content validation and linguistic revision
Karine Lefebvre Translated from French to English
by Nigel Ward ?Science animée, 2001
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In which of these two worlds would you prefer to
live?
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Put some colour in your life!
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Table of contents
Light and colours
Wavelengths and colours
How the eye sees colours
Rainbows
Games
Conclusion and bibliography
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Light and colour
The primary colours are different in art (paint)
and in science (light). Thats because in science
we are mainly interested in adding coloured
lights together but paint works by absorbing
(subtracting) colours.
For paint the primary colours are red, yellow
and blue,
while for light addition they are red, green
and blue.
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By mixing yellow paint with blue paint we obtain
green...
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but by mixing yellow light with blue light we
get
yellow light green light red light
white light!
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By shining red, green and blue light beams onto a
white screen and making them overlap, we obtain
white light.
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By shining red, green and blue light beams onto a
white screen and making them overlap, we obtain
white light.
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By shining red, green and blue light beams onto a
white screen and making them overlap, we obtain
white light.
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Lets concentrate on the colours of light
Here are the secondary colours and how they are
formed
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red blue Magenta
red green yellow
blue green Cyan
Cyan, magenta and yellow are the three secondary
colours.
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Optical filters
Optical filters work like paint, by absorbing
certain colours. For example, a red filter allows
only red light to pass. A cyan filter allows blue
and green light to pass (remember cyan blue
green).
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By superimposing coloured filters (cyan, magenta
and yellow) we get black (the absence of light)
where the three filters overlap. The magenta
filter transmits red and blue light and blocks
green. The yellow filter blocks blue. The cyan
filter blocks red. Where the three filters
overlap every primary colour is blocked.
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By superimposing coloured filters (cyan, magenta
and yellow) we get the primary colours where
pairs of filters overlap. For example, the
magenta filter can transmit red and blue and the
yellow filter can transmit red and green - only
red can pass through both the magenta filter and
the yellow filter.
blue
red
green
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Wavelength and colour
Light consists of electromagnetic waves with
various wavelengths. Wavelengths of light are
usually measured in nanometres (nm).
Longer wavelengths correspond to the colour red.
As the wavelength decreases the light becomes
orange then yellow then green then indigo then
violet. Since we are mainly interested in the
primary colours red, green and blue we can say
long wavelengths correspond to red, medium to
green and short to blue.
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Visible light waves (colours) are part of a
family called the electromagnetic spectrum. All
members of this family share certain properties.
For example, they all travel at the same speed
through a vacuum.
Gamma (?) rays
Radio waves
X rays
Microwaves
Ultraviolet
Infrared
Visible light waves of various colours
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A bit of history...
In 1873, James Maxwell proved that
electromagnetic waves consist of a combination of
an electric wave (an oscillating electric field)
and a magnetic wave (an oscillating magnetic
field).
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At about the same time the German physicist
Heinrich Hertz, with the help of Maxwell, managed
to produce radio waves and showed that they have
all the properties of light reflection,
refraction, interference (superposition of
waves), diffraction, polarisation and speed (?
300 000 km/s).
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But well before them, Isaac Newton had attributed
wave properties to the particles of light.
Whats more, he had discovered that white light
consists of all the colours of the rainbow
combined together.
(1642-1727)
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Working with prisms, he noticed that white light
could be broken up into its different components,
the colours of the rainbow. He had discovered
dispersion. In the diagram below, a prism
disperses white light into the colours of the
spectrum.
prism
whitelight
spectrum
Dispersion
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It is possible of recombine the colours to form
white light again.
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Wavelengths of the colours of the visible spectrum
violet
indigo
blue
green
yellow
Measurements are in angströms. One angström is
one billionth of a metre.
orange
red
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How the eye
perceives colours
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The rod cells are sensitive only to shades of
gray but function even in dim light. There are
about 120 million of these detectors in the
retina.
The cone cells detect colour but dont work well
in dim light. We have only about 7 million cone
cells in the retina.
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Colour blindness
Colour blindness is an anomaly of vision. People
suffering from this condition cannot distinguish
certain colours from one another. For example,
someone suffering from red-green colour blindness
cannot distinguish red and green. Why would this
be a great problem when that person drives a
car? This visual dysfunction can be hereditary,
or a consequence of a disease that affects the
optic nerve.
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Technically, colour blindness is due to a poor
functioning or an insensitivity to colour of the
light-sensitive cells, making the brain unable to
recognise the colour correctly. There are
several types of colour blindness including
"red-green", which affects men more than women,
and yellow-blue", less common, which affects men
and women equally. Certain people can see only
two colours, and everything else looks gray.
Certain people suffer from mono-chromatism
which means they see no colour at all, only
shades of gray.
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We perceive objects to have certain colours
according to which colours they absorb and which
they reflect into our eyes.
This chick appears yellow because the
yellow component is reflected into the eyes of
the observer. The other components of the light
are absorbed.
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This bush appears green because the green
component is reflected into the eyes of the
observer. The other components of the light are
absorbed.
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Optical filters
An optical filter only allows certain colours of
light to pass. Other colours are absorbed by the
filter.
For example, tinted glasses.
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A filter made of a primary colour only allows
that colour to pass.
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A filter made of a secondary colour transmits the
primary colours that make up that secondary
colour.
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What about a colourless filter or a black filter?
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Which colour would our observer see if he looks
at the bush through a red filter?
Click on the bush to check your answer!
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The bush would appear black because the
green component reflected by the bush would be
blocked (absorbed) by the red filter. The filter
can only transmit red light but the bush does not
reflect any red light so no light would reach the
observers eyes (absence of light black).
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Which colour should the filter be so that the
observer sees the bush as green ?
Click on the lenses to check your answer !
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Green since a green filter would allow the green
light reflected by the bush to pass through. The
green light would then arrive in the eyes of the
observer!
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Rainbows
A rainbow is sometimes produced when sunshine
interacts with falling rain.

• In order to see a rainbow, the sun must be
  behind you.

• Sunlight hitting rain does not always produce a
  rainbow. In order for the raindrops to be able to
  form a rainbow they must be between 1 and 2 mm in
  diameter.

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This diagram shows how a ray of sunlight is
dispersed into a spectrum of colours as it passes
through a raindrop.
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The flag game
Stare hard at the red dot for 15 seconds then
look at the white space.
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3
2
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You will notice that the colours of the phantom
image of the flag are the same that those of the
real USA flag.
We see that because red, blue and white are
respectively the complementary colours of cyan,
yellow and black.
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The word game
blue white red gray green Mauve red Orange
yellow Turquoise Pink black
Say out loud the colours of these words do NOT
read the words themselves.
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Conclusion
Colours are part of our daily life Life would
be pretty dull without them! Luckily we find them
everywhere,even in science!
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Bibliography
Internet sites

• Beverly T. Lynds. (Page consulted 05 March
  2001). About rainbows, online.
  http//www.unidata.ucar.edu/staff/blynds/rnbw.html

• H. Jaegle and L. T. Sharpe. (Page consulted 15
  November 2000). Colour and night vision,
  online. http//www.eye.medizin.uni-tuebingen.d
  e/

• the University of Texas, Houston. (Page
  consulted 15 November 2000). Colour vision,
  online. http//eye.med.uth.tmc.edu/MasseyLab/c
  olor20vision/colorvision.htm

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Books

• M. PARAMON, José. Le grand livre de la couleur,
  Italy, Angela Berenuer Gran, 1993, 160 p.

• CHABOUD, René. La météo question de temps,
  France, Nathan, 1993, 286 p.

CD-ROM
Colour Blindness. Microsoft Encarta
Encyclopaedia 2000 CD-ROM. Microsoft
Corporation, 1999.