Transcript 3b Colour - nward.com

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
click here to begin
In which of these
two worlds would
you prefer to live?
The primary colours are different in art (paint) and
in science (light). That’s 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.
By mixing yellow paint with blue paint we obtain …
… green...
…but by mixing yellow* light with blue light we get…
* yellow light =
green light + red light
… white light!
By shining red, green and blue light beams onto
a white screen and making them overlap, we
obtain white light.
By shining red, green and blue light beams onto
a white screen and making them overlap, we
obtain white light.
By shining red, green and blue light beams
onto a white screen and making them overlap,
we obtain white light.
Let’s concentrate on the colours of light…
Here are the secondary colours and how they are formed…
red + blue
= Magenta
red + green
= yellow
blue + green
= Cyan
Cyan, magenta and yellow are the three secondary colours.
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).
red filter
cyan filter
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.
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
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.
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.
Radio waves
Electromagnetic
Spectrum
Gamma () rays
X rays
Microwaves
Ultraviolet
Infrared
Visible light waves of
various colours
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).
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).
But well before them, Isaac
Newton had attributed wave
properties to the particles of light.
(1642-1727)
What’s more, he had discovered
that white light consists of all the
colours of the rainbow combined
together.
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
white
light
spectrum
Dispersion
It is possible of recombine the
colours to form white light again.
violet
3600- 4300
indigo
4300- 4550
blue
4550- 4920
green
4920- 5500
yellow
5500- 5880
orange
5880- 6470
red
6470- 7600
Measurements are
in angströms. One
angström is one
billionth of a
metre.
light
optic nerve
retina
cones and
rods
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 don’t work well
in dim light. We have only about 7 million cone cells
in the retina.
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.
Technically, colour blindness is due to a poor
functioning or an insensitivity to colour of the lightsensitive 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.
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.
This bush appears green
because the green component is reflected
into the eyes of the observer.
The other components of the light are
absorbed.
An optical filter only allows certain colours of light to pass.
Other colours are absorbed by the filter.
For example, tinted glasses.
A filter made of a primary colour only
allows that colour to pass.
red filter
blue filter
green filter
A filter made of a secondary colour transmits the primary
colours that make up that secondary colour.
cyan filter
magenta filter
yellow filter
What about a colourless
filter or a black filter?
colourless filter
black filter
Which colour would our observer see if he looks
at the bush through a red filter?
Click on the bush to check your
answer!
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 observer’s eyes (absence of
light = black).
Which colour should the filter be so that the
observer sees the bush as green ?
Click on the lenses to check
your answer !
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!
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.
This diagram shows how a ray of sunlight is dispersed
into a spectrum of colours as it passes through a raindrop.
Ray of
sunlight
raindrop
reflection
refraction
Dispersion
Stare hard at the red dot
for 15 seconds then look at
the white space.
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.
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.
Colours are part of our daily life …
Life would be pretty dull without them!
Luckily we find them everywhere,
even in science!
• 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.de/
• the University of Texas, Houston. (Page consulted 15 November 2000).
Colour vision, [online]. :
http://eye.med.uth.tmc.edu/MasseyLab/color%20vision/colorvision.htm
• 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.
«Colour Blindness». Microsoft Encarta Encyclopaedia 2000 [CDROM]. Microsoft Corporation, 1999.