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.
INDEX of my physics notes: The
visible spectrum and colour of objects
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.
Use your
mobile phone in 'landscape' mode?
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INDEX of my physics notes: The
visible spectrum and colour of objects
