Light, Atoms
and their
Interactions
Isaac
Newton
1642-1727
2- Prism Experiment:
1st prism:
disperses
white light
into
spectrum
2nd prism:
recombines
spectrum of
coloured
light into
white light
Experimentum
Crucis
(crucial
experiment)
Newton
wrapped the
spectrum to
create the
colour wheel
from
Optiks
1704
Colour Wheel –
Claude Boutet 1708
Unable to represent spectral
red with any pigment,
Boutet substituted two reds
– fire-red and crimson –
omitting one of Newton’s
two blues. To compound the
confusion, the colorist
evidently misread
two of the labels, “orange”
and “violet.”
- John Gage
Colour wheel
by Moses Harris
(c. 1776)
Probably the first
completely
symmetrical circle of
primary and secondary
colours.
Also shows progressive
darkening of each hue
to black at centre (i.e.
shades).
modern
colour
wheel after
Johannes
Itten
from: The
Elements of
Colour
c. 1920
more accurate color
wheels contain
non-spectral colours like
magenta
The Extraspectral or Non-spectral Hues
• these include all hues between blue violet
(left) and orange red (right)
• they are not part of the spectrum, but are
generated in the eye & brain
colour (or more
precisely hue)
and
wavelength:
the electromagnetic spectrum
visible light
ultraviolet light
infra red image
all parts of the electromagnetic spectrum
can be characterized by
1.wavelength (λ)
2.frequency (f)
3.energy (E)
these descriptions are interchangeable
all parts of the EM spectrum travel at the
speed of light “c” (in a vacuum)
“shortest”
wavelength
Blue 400nm
Green 500nm
Red 600nm
“longest”
wavelength
but the rays themselves are not
coloured...
-Newton, Optiks 1704
• light rays have properties like refangibility
• colour is a perception
Colour (or the light rays reflected
off an object) results from the way
the light rays interact with matter
absorption of
colours
(wavelengths)
other than red
happens at
microscopic
level
All matter is composed of atoms
atoms are made
from protons &
neutrons
(in the nucleus)
orbited by
electrons
atomic structure: p and n in nucleus
e- in orbits around nucleus
(Bohr model)
Bohr model: incorrect, yet a powerful visualization tool
Fig. 7-5, p. 131
electrons can move from one orbit to another, but
never between orbits
Absorption:
• e- absorbs a photon
• e- jumps from lower
→ higher orbit
• energy of photon
converted to internal
energy within atom
• energy of photon
absorbed is exactly
equal to the change in
internal energy
Emission:
• e- jumps from higher →
lower orbit
• e- emits a photon
• internal energy within
atom converted to
energy of photon
• energy of photon
emitted is exactly equal
to the change in internal
energy
From sources that produce their own light, 3 types
of spectra are possible:
Coloured objects that require an illuminating source contain
colorants (pigments & dyes) that absorb specific wavelengths
(called selective absorption).
The colours of pigments and dyes arise due to selective
absorption by molecules,
Molecules are collections
of atoms, held together by
shared electrons
water
H20
methane
caffeine
Benzene molecule: C6H6
Direct Blue 86
Coloured surfaces made of molecules do not reflect
single wavelengths, rather they reflect a range of
wavelengths across the spectrum.
In order to describe more precisely how a surface
reflects light, a spectral reflectance curve is drawn.
= graph of
how much light
is reflected
for each wavelength
% Reflected
wavelength
spectral reflectance curve for a scarlet red paint
(blue, green and red lines represent response of 3 types
of cones in human eye)
reflectance curve of a red-purple paint
reflectance curves of two orange yellow paints
(left) cadmium yellow deep, PY65; (right)
quinacridone gold, PO49, with visual color samples
matched in Lab color space
Subtractive mixture: magenta + yellow = red
→ mixed colour results from wavelengths mutually
reflected by both paints in mixture