COLOUR
YEAR 11 - UNIT ONE PHYSICS
Mr. Bowen
East Doncaster Secondary College
Introduction
In Chapter One we looked at some of the
properties of light associated with its
straight line propagation and its ability to
reflect off materials. We are now going to
explore the nature and behaviour of light and
develop explanations for the images and
colours seen around us.
Summary
By the end of this presentation you should know:
the
component colours of white light
why objects look coloured
the factors that effect the colour of
an object
the primary colours of light
how colour filters work
that visible light is just a small part
of the electromagnetic spectrum
Component Colours of light (1)
 The
“rays” that we used to indicate the
path of light in Chapter 1 were a simplified
representation to indicate direction only.
They told us little about the nature of light.
 Light is actually a series of complex
electromagnetic waves that are able to
detected by our eyes.
 Light exhibits some wave-like properties.
 The “wavelength” of light (distance between
consecutive wavefronts) determines its
colour.
Component Colours of light (2)
 The
range of wavelengths of light that our
eyes are responsive to is known as the
“visible spectrum”.
 Each variation in colour is caused by light of
a different wavelength.
 The colours that make up the visible
spectrum are (in order from longest to
shortest wavelength): red, orange, yellow,
green, blue, (indigo), violet.
 Acronym: ROYGBIV
Wavelengths of light
(nm = x 10-9 m)
Colour
red
orange
yellow
green
blue
violet
Wavelength
780 nm
650 nm
600 nm
550 nm
450 nm
390 nm
Seeing coloured objects
 “White”
light contains roughly equal amounts
of each of the colours of the visible
spectrum.
 Sunlight, incandescent light and fluorescent
light all produce the same general sensation
(see fig. 2.2 page 35).
 When light falls on an object it may be
reflected, transmitted or absorbed.
 We see light from luminous objects directly
but we see most objects because light is
reflected off them.
Viewing objects under white light
 An
object’s colour is predominantly
determined by the colour/s of the light
reflected from it.
 White light incident on a material that
reflects predominantly red light will appear
red. The other colours are absorbed.
white light in
(ROYGBIV)
OYGBIV
absorbed
red reflected
Black, white and grey objects
 An
object appears white if all of the colours
of the spectrum are reflected in roughly
equal proportions.
 An object appears black if all of the white
light falling on it is absorbed (i.e. ROYGBIV
are all absorbed).
 An object appears grey if only a small
proportion of the incident white light is
reflected with all colours in roughly equal
proportions.
Colours other than ROYGBIV
 When
you see reflected colours other than
the main ROYGBIV colours, the pigment in
the material is reflecting back a range of
wavelengths not just one predominant
colour.
red + yellow = orange
red + blue = purple
yellow + blue = green
 Most
pigment reflection is not pure (one
colour only ) but often a range of colours
centered on the most dominant reflection.
Primary pigment colours
 When
we mix paints, yellow plus blue gives
green. This occurs because when they are
mixed the yellow pigment reflects orange,
yellow and green; and the blue pigment
reflects green, blue and violet. Green is
reflected by both and becomes the
dominant reflected colour.
 The three primary pigment colours are red,
yellow and blue. These three can be mixed
to make all other colours but they cannot be
produced by mixing other pigments.
Viewing objects under coloured light
 When
an object is illuminated by white light
it is said to be showing “true colour”.
 If the incident light is not white, the object
may look completely different.
 The “impure” colour depends on the colour/s
of the incident light and the colours that
are reflected back to you.
what colours are
incident?
what colours are
reflected?
Example 1
white light
(ROYGBIV in)
Y G
O
orange, yellow(dominant)
& green reflected back
- looks yellow
pure green light
incident
G
green reflected back
- looks green
Example 2
The French flag under white light.
The French flag under pure yellow light?
(the yellow is absorbed by the red and blue,
the white section reflects the yellow)
Colour Addition & Primary Colours
 When
you combine coloured light sources
you are performing colour addition.
 Red, green and blue light combined together
on to a screen produce white light. These
are known as the “primary colours” of light.
 You can combine red, green and blue light in
varying percentages to produce all other
colours of the spectrum.
RGB values for common colours
 Table:
RGB values for some common colors
Colour
Red
Green
Blue
sky blue
0.5
0.5
1.0
brown
0.5
0.4
0.3
orange
1.0
0.5
0.0
pink
1.0
0.7
0.7
purple
0.75
0.5
1.0
gold
0.9
0.8
0.3
Primary & secondary colours
 If
the three primary (p) colours partially
overlap they produce white and three
secondary (s) colours.
magenta (s)
red (p)
blue (p)
cyan (s)
green (p)
yellow (s)
white
Secondary colours
 Cyan
= green + blue
 Magenta = red + blue
 Yellow = red + green
Complementary Colours
 Colours
that combine to produce white light
are said to be complementary.
 cyan + red = white, magenta + green = white,
yellow + blue = white (these are pairs of
complementary colours)
Filters and colour subtraction
A
filter allows some colour(s) to be
transmitted but all other colours are
absorbed.
 A filter changes the colour of light through
a process called “colour subtraction”.
Red filter
White light
Red light
transmitted
OYGBIV absorbed
Summary
 In
Summary, our perception of the colour of
an object is not solely the result of the
properties of light.
 There are many factors which influence the
perceived colour of an object.
 Stick the photocopy of this presentation in
your module and copy Worked Examples
2.1A, 2.1B and 2.1C into it. Also do 2.1
Questions Nos, 1, 2, 3, 5, 6, 7, 8 and 10.
THE END