YEAR 11 - UNIT ONE PHYSICS
East Doncaster Secondary College
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.
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
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
Component Colours of light (2)
The range of wavelengths of light that our
eyes are responsive to is known as the
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.
Wavelengths of light
(nm = x 10-9 m)
red 780 nm
orange 650 nm
yellow 600 nm
green 550 nm
blue 450 nm
violet 390 nm
Seeing coloured objects
“White” light contains roughly equal amounts
of each of the colours of the visible
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
Black, white and grey objects
An object appears white if all of the colours
of the spectrum are reflected in roughly
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
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
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 what colours are
pure green light
orange, yellow(dominant) green reflected back
& green reflected back - looks green
- looks yellow
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) blue (p)
red (p) cyan (s)
Cyan = green + blue
Magenta = red + blue
Yellow = red + green
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
Filters and colour subtraction
A filter allows some colour(s) to be
transmitted but all other colours are
A filter changes the colour of light through
a process called “colour subtraction”.
Red filter Red light
White light transmitted
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.