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180

FIND OUT MORE. Eye 140 • Light 178–179

Light has different wavelengths, which

we see as colours. The range of wavelengths

we see is called the visible spectrum. We separate

the colours of the spectrum by. DISPERSION.

When white light shines through a

specially shaped piece of glass called a

prism, it separates into its different

wavelengths by dispersion. The

wavelengths show up as a range

of colours called a spectrum. The

English scientist Isaac Newton

first used a prism to disperse

sunlight in the late 1600s.

HOW DOES A RAINBOW FORM?

Rainbows appear when there are water drops in the

atmosphere and bright sunshine at the same time. The

water drops act like tiny prisms, refracting and reflecting

the sunlight, and dispersing it into the colours of the

spectrum. To see a rainbow, you have to be standing at a

particular angle to the water drops and the Sun.

HOW MANY COLOURS CAN WE SEE?

Different wavelengths of light blend to produce

millions of shades of colour. The human eye is able

to pick out over 10 million of them – some of which

can be shown by a. (^) COLOUR TREE. The amount of
colour we see depends on how much light there is. In
dim light we see no colours at all, only shades of grey.
WHAT IS THE VISIBLE SPECTRUM?
Light waves are just one type of electromagnetic wave.
They belong to an electromagnetic spectrum that
includes radio waves, X-rays, and gamma rays. The
visible spectrum is the only part the human eye can
see. To our eyes, the colours in the visible spectrum
range from violet at one end to red at the other.
WHAT ARE PRIMARY COLOURS?
The light-sensitive cells in the human eye react to just
three types of light: red, green, and blue wavelengths.
These are the three primary light colours. If all three
types of wavelength enter the eye with equal strength,
we see white light. When just red and green light are
present, we see the mixture as yellow.
HOW IS COLOUR CREATED ON A COMPUTER?
Most computer drawing and graphics programs
include an electronic version of a colour tree for
choosing colours. This may be done by selecting
pre-set colours from a given range, or by setting
the percentages of red, green, and blue in the colour.
A colour tree is one way of grading or classifying colours. Using a
colour tree, it is possible to describe and then match a particular
shade of colour (of paint or fabric, for example).
4 THE MUNSELL COLOUR TREE
This system describes a colour according to three characteristics: its hue
(basic colour, such as blue), its chroma (strength of colour), and its value
(lightness or darkness). There are ten levels of value going up (like the
trunk of a tree) from black to white. At each level, each hue has a range
of shades depending on how far out it is from the central trunk. Today,
this system is used to specify colour on computers.
BENDING LIGHT THROUGH A PRISM 1
Different wavelengths of light travel at different
speeds through glass. A prism refracts (bends) the
shortest wavelengths the most. We see these as violet
light. The longest wavelengths are refracted the least. We
see these as deep red. All the other colours are in between.
1 PRIMARY LIGHT
The three primary light colours,
red, green, and blue, combine in
pairs to create secondary colours.
Red and green make yellow, blue
and green make cyan, and red
and blue make magenta. All
three together make
white light.
COLOUR TREE
4 RAINBOW OF LIGHT
Sunlight is a mixture of all the colours of the
spectrum – red, orange, yellow, green, blue, indigo, and
violet. Some animals can see colors that we cannot see.

Colour

DISPERSION

The different

wavelengths

(colours) in the

light beam are

refracted at

different angles

On a computer,

moving a slide up

or down changes

the level of value

Range of

colour shades

at the highest

value level

Some light

is reflected off

the bottom of

the prism

Triangular

glass prism

Beam of

white light

visible

spectrum