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Unit 4: Science and Materials in Construction and the Built Environment
Principles of Light
12.1 Nature of Light
Light is energy in the form of electromagnetic radiation. This energy is
radiated by processes in the atomic structure of different materials and
causes a wide range of effects. The different forms of electromagnetic
radiation all share the same properties of transmission although they
behave quite differently when they interact with matter.
Light is that particular electromagnetic radiation which can be detected by
the human sense of sight. The range of electromagnetic radiation to
which the eye is sensitive is just a very narrow band in the total spectrum
of electromagnetic emission, as illustrated in figure 12.1
Figure 12.1: Electromagnetic Spectrum
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12.2 Electromagnetic Waves
The transmission of light energy can be described as a wave motion or as
‘packets’ of energy called photons. The two theories co-exist in modern
physics and are used to explain different effects. The most convenient
theory for everyday effects is that of electromagnetic wave motion. This
can be considered as having the general properties listed below.
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The energy resides in fluctuations of electric and magnetic fields,
which travel as a transverse wave motion.
The waves require no medium and can therefore travel through a
vacuum.
Different types of electromagnetic radiation have different
wavelengths or frequencies.
All electromagnetic waves have the same velocity, which is
approximately 3 × 108 m/s in vacuum.
The waves travel in straight lines unless affected by reflection,
refraction, and diffraction.
Although not visible to us, the wave motion of light and other
electromagnetic waves is the same mechanism found in sea waves, sound
waves and earthquakes. All waves can change their direction when
subject to the following effects.
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
Reflection is reversal of direction which occurs at a surface.
Examples include mirrors, coloured surfaces. See figure 12.2
Refraction is deflection that occurs at the boundaries of different
materials. For example: prism effects occur at the edge of air and
glass, red sunsets are caused by differing layers of the atmosphere.
See figure 12.3
Diffraction is deflection that occurs at apertures, at edges and in
thin layers. Examples include coloured patterns in thin layers of oil,
coloured spectrums caused by narrow slits. See figure 12.4
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Figure 12.2: Examples of Reflection
Figure 12.3: Examples of Refraction
Figure 12.4: Examples of Diffraction
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12.3 Visible radiation
The wavelengths of electromagnetic radiation that, are visible to the eye
range from approximately 380 nm to 760 nm. If all the wavelengths of
light are seen at the same time the eye cannot distinguish the individual
wavelengths and the brain has the sensation of white light.

White light is the effect on sight of combining all the visible
wavelengths of light.
White light can be separated into its component wavelengths. One
method is to use the different degrees of refraction of light that occur into
a glass prism, as shown in figure 12.5. The result is a spectrum of light,
which is traditionally described in the seven colours of the rainbow
although, in fact, there is a continuous range of hues (colours) whose
different wavelengths cause different sensations in the brain.

Monochromatic light is light of one particular wavelength and
colour.
If the colours of the spectrum are recombined then white light is again
produced. Varying the proportions of the individual colours can produce
different qualities of ‘white’ light.
Figure 12.5: Dispersion of white light
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Unit 4: Science and Materials in Construction and the Built Environment
12.4 Non-Visible radiation
Electromagnetic radiation with wavelengths outside the range of visible
wavelengths cannot by definition, be detected by the human eye.
However, there are wavelengths emitted by the sun which are adjacent to
the visible range of wavelengths, and although they cannot be seen they
are relevant to lighting processes.
12.4.1 Infrared
Infrared (IR) radiation has wavelengths slightly greater than those of red
light and can be felt as heat radiation from the sun and from other heated
bodies. Infrared radiation is made use of in radiant heating devices, for
detecting patterns of heat emissions, for ‘seeing’ in the dark, and for
communication links.
12.4.2 Ultraviolet
Ultraviolet (UV) radiation has wavelengths slightly less than those of
violet light. It is emitted by the sun and also by other objects at high
temperature. Ultraviolet radiation helps keep the body healthy but
excessive amounts can damage the skin and the eyes. The composition of
the earth’s atmosphere normally protects the planet from excessive UV
radiation emitted by the sun.
Ultraviolet radiation can be used to kill harmful bacteria in kitchens and in
hospitals. Certain chemicals can convert UV energy to visible light, and
the effect is made use of in fluorescent lamps.
12.5 Nature of Vision
The portion of the electromagnetic spectrum known as light is of
environmental interest to human beings because it activates our sense of
sight, or vision. Vision is a sensation caused in the brain when light
reaches the eye. The eye initially treats light in an optical manner
producing a physical image in the same way as a camera. This image is
then interpreted by the brain in a manner which is psychological as well
as physical.
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12.5.1
The eye
Figure 12.6 shows the main features of the human eye with regard to its
optical properties. The convex lens focuses the light from a scene to
produce an inverted image of the scene on the retina. When in the
relaxed position the lens is focused on distant objects. To bring closer
objects into focus the ciliary muscles increase the curvature of the lens: a
process called accommodation. The closest distance at which objects can
be focused, called the near point, tends to retreat with age as the lens
become less elastic.
Figure 12.6: Structure of the eye
The amount of light entering the lens is controlled by the iris, a coloured
ring of tissue, which automatically expands and contracts with the amount
of light present. The retina, on which the image is focused, contains light
receptors which are concentrated in a central area called the fovea; and
are deficient in another area called the blind spot.
12.5.2
Operation of vision
The light energy falling on the retina causes chemical changes in the
receptors which then send electrical signals to the brain via the optic
nerve. A large portion of the brain is dedicated to the processing of the
information received from the eyes and the eyes are useless if this sight
centre in the brain is damaged.
The initial information interpreted by the brain includes the brightness
and colour of the image. The stereoscopic effect of two eyes gives further
information about the size and position of the objects. The brain controls
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selection of the many items in the field of view and the sense of vision
greatly depends on interpretations of images learned from previous
experience.
12.5.3
Sensitivity of vision
The light-sensitive receptors on the retina are of two types. These
receptors respond to different wavelengths of light in the manner shown
in figure 12.7 and they give rise to two types of vision.
Cone vision
The cones are the light receptors that operate when the eye is adapted to
normal levels of light. The spectrum appears coloured. There is a
concentration of cones on the fovea at the centre of the retina and these
are used for seeing details.
Rod vision
The rods are the light receptors that operate when the eye is adapted to
very low levels of light. The rods are much more sensitive than the cones
but the spectrum appears uncoloured. The colourless appearance of
objects in moonlight or starlight is an example of this vision. There is a
concentration of rods at the edges of the retina, which cause the eyes to
be sensitive to movements at the boundary of the field of view.
Figure 12.7: Sensitivity of eye
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12.6 Colour reproduction
The human vision can distinguish many hundreds of different colours and
intensities of colour. Modern systems of colour representation by ink,
photography and television can reproduce these many colours to the
satisfaction of the eye, yet such systems use just a few basic colours.
It has been shown that white light contains all the colours of the spectrum
which can be recombined to give white light again. White light can also be
split into just three colours which recombine to give white light again. In
addition to white light, any colour can be reproduced by various
combinations of three suitable primary colours. Most colour systems use
this trichromatic method of reproduction and the eye is believed to send
its information to the brain by a similar method of coding.
Newton’s discoveries concerning the combination of colours initially
seemed to disagree with the experience of mixing paint. This confusion
about combining colours can still arise because they can be mixed by two
different methods which have different effects: additive mixing and
subtractive mixing.
12.6.1
Additive Colour
If coloured lights are added together then they will produce other colours.
When the tree primary additive colours are combined in equal proportions
they add to produce white light.
Additive primary colours
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Red + Green + Blue
Red + Green
Green + Blue
Red + Blue
=
=
=
=
White
Yellow
Cyan
Magenta
Application of additive colour
Some of the important applications of additive colour mixing are
described below.
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Stage lighting: By using three or more coloured light sources, on
dimming controls, any colour effect can be obtained.
Colour television: A video screen has many small red, green, and
blue phosphors, each type controlled by a separate beam of
electrons. A close look at a colour television will reveal that white
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
doesn’t actually exist on the screen but is an effect produced in the
brain.
Colour printing: Some processes, such as gravure, produce a
mosaic of ink dots which effectively act as an additive system.
12.6.2
Subtractive Colour
If colours are subtracted from white light then other colours will be
produced. When the three primary subtractive colours are combined in
equal proportions they subtract (absorb) components of white light to
produce darkness.
Subtractive primary colours
Cyan
subtracts
Red
Magenta
subtracts
Green
Yellow
subtracts
Blue
These pairs are termed
complementary colours
White light can be considered as a combination of red, green and blue
light. Materials that transmit or reflect light absorb selected wavelengths
and pass the remaining light to the eye. Combining two or more paints, or
coloured layers, has a cumulative effect, as shown in figure 12.8.
Figure 12.8: Combination of subtractive colours
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Application of subtractive colour
Most colours are seen as a result of subtractive processes and some
common applications are given below:
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Paint Pigments: Paint colours are mixed according to subtractive
principles, even though, for simplicity, the primary colours are often
called red, yellow and blue instead of magenta, yellow and cyan.
Colour photographs: All colour transparencies and colour prints
are finally composed of different densities of three basic dyes: cyan,
magenta and yellow.
Colour printing: White paper is over-printed three times with the
three basic colours of printing ink which are cyan, magenta and
yellow. Black ink is also used to achieve extra density.
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