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4.2 Properties of Visible Light

Visible light is a mixture of all the colours of the rainbow. A prism refracts light,
separating the colours. A second prism can recombine the colours to form white
light. Different colours of light are carried by light waves that have different
wavelengths. An object looks blue in sunlight because it reflects blue and absorbs
colours other than blue from the sunlight.

After a rainstorm you might

Key Terms
step outside to a dazzling
light display of colour in the sky
reflection (see Figure 4.9). There
refraction might be a huge arc of
spectrum colours curving through the
visible light sky in front of you.
wave model of light

Figure 4.9 When the sunlight is

behind you and the air is full of water
droplets, you may be lucky enough to
see a spectacular rainbow.

4-5 Rainbows of Light Find Out ACTIVITY

The ability to see colour depends on the cells in your What to Do

eyes that are sensitive to different wavelengths of 1. In a darkened room, shine a flashlight through a
light. In this activity, you will observe the colours of the glass prism. Project the resulting colours onto a
light produced by a flashlight. white wall or ceiling. What colours do you see?
Materials 2. In a darkened room, shine a flashlight over the
surface of water with dishwashing liquid bubbles
• flashlight
in it. What do you see?
• glass prism
3. Clean up and put away the equipment you have
• water
used.
• dishwashing liquid
What Did You Find Out?
1. How did your observations in each case differ?
Explain where you think the colours came from.

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Wave Model of Light

Scientists have developed a model of light by looking at how light
behaves, and then trying to explain what they see. As you learned in
earlier science studies, a model is a way of representing something in
order to understand it better and to make predictions. One
explanation of light behaviour is the wave model of light, which
pictures light travelling as a wave. In this model, light is a type of
wave that travels through empty space and transfers energy from one
place to another, such as from the Sun to Earth. In the simplest
terms, visible light is a wave that you can see.

Refraction of Light
What occurs when a light wave passes from one material to another— Did You Know?
from air into water for example? If the light wave is travelling at an
The fastest known form of
angle and the speed that light travels is different in the two materials,
energy in our universe is a light
the wave will be bent, or refracted. Refraction is the bending or
wave travelling through space.
changing direction of a wave as it passes from one material to another. The speed of light is
White light, such as sunlight, is made up of waves having different approximately 300 000 km/s.
wavelengths and frequencies. If a light wave is refracted, such as by The distance 300 000 km is
passing through a prism (see Figure 4.10), the different wavelengths equal to about seven times the
bend by different amounts. Because the longer wavelengths are distance around Earth.
refracted less than the shorter wavelengths, different colours are
separated when they emerge from the prism.

Figure 4.10 A prism refracts light into different colours.

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Colours of the Rainbow

Word Connect Does the light leaving the prism in Figure 4.10 remind you of a
rainbow? Like prisms, water droplets also refract light (see Figure
The word “spectrum” comes 4.11). In a rainbow, the human eye can distinguish a range of colours
from Latin and means spectre, that are often described as falling into seven broad categories. In order
which is another word for of decreasing wavelength, and increasing frequency, these colours are
ghost. The plural of spectrum red, orange, yellow, green, blue, indigo, and violet. This range of
is spectra.
colours or frequencies of visible light is called the visible spectrum
(see Figure 4.12). The seven colours most easily seen in a rainbow are
sometimes abbreviated in the
form of a person’s name:
ROY G BIV (Red, Orange,
sunlight Yellow, Green, Blue, Indigo,
Violet).

Figure 4.11 As white light

passes through the water droplet, Figure 4.12 The visible spectrum
different wavelengths are bent by is made up of different colours,
different amounts. each having its own wavelength.
water droplet Red has the longest wavelength, about
700 nanometres (nm), while violet has
the shortest, about 400 nm. ▲

700

600

580

500

400

Reading Check
1. How does a prism separate light into different colours?
2. Which colour has the longest wavelength?
3. Which colour has the shortest wavelength?
4. Which colour has the highest frequency?
5. Which colour has the lowest frequency?

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Producing the Visible

Spectrum
At one time, people believed that
colour was something added to
light. When white light struck a
green leaf, people believed that the
leaf was adding green to the light.
Is colour picked up when light
strikes a coloured object? Or does
light itself contain colour? In the
17th century, English scientist Sir
Isaac Newton (see Figure 4.13)
conducted a famous experiment in
search of the answer to these
questions.
Newton placed a prism so that Figure 4.13 Sir Isaac Newton
a thin beam of white light could pass through it. When white light
travelled through the prism, he saw bands of colour emerge.
He observed that each band of colour was refracted at a different
angle. Newton concluded that the prism was not the source of the Did You Know?
colours. The different colours must have been present already in the
The Sun’s maximum output is in
white light. the red to yellow part of the
Next, Newton passed these colours through more prisms. visible spectrum. Plants have
This time, only white light emerged, as shown in Figure 4.14. In evolved to make maximum use
this way, Newton showed that colour was a property of visible light. of these wavelengths. Various
He proposed that white light such as sunlight is the result of mixing kinds of chlorophyll, the
together all the different colours of light. Figure 4.15 shows what pigments that capture sunlight
happens to the recombined light if one colour is removed from during photosynthesis, absorb
red and yellow pigments
the spectrum.
especially well.

white light
white light

Figure 4.14 A prism causes white light to split into a spectrum. Two more prisms Figure 4.15 If one colour is
can recombine the colours, producing white light again. removed from the spectrum, the
recombined light is no longer white.

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Colour and Reflection

Reflection occurs when a light wave strikes an object and bounces off.
When sunlight strikes coloured clothing, some colours are reflected
while other colours are absorbed (Figure 4.16). Only the reflected
colours can be seen.

Figure 4.16 Yellow cloth reflects yellow and absorbs other colours. Red cloth reflects red and
absorbs other colours.

Why does a bright red shirt look black when it is placed in a dark
internet connect room? The answer is that since a shirt does not produce its own light,
Only three numbers are needed
but merely reflects the light in the room, the shirt appears to be black
to specify every colour that can when there is no source of light.
be produced on a computer Only three colours of light, such as red, green, and blue, are
screen. People who create web needed to produce all the colours of the rainbow. The colours red,
pages sometimes specify colours
this way. For more information
green, and blue are sometimes called the additive primary colours.
go to www.bcscience8.ca. They are called additive colours because adding all three together in
the proper amounts will make white light, as shown in Figure 4.17A.
The light of two additive primary colours will produce a secondary
colour. The three secondary colours are yellow, cyan, and magenta
(see Figure 4.17B).

cyan

Figure 4.17(B) Each

white secondary colour is
light created by combining
red light blue light two of the primary
yellow
additive colours.
Figure 4.17(A) Combining
the additive primary colours green light
produces white light.
magenta

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Figure 4.18 When light

waves reflect from the ridges
on a CD, the light waves can
It takes three colours of light
add together to make some
colours brighter. Light waves to produce white light, but
can also cancel each other, they do not have to be the
removing some colours. primary colours. For example,
the secondary colours of
yellow, magenta, and cyan,
could do the same job. Red,
green, and blue (RGB) are
Reading Check usually used in computer
1. Why did Newton conclude that the prism was not the source of monitors, while cyan,
colours? magenta, yellow, and black
(CMYK) are usually used in
2. How could you use primary colours to produce secondary
printers. Find out why these
colours?
colour systems are used.
3. Why does a green shirt look green? Visit www.bcscience8.ca.
4. Why does a blue hat look black when it is in a dark room?

4-6 Colour Your Rainbow Find Out ACTIVITY

In this activity, you will create a rainbow using a bright What Did You Find Out?
light source and a CD. Then you can “colour your 1. Compare your findings with your classmates’
rainbow” as you observe it through different coloured findings.
filters.
2. Write a short paragraph that answers the following
Materials questions:
• CD (a) Which colours do you see when you look at
• coloured filters the rainbow made from white light?
(Remember, you might see more or fewer
• coloured pencils or felt pens
colours than a classmate.)
• white light source
(b) What is the effect on the appearance of the
What to Do rainbow when a red filter is held in front of it?
1. Hold a CD up to a light and adjust it until you can (c) Does it make any difference whether the filter
see a rainbow of colours. Keep in mind that is held between the CD and your eye or
different individuals see colours differently. between the CD and the light source? Explain.
2. Select a red coloured filter. Hold it between your (d) What does a coloured filter do to the light
eye and the CD. Then try holding the filter between coming from the light source?
the CD and the light source. Use coloured pencils or
felt pens to draw what you see in colour.
3. Repeat with several different coloured filters.

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Career Connect

Concert Lighting Designer happening at the same time, such as live feed
video. Once the concept is firm, we videotape
the band doing the songs. We work through the
night programming the lights so they give the
songs the right feel. We can usually program
two to four songs per night. Today you need a
lot of programming skills so you can use
computer-aided design programs such as
AutoCAD to design the light show. You also
need to understand how the lights and lenses
work.
Q. What do you need to know about lenses and
prisms to be able to design a light show?
A. These days we usually use automated lights that
have all the different lenses and prisms
integrated into one unit. The lenses spread the
beam size to make it cover a larger or smaller
Garry Waldie
area. Prisms break up the patterns that we put
in the lights. These “gobo patterns” can be
It is pitch black when you hear the first notes of anything from a simple spiral to the name of a
that hit song. Suddenly the lights come up and your band or hockey team.
favourite band is in the spotlight. Garry Waldie has
designed, programmed, and run thousands of light
shows at concerts. He has worked with Justin
Timberlake, Metallica, John Mellencamp, Christina
Aguilera, P. Diddy, and many more.

Q. How did you become a lighting designer?

A. As a kid, I was always going to the theatre. I
managed to get on tour after working with
some local acts. Then I got to do the lighting for
the opening act. Eventually I worked up to An example of gobo patterns
lighting the main act, and today I do about
200 shows a year. Questions
Q. Why is lighting important to a concert? 1. How are the lights for a concert controlled?
A. It sets the mood and enhances the whole show. 2. Why does the designer need to understand
Q. How do you create a light show? different lenses and prisms?

A. First I listen to the material and come up with a 3. Why would a computer-aided design
concept for each song. I pitch the concept to program be good to learn if you wanted to
the band and find out what else will be become a lighting designer?

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Checking Concepts 12. (a) A certain electromagnetic wave has a

wavelength of 200 nm. Is this wave
1. You can understand some properties of
visible to humans? Explain your answer.
light by using the wave model.
(b) A micrometre (µm) is one millionth
(a) What is a model in science?
of a metre. State the wavelength
(b) What is light, as described by the
of the wave
wave model?
(i) in micrometres
2. Which has a longer wavelength, red light or
(ii) in millimetres
green light?
(iii) in metres
3. Which colour refracts more in a prism,
13. Suppose that a device is built that converts
yellow or blue?
colour into sound. For example, if this
4. Explain how a prism is able to break
device detects the colour yellow, it will
sunlight up into its component colours.
produce the tone equivalent to middle C
5. Contrast refraction and reflection.
on the piano. If it detects blue, then it will
6. (a) What is the minimum number of
produce the same tone one octave higher in
coloured lights needed to produce all
pitch (C above middle C).
possible colours, including white?
(a) How will the sound for red light
(b) List some colours that will work
compare with the two tones mentioned
together to produce white light.
above?
7. What do the letters B and V stand for in
(b) How will green, orange, and violet
the acronym ROY G BIV?
compare?
Understanding Key Ideas
8. A light beam that is composed of red and
green light is passed through a red- P ause and R eflect
coloured filter.
(a) What is the colour of light that passes Some green paints are green because the
through the filter? pigment is made out of green chemicals.
(b) What colour is absorbed by the filter? Other green paints are made by blending two
9. Explain how a shirt can look green even or more non-green pigments such as blue
though the light falling on it contains red, and yellow. Why do you think that many
blue, and green. artists prefer to blend their own green paints
10. Describe a way to use two prisms to remove rather than use ready-made green paints?
the colour yellow from a beam of sunlight.
11. Make a concept map that links wavelength,
frequency, amplitude, brightness, and
colour.

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