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An astronomical telescope
can detect light from the
depths of space.

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Key Ideas

Many properties of light can

4 be understood using a wave
model of light.

4.1 Properties of Waves

4.2 Properties of Visible Light
4.3 Light and the
Electromagnetic Spectrum

Optical systems make use of

5 mirrors and lenses.

5.1 The Ray Model of Light

5.2 Using Mirrors to Form Images
5.3 Using Lenses to Form Images

Human vision can be

6 corrected and extended
using optical systems.

6.1 Human Vision

6.2 Extending Human Vision

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An aerobatics team produces colourful vapour trails.

igh-performance jets fly low in front of a crowd. The jets are at

H high speed and in tight formation, moving at 300 km/h and
holding steady at a separation of just 3 m from wing tip to wing tip.
On a radio command, each pilot squeezes a trigger, releasing billowing
clouds of red, white, and green. The spectators see the sky fill with
brightly coloured clouds. A thunderous roar rips through the air as the
jets scream by. As the jets move off, the clouds begin to mix and break
up.

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A performance like an air show depends on many things, including

the effective use of light. The pilots produced the coloured clouds at
the same instant using radio transmissions to communicate. Radio
internet connect
communication, colours, cameras, binoculars, eyeglasses, and our own Four main types of light sources
human vision system all depend on predictable properties of light. are incandescence, electrical
Ancient societies used light energy from fire and sunlight. In our discharge, fluorescence, and
modern society we also use laser light, radio waves, infrared light, and phosphorescence. Find out how
each of these processes
other forms of light energy. In this unit, you will learn about how we produces light. Start your
see and use visible search at www.bcscience8.ca.
light as well as
invisible kinds of
radiation. You will
learn about
optics—the branch Word Connect
of physics that
studies the Light comes from the Greek
properties of light word leukos, which means
and vision. white, and later from a
German word that means
to shine.
A laser light show

Light Is Energy Find Out ACTIVITY

Solar calculators use light from a light source such as a What Did You Find Out?
light in the room or the Sun to operate without 1. What happened to the display when light was
batteries. In this activity, you can observe evidence that prevented from reaching the solar panel?
light is a form of energy.
2. Was the calculator able to retain the numbers that
Materials were entered before the solar panel was covered?
Explain.
• calculator with solar panel that does not use
batteries 3. How would you explain to a younger student how
this experiment does or does not show that light
What to Do is energy?
1. Enter some numbers into the calculator and then
block any light from getting to the solar panel.
Note the result.
2. Uncover the panel and look again at the display.
Note the result.

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magine standing at the edge of a lake. The lake is calm and flat. It acts like a
I mirror, reflecting the far shore and the mountains beyond. Suddenly a fish
jumps. You hear a splash, and circles of water waves radiate out from where the
fish re-entered. These waves carry the energy that the fish transferred to the
water surface by its jump. The size of the waves and the amount of energy they
carry give you information about the size of the fish and how far out of the
water it jumped. Light is also a wave that carries energy a long way, as it travels
from its source, such as a flashlight or a star. All waves, including water waves
and light waves, share many common characteristics.

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FOLDABLES TM
Reading & Study
Skills

Make the following Foldable to guide your

study of the wave model of light.

What You Will Learn STEP 1 Draw a mark at the midpoint of a

sheet of paper along the side edge.
In this chapter, you will Then fold the top and bottom edges
• recognize that waves carry energy in to touch the midpoint. (If you are
• describe ways in which water waves can using notebook paper, use the centre
of the middle hole to mark the
explain properties of light
midpoint.)
• explain why a prism separates white light
into different colours
• describe properties and uses of
electromagnetic waves

STEP 2 Fold in half from side to side.

Why It Is Important

You can see and hear the world around you

because of the energy carried by waves. Waves
with different properties can be used in different STEP 3 Open and cut along the inside fold
ways. Electromagnetic waves can be used to lines to form four tabs.
make different kinds of images of the world
around us.

STEP 4 Label each tab as shown.

Skills You Will Use Properties Water Waves
of Waves and Light

In this chapter, you will

Prisms Electromagnetic
• observe how light can be separated into and Light Waves
colours
• model the properties of light
Read and Write As you read the
• communicate using diagrams and colours
chapter, record notes under the appropriate
tabs.

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4.1 Properties of Waves

Waves transfer energy through matter or space. Amplitude is the height of a wave
crest or depth of a wave trough, as measured from its rest position. A wavelength is
the distance over which the wave repeats. As the wavelength decreases, the
frequency increases. Waves can differ in how much energy they carry and in how fast
they travel.

A surfer bobs in the ocean

Key Terms
waiting for the perfect wave
amplitude (Figure 4.1), microwaves
crest warm up your leftover
energy pizza, and sound waves
frequency from your CD player
trough bring music to your ears.
wave These and other types
wavelength of waves have many
properties in common. Figure 4.1 Waiting for a wave. A wave transfers
energy through matter or space.

4-1 Watching Water Waves Find Out ACTIVITY

You do not need to visit the ocean to make waves. In 3. Lightly tap your pencil once per second on the
this activity, you can make waves right in your surface of the water. Observe the spacing of the
classroom. water waves.
4. Increase the rate of your tapping. Observe the
Materials
spacing of the water waves.
• pie plate or wide pan
5. Clean up and put away the equipment you have
• water used.
• pencil
What Did You Find Out?
What to Do 1. In what direction did the waves travel when you
1. Fill a pie plate or other wide pan with water about tapped the water lightly with your pencil?
2 cm deep. 2. How did the spacing of the water waves change
2. Lightly tap the bottom of a pencil once in the when the rate of tapping increased?
middle of the surface of the water. Observe the
waves that form.

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Features of a Wave
A wave is a disturbance or movement that transfers energy through
matter or space, without causing any permanent displacement. Sound
waves disturb the air and transfer energy through it. Ocean waves Did You Know?
disturb the water and transfer energy through it. Energy is the Sound waves can be used to
capacity to apply a force over a distance. A force is a push or pull on make an image of an unborn
an object. child during an ultrasound
To visualize the features of a wave, examine Figure 4.2. The procedure. Sound waves can
also be used for cleaning lenses
dotted line shows the equilibrium or rest position. The rest position
and other optical equipment,
is the level of the water when there are no waves. Notice the labels
dental instruments, and surgical
in the illustration. A crest is the highest point in a wave. A trough instruments.
is the lowest point in a wave.

The duck and the water

crest trough around it do not move
forward. They just move
up and down as the
wave passes.

A crest comes
through, and the
duck rises.

A trough comes
through, and the
duck drops.
Figure 4.2 The wave is moving from left to right.

Wavelength
The wavelength is the distance from crest to crest or from trough to
trough. You can also think of a wavelength as the distance covered by
one complete crest plus one complete trough (see Figure 4.3).
Wavelength is measured in metres.
wavelength

crest

trough

wavelength

Figure 4.3 A wavelength is the distance over which the wave repeats.

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Amplitude
If a breeze picks up on the lake where the duck is sitting, the height
of the waves can increase. This means that the duck floats higher and
lower as the crests rise and the troughs deepen. When the crests are
high and the troughs are low, we say the wave has a larger amplitude.
The amplitude is the height of a wave crest or depth of a wave
trough, as measured from its rest position (see Figure 4.4).
crest
amplitude

trough amplitude

Figure 4.4 The amplitude

of the wave crest equals the
amplitude of the wave trough.
The amplitude is related to the amount of energy carried by the
wave. The larger the amplitude, the greater the energy transported. A
light wave that has a large amplitude carries more energy and is very
bright. A dim light has a lower amplitude and carries less energy. The
next time you lower the brightness of a light using a dimmer switch,
think of the switch as a light wave amplitude adjuster.

Frequency
As the wavelength decreases, the duck and the water move up and
internet connect
down more frequently. Every cycle of bobbing up and down is called
With sound waves, frequency is an oscillation or a vibration. Frequency is the number of repetitive
related to musical pitch. Find motions, or oscillations, that occur in a given time. Frequency is
out more about the frequencies usually measured in hertz (Hz), or cycles per second. In our example,
of musical notes. Start your
it is the number of times per second the duck bobs from crest to
search at www.bcscience8.ca.
crest. For example, if two wave crests were to pass under the duck
every second, then the duck is said to be vibrating or oscillating at a
frequency of 2 Hz.
When the duck is sitting in water waves with short wavelengths, it
will bob up and down frequently. When the duck is sitting in waves
with long wavelengths, it will bob up and down less frequently. The
Suggested Activities
shorter the wavelength, the greater the frequency (see Figure 4.5).
Find Out Activity 4-2 When one value increases as the other decreases, scientists call this an
on page 138
inverse relationship.
Find Out Activity 4-3
on page 139
distance travelled in 1 s distance travelled in 1 s

Figure 4.5 The wavelength

rest position
of a wave decreases as the
frequency increases. All
waves share this property.
one one one
wavelength wavelength wavelength

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A Water Wave Moves Energy, Not Water

water
A water wave does not carry water along with it. Only the energy movement
carried by the water wave moves forward (see Figure 4.6). Many
important types of waves share this property—they carry energy wave
without transporting matter. Think of being out in the middle of a
lake and bobbing straight up and down as the wave passes underneath.
Only the energy in the wave moves forward toward the shore. You do
not move forward and neither does the water. Once the waves have
passed, the water returns to its original, or rest, position. Figure 4.6 The energy carried by
the wave moves forward. The water
Two Types of Waves moves up and down.

Waves can differ in how much energy they carry and in how fast they
travel. Waves also have other characteristics that make them different
from each other.
Sound waves travel through the air to reach your ears. Ocean
waves move through water to reach the shore. In both cases, the
matter the waves travel through is called a medium. The medium crest
can be a solid, liquid, or gas, or a combination of these. For sound
waves, the medium is air, and for ocean waves the medium is water.
The two types of waves that travel through a medium are transverse
waves and compression waves. trough rest position

Transverse waves
Figure 4.7 A transverse wave
In a transverse wave, matter in the medium moves up and down travels horizontally along the rope,
perpendicular to the direction that the wave travels (see Figure 4.7). and the rope moves up and down.
When you shake one end of a rope while your friend holds the other
end, you are making transverse waves. The wave and its energy travel
from you to your friend as the rope moves up and down.

Compression waves
Sound waves are compression waves. In a compression
wave, matter in the medium moves back and forth
along the same direction that the wave travels. You can
model compression waves with a coiled spring with a
piece of string tied on a coil (see Figure 4.8). Squeeze
several coils together at one end of the spring. Then let Figure 4.8 A compression wave
go of the coils, still holding onto the other end of the travels horizontally along the spring,
spring. A wave will travel along the spring. As the wave and the coils in the spring move back
moves, it looks as if the whole coil spring is moving and forth horizontally.

toward one end. The string moves back and forth as

the wave passes, and then stops moving after the wave has passed.
The wave carries energy, but not matter, forward along the spring.

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Water waves and seismic (earthquake) waves are a combination of

transverse and compression waves. Seismic waves can travel through
Earth and along Earth’s surface. When objects on Earth’s surface
absorb some of the energy carried by seismic waves, the objects move
Traffic waves are a form of
traffic jam on highways that
and shake.
can occur when cars are more Not all waves need a medium to travel through. Some waves, such
densely packed in some places as visible light waves and radio waves, can travel through space where
and less densely packed in there is no material.
others. A traffic wave can
move through a lane of cars
causing the whole lane to Reading Check
slow down. Find out what
1. What is the difference between a crest and a trough?
causes these waves and what
can be done to prevent them.
2. What are three ways to measure wavelength?
Start your search at 3. What property of a wave is measured in hertz?
www.bcscience8.ca. 4. How are the wavelength and frequency of a wave related?
5. What is the difference between a transverse wave and a
compression wave?
Suggested Activity
Conduct an Investigation 4-4
on page 140

4-2 Frequency Formula Think About It

Examples of frequency exist all around you. In this What to Do

activity, you can calculate frequency by using the 1. Use the following equation to calculate frequency
number of cycles, the time, and an equation. (in hertz) for each of the examples below. The first
example is done for you.
frequency ⫽ cycles ⫼ seconds
(a) pendulum: 24 swings in 6 s
frequency ⫽ cycles/s
⫽ 24 swings/6 s
⫽ 4 Hz
(b) merry-go-round: 12 revolutions per 2 min
(c) flashing red light at an intersection:
30 flashes in 0.5 min
(d) heart rate: 18 beats per 20 s
(e) car drive shaft: 2000 rpm (revolutions per min)

What Did You Find Out?

1. In order to calculate frequency measured in hertz,
what must be done with the time unit
before dividing?
The pendulum on a grandfather clock

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4-3 Catch a Waveform Find Out ACTIVITY

A waveform is a visual record of waves. In this activity,

you will make a waveform using the motion of a
vibrating metre stick.

Materials
• felt pen
• metre stick
• C clamp
• cardboard or manila card stock
• masking tape

What to Do
Part 1, step 4
1. Tape the felt pen to the end of the metre stick.

Part 1
What Did You Find Out?
2. Clamp the metre stick to a desk with 40 cm of the
1. What did you observe about the sound of the
metre stick (and the pen) extending out from the
metre stick vibrating?
desk. Hold the end firmly in place on the desk.
2. Measure the distance between two adjacent crests
3. Gently press down on the metre stick and let it go
on each waveform. Which trial produced waves
so that it can vibrate gently.
with the longest wavelengths?
4. Have a partner hold the cardboard and walk slowly
3. Which trial produced the most vibrations?
next to the vibrating pen. The waveform should be
recorded on the cardboard. Make sure that several 4. As the wavelength increases, what happens to the
waves are recorded. You may need to practise this frequency?
several times to get it right. Your partner can 5. What is the relationship between wavelength and
follow a masking tape line on the floor in front of frequency?
the desk to make it easier to walk in a straight line. 6. Is it possible for the wave with the greatest
wavelength to also have the greatest frequency?
Part 2
Explain.
5. Make a new waveform on a new piece of
cardboard by repeating steps 3 and 4.
This time, increase the length that the metre stick
extends out from the desk to 60 cm.
6. Label each waveform with crest, trough, and
wavelength.
7. Clean up and put away the equipment you have
used.

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4-4 Wire Waves

Skill C h e c k A coiled metal spring can be stretched along the floor and moved back and forth
• Observing to generate waves. When you make compression waves in a coiled spring, a
• Classifying compression is the region where the coils are close together. The less-dense region
of a compression wave is called a rarefaction.
• Communicating
• Modelling Question
How can a coiled metal spring be used to investigate amplitude, wavelength, and
frequency?
Safety

• Do not let go of the spring

when it is stretched out.
• The end of the spring might
be sharp.
Materials
• coiled metal spring or
Slinky®
• piece of masking tape or
string

Step 1 Attach tape or string to the spring.

Step 2 Carefully stretch the spring out on the floor.

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Conduct an INVESTIGATION
Inquiry Focus

Step 3 Hold each end of the spring firmly.

Procedure Analyze
1. Work with a partner. Attach a piece of tape or 1. How did the wavelength in the spring change as
string at about the halfway mark of the spring. it moved from side to side more quickly?
2. Stretch the spring out on the floor, with you 2. How did the marked coil move in each of your
and your partner each holding an end. Be very waves?
careful not to overstretch the spring, as it is 3. (a) How are the frequency and amplitude of a
easily damaged. Also, be careful not to allow the wave related?
spring to get knotted up. Always keep the spring
(b) Can a low frequency wave sometimes have
on the floor when generating waves.
a large amplitude, and sometimes have a
3. Hold one end of the spring firmly in place as small amplitude? Explain.
your partner moves the other end slowly from
side to side. Observe and draw a diagram of the Conclude and Apply
wave that results. Label it “low frequency 1. (a) Draw a diagram to illustrate:
wave,” and indicate its wavelength. Use arrows (i) a wave with a high frequency, a short
to show the directions in which the marked coil wavelength, and a large amplitude
moves. Note whether you feel a side-to-side
(ii) a wave with a low frequency, a long
force as you hold the spring firmly in place.
wavelength, and a small amplitude
4. Repeat step 3 but have your partner move the
(b) Use labels to show crests, troughs,
end of the spring quickly from side to side to
wavelength, and amplitude on both
provide a higher frequency. There will be more
diagrams you drew in (a).
places on the spring that do not move very
much, and other places that move a lot. What 2. The amount of energy transferred by the spring
has happened to the frequency? Observe and changes with frequency, and also with
draw a diagram of the resulting wave. Indicate wavelength.
the wavelength. Label this diagram. (a) What happens to the amount of energy
5. Try to do the following. Draw and label a transferred through the spring as the
diagram for each of your results. frequency increases?
(a) Increase the amplitude of the wave. (b) What happens to the amount of energy
transferred through the spring as the
(b) Make a low frequency, high amplitude wave.
wavelength increases?
(c) Make a high frequency, high amplitude wave.
(d) Make a low frequency and low amplitude
wave.

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Noise Cancellation Headphones

positive sound

cancelled sound

The positive and negative

sound waves cancel
each other in noise
cancellation headphones. negative sound

Have you ever tried to listen to music on a noisy bus? Noise cancellation headphones have tiny
Earphone plugs help to keep out the background microphones mounted into the headsets that detect
noise, while larger headphones have foam pads that the background noise. The background noise is called
help block noise. Noise cancellation headphones, also the positive sound, because it is the sound that is
called noise reduction headphones, use properties of normally heard. A digital signal processor analyzes the
sound waves to reduce noise by cancelling out shape of the positive sound wave and then generates
unwanted waves. Noise cancellation headphones work another sound wave that has the exact opposite
best against constant noise, such as the sounds of a shape. This cancellation wave is called the negative
school cafeteria or an aircraft engine. sound. The negative sound is then amplified and
played through the headphones. The positive and
Sound is carried by a series of high and low
negative sound waves combine and effectively cancel
pressure waves that move from the source of the
each other out.
sound to your eardrum. The changing pressures cause
your eardrum to vibrate. Sound waves have a Some people use noise cancellation headphones
particular shape that simply to listen to silence. Others use them to listen to
is determined by their music. Using noise cancellation allows you to listen to
wavelength and music at a lower volume than you would otherwise be
amplitude. The wave- able to.
length determines the
Noise cancellation does not remove all sounds that
pitch of the sound and
you might hear—which is a good thing, because you
the amplitude
want to be able to hear the approach of the school
determines how loud
bus you are waiting for!
the sound is.

Noise cancellation headphones can help you study when you

are in a noisy place.

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Checking Concepts 8. A student performs a frequency experiment

on three different pendulums and obtains
1. Draw a wave with a wavelength of 4 cm and
the following results:
an amplitude of 1 cm. Label the crest, the
trough, the amplitude, and the wavelength.
Pendulum Number Time to Complete
2. (a) A buzzer vibrates 900 times in 1 s. What
of Swings All the Swings
is its frequency?
(b) A guitar string vibrates 880 times in 2 s. (s)
What is its frequency?
A 32 8s
(c) A ball bounces on the floor 10 times in
50 s. What is its frequency? B 72 9s
3. (a) Draw a transverse wave and a C 210 1 min 20 s
compression wave.
(a) Calculate the frequency of each
(b) Give an example of each type of wave.
pendulum in Hz.
4. A speedboat zips by on a lake and sends
(b) Rank the pendulums from lowest to
a series of waves toward a dock. The
highest frequency.
frequency of the waves is 5 Hz. How
9. A female soprano sings at a higher frequency
many wave crests will pass by the dock
(higher pitch) than a male baritone.
in 8 s?
(a) Which singer is producing waves of
longer wavelength? Explain your answer.
Understanding Key Concepts (b) If both singers sing at an equal volume,
5. You can make a wave by shaking the end of a which singer is sending more energy out
long rope up and down. through his or her voice? Or are they
(a) Explain how you would shake the end of both sending out the same energy?
the rope to make the wavelength shorter. Explain your answer.
(b) State two different ways you could shake
the rope to increase the energy carried by
the wave.
6. Explain why water waves travelling under a
P ause and R eflect
raft do not move the raft horizontally.
7. A wave in the open ocean between Prince Write a paragraph or develop a table to
Rupert and Haida Gwaii has an amplitude of explain how a wave with a length of 6 cm
15 m in a large storm. The wavelength is and a frequency of two waves per second
1000 m. changes when the frequency is changed to
(a) How high will the crest of the wave be four waves per second.
above a boat that is in the trough?
(b) If the wave travels at a speed of 10 km
per hour, how often will a wave pass
under the boat?

Chapter 4 Many properties of light can be understood using a wave model of light. • MHR 143