What Is Refraction? 12.

1
Light travels in straight lines through air. But what happens when it
travels from one material into another? You likely have noticed the strange
phenomenon of a spoon or stir stick, when placed in a glass of water,
looking somewhat disconnected at the surface of the water. The spoon is not
really disconnected. It is made of a solid material. So what is happening?
Why does an object appear disconnected, or broken, at the water’s surface?
Find out by doing the “Exploring With Light” activity.

T RY THIS EXPLORING WITH LIGHT

SKILLS: Observing, Analyzing SKILLS HANDBOOK
3.B.
Equipment and Materials: beaker or other transparent
container; stir stick; coin; water
1. Place a coin in the middle of a beaker. Fill the beaker with water.
2. Look at the coin from the edge of the beaker. Be sure that
you are looking above the beaker through the surface of the
water as in Figure 1.
stir stick
3. While looking above the beaker, aim a stir stick just inside
the outer edge of the coin so that it looks as though you are
going to just touch the coin. Place the stir stick in the water
and attempt to touch the coin.
4. Now look at the beaker from the side and notice the position
of the stir stick and the position of the coin.
A. Did you touch the outer edge of the coin when you placed the
stir stick in the water? T /I
B. Describe where the stir stick actually went. T/I

coin
C. Why do you think the stir stick missed the coin even though
you had aimed the stick directly at the coin while looking
above the beaker? Write a brief explanation. T/I Figure 1

Bending Light
From the previous activity, you noticed that, even though you had aimed
the stir stick at the coin, you missed touching the coin. The light coming
from the coin to your eyes became bent. The light from the coin went
through the water and then through the air into your eyes. Along this path,
the light changed direction as it went from water into air. This bending of
light when it travels from Ontario Science
one material 10 SB into another is called
(medium)
0-17-635528-6
refraction. Refraction causes interesting effects whenever light travels refraction the bending or change in
from one medium to another. FN C12-F04-UDOS10SB direction of light when it travels from one
CO Allan Moon medium into another
Pass 2nd pass
Approved
Not Approved

NEL 12.1 What Is Refraction? 515

 LEARNING TIP What Causes Refraction?
Using an Analogy The refraction of light becomes obvious when you shine a powerful beam
An analogy allows you to make a
of light into water at an angle (Figure 2). Why does the direction change?
comparison by using as your example
something that is simpler and easier to A useful analogy is to think of a wagon travelling at an angle from pavement
understand. Although a wagon moving onto sand (Figure 3). When the right front wheel hits the sand, it slows
from pavement onto sand is not a beam down. The left front wheel, however, does not slow down because it is
of light, this scenario allows you to still on the pavement. This results in the wagon pivoting about the slower
understand the principle of refraction
right front wheel. So the direction of the wagon changes as it moves from
using everyday objects.
pavement onto sand. You might notice a similar effect when a car travels at
an angle from pavement onto mud or snow.

pavement

sand

Figure 2 The refraction of light going from air into water Figure 3 A wagon changes direction when travelling at an angle from
pavement onto sand because one front wheel slows down while the
other wheels continue moving at a higher speed.

DID YOU KNOW? How Fast Is the Speed of Light?

Measuring the Speed of Light The analogy of refraction using the wagon involves one of the front wheels
The first calculation of the speed
slowing down as the wagon travels from one surface onto another. Is this
of light that was close to today’s
accepted value of 3.00 × 108 m/s the case with light? That is, is the speed of light in water different from the
was done by Dutch mathematician speed of light in air? Measurements of the speed of light clearly indicate
pavement
Christian Huygens. He based that this is indeed the case. Light travels at a speed of 3.00 × 108 m/s
his calculations on astronomical in a vacuum, at a speed of 2.26 × 108 m/s in water, and at a speed of
observations of Io, one of Jupiter’s
moons, collected by Danish
1.76 × 108 m/s in acrylic. (Note that the speed of light in air is slightly less
astronomer Olaus Roemer in 1676. than the speed of light in a vacuum. This difference, however, is so small that
it is not significant. Therefore, we use the same value for the speed of light in
GO TO NELSON SCIENCE a vacuum and the speed of light in air.)sand

The Rules for Refraction

Because the speed of light changes depending on the medium through
which it is travelling, it is possible to make two statements about refraction
angle of refraction the angle between in general and about the angle of refraction in particular. The angle of
the refracted ray and the normal refraction is the angle between the refracted light ray and the normal.
1. The incident ray, the refracted ray, and the normal all lie in the same
plane. The incident ray and the refracted ray are on opposite sides of
the line that separates the two media.C12-F05-UDOS10SB.ai
The front wheels are turning into the
516 Chapter 12 • The Refraction of Light sand in top illustration. NEL

Ontario Science 10 SB
 2. Light bends toward the normal when the speed angle of
of light in the second medium is less than the inc incidence
ide
nt r
speed of light in the first medium (Figure 4). ay normal
Light bends away from the normal when the
speed of light in the second medium is greater.

re
fra
(This second statement can also be predicted from

ct
ed
the principle of the reversibility of light; light angle of

ra
y
still follows the same path even if you switch its refraction
original direction. Recall that this principle also
applies to the reflection of light.)
Figure 4 Light bends toward the normal when its speed decreases
in a material.

The Bent Spoon

Using the concept of refraction, it is possible to explain why a spoon in a glass READING TIP
of water appears bent (Figure 5). Light coming from the part of the spoon below Evaluating
the water’s surface must travel through the water into air. The speed of light To determine whether a main idea
increases as light goes from water into air so, if it hits the water–air boundary at seems reasonable, think about examples
that you know. For instance, you may
an angle, light will bend away from the normal. The human brain perceives light
have seen a fallen tree branch appear
to travel in a straight line, so it will project to be bent in a river. By applying the
these light rays backwards to a virtual light explanation of refraction to a similar
source behind the real spoon (Figure 6). example from your own experience, you
This is similar to how the brain projects might determine if the explanation is
reasonable.
light rays to form a virtual image in a
plane mirror.

Figure 5 The familiar “bending” of Figure 6 The brain thinks that the spoon
a spoon in a glass of water is behind where it really is because light
is refracted away from the normal when
travelling from water into air.

Partial Reflection and Refraction

C12-F27-UDOS10SB.ai
Light has some very interesting and unique properties that are not observed
with most other matter in nature. Do the “Examining Light in a Window”
activity on the next page to observe one of these properties.

NEL 12.1 What Is Refraction? 517

 T RY THIS EXAMINING LIGHT IN A WINDOW
SKILLS: Questioning, Observing SKILLS HANDBOOK
3.B.
1. Stand directly in front of a clean window and look through it. C. Light coming from you appears to have two different
This experiment works better if it is dimmer outside or the behaviours based on questions A and B. Do you find this to
window is in shadow. Look at a tree, a person, a building, or be surprising? Explain.
some other object outside. D. A thought experiment is one that you do in your head; it
2. Continue to stand directly in front of the window, but this is not one that you actually do. Let us do a brief thought
time focus on the glass directly in front of you. You should experiment. Imagine that you were to throw a tennis ball at a
be able to see a faint reflection of yourself in the window. window. Predict the two possible outcomes when the ball hits
(Try turning the light off or on in the room if you have trouble the glass. Is it possible that these two outcomes could both
seeing your reflection.) occur at the same time? T/I
A. During late dawn or early dusk, someone standing outside E. Unlike the ball, light travelled through the window (to the
the window would be able to see you clearly through the outside observer) and also bounced back (allowing you to
window. What does this tell you about how light has travelled see your reflection) at the same time. This property does
from you toward the outside observer? T /I not appear to happen with most other things (such as balls).
B. You are also able to see your reflection in the glass. What Does this imply that light has at least one property that might
does this tell you about the light that travels from you toward make it special or unique? Explain. T/I
the glass surface directly in front of you? T /I

Refraction is often accompanied by reflection. Some of the light that

strikes water is reflected off the water, but a great deal of light is also
refracted as it enters the water and illuminates the water below the surface
(Figure 7). A transparent window exhibits the same property that light
can be both reflected and refracted at the same time. This is called partial
reflection and refraction. This effect is enhanced if glass has a special film
coating behind it that allows some of the incident light to be refracted
but that also reflects much of the incident light. This results in a mirrored
To see some interesting
surface that you can see through, but others cannot. This type of surface is
simulations involving refraction, called a silvered two-way mirror, and it is exactly how mirrored sunglasses
GO TO NELSON SCIENCE are made (Figure 8).

Figure 7 A beam of light is both reflected and refracted when it Figure 8 Mirrored sunglasses allow you to see out but do not allow
strikes water. The tree is visible as a result of reflection, whereas the others to see in.
fish is visible due to refraction.

518 Chapter 12 • The Refraction of Light NEL

 Two-way mirrors are used in the windows of many buildings (Figure 9).
In the summer time, these windows reflect some of the incident sunlight,
which reduces air-conditioning costs. The reflection of clouds and the blue
sky off these windows also makes them very visually appealing.

UNIT TASK Bookmark

You can apply what you learned about refraction at the boundary of two media as you think
about the Unit Task described on page 588.
Figure 9 Many modern buildings
use windows that illustrate the partial
transmission and reflection of light.

IN SUMMARY
• Refraction is the bending or change in direction • Light bends toward the normal when it slows
of light when it travels from one medium into down in a medium and away from the normal
another. when it speeds up in a medium.
• The speed of light depends on the medium that it • Light can undergo partial reflection and
is passing through. refraction at the same time at a surface.

CHECK YOUR LEARNING

1. Clearly explain what is meant by the term 4. Which way will light bend if it is travelling
“refraction.” K/U (a) faster in a medium?
2. (a) Explain why refraction takes place. (b) slower in a medium? K/U

(b) What conditions must be present for refraction to 5. What property of light is illustrated in Figure 11? K/U

take place?
(c) From your answers to (a) and (b), make a prediction
about the speed of light in water as compared to the
speed of light in air. K/U
3. Figure 10 represents a beam of light going from one
medium into another. T/I

medium A
Figure 11

6. Give some examples of where you have seen two-way

medium B
mirrors used. A
7. In your own words, explain some practical applications of
partial reflection and refraction. A C

Figure 10

(a) One medium is air, in which light has a speed of

3.00 × 108 m/s. The other medium is ice, in which light
has a speed of 2.29 × 108 m/s. Use this information to
identify medium A and medium B. Explain.
(b) Do you know in which direction the light beam is
travelling? Does it matter? Explain.

NEL 12.1 What Is Refraction? 519