12.7
Phenomena Related to Refraction
OVERALL EXPECTATIONS
• demonstrate scientific investigation skills in the four areas of skills
• investigate, through inquiry, the properties of light, and predict its behaviour,
particularly with respect to reflection in plane and curved mirrors and
refraction in converging lenses
• demonstrate an understanding of various characteristics and properties of
light, particularly with respect to reflection in mirrors and reflection and
refraction in lenses
Time
45–60 min
Vocabulary
• apparent depth
• mirage
• dispersion
Skills
SPECIFIC EXPECTATIONS
Researching
Communicating
Scientific Investigation Skills
Other Program Resources
• identify and locate print, electronic, and human sources that are relevant to
research questions
• select, organize, and record relevant information on research topics from
various sources, including electronic, print, and/or human sources using
recommended formats and an accepted form of academic documentation
• draw conclusions based on inquiry results and research findings, and justify
their conclusions
• communicate ideas, plans, procedures, results, and conclusions orally, in
writing, and/or in electronic presentations, using appropriate language and a
variety of formats
Developing Skills of Investigation and Communication
• use appropriate terminology related to light and optics
Understanding Basic Concepts
• identify the factors, in qualitative and quantitative terms, that affect the
refraction of light as it passes from one medium to another
• describe properties of light, and use them to explain naturally occurring
optical phenomena
KEY CONCEPTS
• Light changes direction predictably as it travels through different transparent media.
• Light bends toward the normal when it slows down in a medium with a higher index
of refraction.
• Total internal reflection may occur when an incident ray is aimed at a medium with a
lower index of refraction.
• The refraction and reflection of light can be used to explain natural phenomena.
Skills Handbook 4. Research
Skills
Science Perspectives 10
website www.nelson.com
/scienceperspectives/10
Related Resources
Maunder, Michael. Lights
in the Sky: Identifying
and Understanding
Astronomical and
Meteorological
Phenomena. Springer,
2007.
Naylor, John. Out of
the Blue: A 24-Hour
Skywatcher’s Guide.
Cambridge University
Press, 2002.
Science Perspectives 10
ExamView ® Test Bank
Science Perspectives 10
Teacher eSource SUITE
Upgrade
Science Perspectives 10
website www.nelson.com
/scienceperspectives/10
EVIDENCE OF LEARNING
Look for evidence that students can
• use the terms apparent depth, mirage, and dispersion correctly
• explain phenomena such as apparent depth, the flattened sun, mirages, shimmering,
and the rainbow in scientific terms
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SCIENCE BACKGROUND
Optical Phenomena
• The phenomenon of apparent
depth occurs when the true depth
of an object (usually under water)
is greater than the depth at which
the brain perceives it to be. Students
will most likely have experienced this
phenomenon before, perhaps when
looking down at their feet in a shallow
swimming pool and having them
appear to be much closer to their torso
than they actually are.
• The appearance of the Sun being
flattened when it is close to the
horizon is another phenomenon that
results from the refraction of light.
There are two factors at play in the
refraction. First, light from the bottom
of the Sun is refracted more than the
light from the top of the Sun. Second,
light rays from the bottom of the Sun
have a greater angle of incidence than
the light rays from the top of the Sun.
• The most common mirage appears to
be a puddle of water or oil on the road
ahead. As you approach the mirage,
it quickly “evaporates.” In fact, it was
simply a reflection of the sky. The light
rays are reflected—or, more precisely,
intensely refracted—by a layer of very
hot air close to the ground. Because
tarmac and sand both absorb sunlight
easily and then radiate heat energy
back out, mirages most often occur on
roadways and in deserts.
• Shimmering occurs when light passes
through several layers of air that have
different temperatures. The light
travels more slowly through the colder
layers of air and more quickly through
the warmer layers of air. As the layers
get warmer and the speed of light
increases, the light ray bends further
and further away from the normal
until total internal reflection takes
place in the warmest layer of air.
• The most common optical
phenomenon is probably the
rainbow. Rainbows often occur
after a rainstorm, but they can be
seen pretty much anywhere there is
sunlight and water droplets in the
air, such as near a waterfall or even
a garden hose. When seen from an
airplane, rainbows are circular. When
seen from the ground, though, the
landscape blocks some of the circle,
leaving an arc.
POSSIBLE MISCONCEPTIONS
Identify
• Students have preconceptions of mirages based on cartoon-style images of
mirages depicting them as images of lush, verdant areas in the middle of a
desert.
Clarify
• Explain that an actual mirage simply looks like a pool of water—nothing
more.
Ask What They Think Now
• At the end of the lesson, ask students to describe a mirage. (It is a reflection
of the sky, and it looks like a pool of water.)
TEACHING NOTES
Engage
• Engage students by asking if they have ever tried to pick up a coin (or some
other object) from the bottom of a shallow pool. Ask, Did the water appear
to be shallower than it actually was? How do you explain this? Have students
suggest reasons for objects in water appearing shallower than they actually
are. Tell students that they will be able to confirm their ideas about this and
other illusions in this section.
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Explore and Explain
• To help students better understand the phenomenon of apparent depth, go
over Figure 2 on page 535 of the Student Book in class. Draw the diagram
without labels or rays on the board, showing a close-up of the boundary
area. Ask, Is the light leaving the tank going toward a medium with a lower or a
higher refractive index? (lower) Which way will the light rays bend? (away from
the normal) Have a volunteer draw on the diagram to show how a light ray
from the pencil point will bend away from the normal, causing the pencil to
appear to be closer than it actually is. An analysis of the figure should help
students gain a better understanding of how this illusion is formed.
• Perform a similar analysis with Figure 3. Have a student volunteer explain
why the fish appears to be closer than it actually is.
• Introduce another phenomenon related to refraction by leading a discussion
of the “flattened Sun” phenomenon. Challenge students to explain Figure 5(b)
on page 536 in their own words. (Light from the Sun is refracted by Earth’s
atmosphere. It is bent downward, so the Sun appears to be higher than it
actually is. Light from the bottom of the Sun’s disc is refracted more than light
from the top of the disc. The image of the Sun is shifted upward, but the bottom
of the Sun is shifted a greater amount than the top is. This makes the bottom
appear to be closer to the top, which means the Sun appears to be flattened.)
• Move on to the topic of mirages. After drawing the diagrams below on the
board, discuss how mirages are formed. (Students can also refer to Figure 6(b)
on page 537 of the Student Book.)
object
object
cool air
cool air
warm air
warm air
toward the
normal
away from
the normal
image
Ask, Which layer of air is warmer? (the bottom layer) How does being warm
change its index of refraction? (the index gets lower) How will light crossing this
“boundary” bend? (It will go from cool to warm and speed up, so it will bend
away from the normal.) Point to the rising part of the curve in Figure 6(b).
Why is the light bending toward the normal here? (It is slowing down as it goes
from warm to cool air.)
• Warmer air has a lower index of refraction than cooler air, so a boundary
forms in the diagram on the left. Light entering the warmer air speeds up and
turns away from the normal. The second diagram shows what happens as the
light continues along. Total internal reflection occurs and over a distance,
and the light returns to a cooler area. Since the light is now slowing down, it
bends toward the normal, causing an even greater curve. The eye perceives the
image of the sky (the object) as a straight-line path from the ground, creating
the illusion.
• Continue by explaining that shimmering occurs because light is refracted
in different directions as it travels through air of different temperatures.
Challenge students to think about the role of total internal reflection and
refraction in optical phenomena. Ask, In addition to shimmering, what
other phenomenon is caused by light passing through sections of air at different
temperatures? (The twinkling of stars) Does total internal reflection occur in that
case? (No)
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Reading Tip
Challenging Beliefs
Explain to students that
reading something that
challenges their beliefs is
an opportunity for learning
and better understanding.
Sometimes our beliefs are
not based on science, so
times when our beliefs
are challenged present an
opportunity to engage in
critical, scientific thought.
• Introduce the formation of rainbows by directing students’ attention to
Figure 10 on page 538. Review with students how prisms split light into its
different colours. Explain how this happens in the natural world. Encourage
students to use the rays and angle markings in the figure to help them in
analyzing the situation and in producing answers to the following questions.
Ask, What happens to light at the red end of the spectrum as it exits the upper
raindrop? (It travels to the viewer’s eye, creating a virtual image of a tiny part
of the rainbow.) Ask, What happens to light at the blue end of the spectrum
as it exits the upper raindrop? (It does not reach the viewer’s eye.) Ask, What
happens to light at the red end of the spectrum as it exits the lower raindrop? (It
does not reach the viewer’s eye.) Ask, What happens to light at the blue end
of the spectrum as it exits the lower raindrop? (It travels to the viewer’s eye,
creating a virtual image of a tiny part of the rainbow.)
• Have students complete Research This: Other Atmospheric Optical
Phenomena.
RESEARCH THIS OTHER ATMOSPHERIC PHENOMENA
Skills
• Researching, Communicating
Purpose
• To explore atmospheric phenomena not described
in the lesson and determine how refraction and
reflection cause them.
Notes
• You may want to save class time by preparing
research materials—books and magazines from the
school library and/or websites—in advance.
• The Belt of Venus is actually the shadow of the Earth
cast by the setting Sun.
Suggested Answers
A. Sun dogs occur when the Sun is relatively low in
the sky and its rays pass through atmospheric ice
crystals, such as those in cirrus clouds, on the way
to the observer. Moon dogs occur under the same
conditions with the Moon taking the place of the
Sun.
B. An ice bow is observed in the direction of the
Sun, whereas a rainbow is observed in a direction
opposite to the Sun.
C. A green flash occurs when the Sun is rising or
setting. After the Sun’s disk is actually below the
horizon, the atmosphere can refract sunlight
towards our eyes and “lift” the Sun into view.
Blue and green colours get refracted the most, so
a green “flash” is green light being “lifted” into
view more than the orange and red colours. Green
flashes are hard to see because they usually last
only a few seconds and require a strongly layered
atmosphere to be bright.
D. Answers will vary.
Extend and Assess
• To conclude this section, go back to the depth illusions (Figures 2, 3, and 4)
and ask, Why does the depth of an object always appear to be shallower than its
actual depth? (Light rays coming from a slower medium bend away from the
normal, causing the image of the object to appear to rise in the water.) Now
invite students to surmise when an object might appear to be deeper than it
actually is. (Guide students to recognize that if light goes from a denser to a
less dense medium, it will appear to be deeper than it actually is. This might
occur if one looked upward from underwater into air.)
• Have students complete the Check Your Learning questions on page 539 of
the Student Book.
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CHECK
YOUR LEARNING
Suggested Answers
1. (a) The apparent depth of a submerged object is the distance from the surface of the water to the virtual image.
(b) Light rays coming from a submerged object are refracted away from the normal upon leaving the water. An
observer’s brain traces these rays back to an apparent point of origin, forming a virtual image of the object that
appears closer.
2. You should aim below the apparent position of the fish and deeper because its virtual image lies closer to the
surface than the fish itself.
3. Light travels more slowly in a medium with a high index of refraction and vice versa. Thus, as air gets warmer and
its index of refraction decreases, light travels faster.
4. You are looking at a virtual image of the sky that creates the illusion of a pool of water. Layers of air above a hot
road grow cooler as molecules’ distance from the road increases. Light from the sky passing through these layers
gets refracted and reflected to an observer’s eyes to create a virtual image of the sky on the road.
5. Light refracts more when it is slowed more. A slower speed for light in a medium means a higher index of
refraction. Since violet light is refracted more than red light, violet light has a higher index of refraction.
6. The light is first refracted and dispersed upon entering a raindrop. Then the light undergoes partial internal
reflection at the back of the drop and is refracted once again as it exits the drop.
7. A rainbow of colours could not form if all colours of visible light travelled at the same speed in raindrops. The
colours must have different speeds in the raindrops in order to be refracted and dispersed.
DIFFERENTIATED INSTRUCTION
• All students, especially visual/spatial learners, will benefit from explaining
the depth illusion using diagrams on the board. Have students draw a
diagram of Figure 2 on the board. First, ask them to draw the pencil as if
no refraction were taking place. (No illusion will result as light goes straight
from the pencil tip to the eye.) Now have students consider the water-air
boundary. Ask them to draw the normal. Ask, Which way will the light rays
bend? (toward the normal) Students should then draw in the bent rays. Where
does the light appear to be coming from? (higher in the tank than it actually is)
Where is the light actually coming from? (lower in the tank) Finally, have verbal/
linguistic learners explain the entire process that was just drawn on the board.
(Light exiting the slower water turns away from the normal, causing the pencil
to appear higher up in the water than it actually is.)
• Have mathematical/logical learners prepare a chart that presents the
similarities and differences among the phenomena described in this section.
Tell them to consider the causes as well as the appearance of each one. Hang
the completed charts in the classroom.
ENGLISH LANGUAGE LEARNERS
• Focus on making comparisons, beginning with examples in the section such
as warmer and warmest. Make a chart on the board. Invite students to find
examples in the chapter (or elsewhere in the book) of comparison words.
Examples might include warm, warmer, warmest, deep, deeper, deepest, short,
shorter, shortest, close, closer, closest, shallow, shallower, shallowest, hard, harder,
hardest. Ask students to use each word in a sentence.
• Give a few examples in which the –er and –est formations do not work (words
with multiple syllables): difficult, more difficult, most difficult, important, more
important, most important, bent, more bent, most bent, unusual, more unusual,
most unusual. Again, have students use each term in a sentence.
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