Download 12.5 Answers

12.5
Time
45–60 min
Vocabulary
• critical angle
• total internal reflection
• retro-reflector
Assessment Resources
Assessment Rubric 1:
Knowledge and
Understanding
Assessment Summary 1:
Knowledge and
Understanding
Other Program Resources
Science Perspectives 10
website www.nelson.com
/scienceperspectives/10
Total Internal Reflection
OVERALL EXPECTATIONS
• identify and describe a variety of careers related to the fields of science
under study, and identify scientists, including Canadians, who have made
contributions to those fields
• 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
SPECIFIC EXPECTATIONS
Career Exploration
• identify and describe a variety of careers related to the fields of science under
study and the education and training necessary for these careers
Developing Skills of Investigation and Communication
• use appropriate terminology related to light and optics
Understanding Basic Concepts
Related Resources
Gizmo: Refraction
Vasan, Srini V. Basics of
Photonics and Optics.
Trafford Publishing, 2006.
• explain the conditions required for partial reflection/refraction and for total
internal reflection in lenses, and describe the reflection/refraction using
labelled ray diagrams
• identify ways in which the properties of mirrors and lenses determine their
use in optical instruments
• identify the factors, in qualitative and quantitative terms, that affect the
refraction of light as it passes from one medium to another
Johnson, B. K. Optics and
Optical Instruments:
An Introduction. Dover
Publications, 1960.
KEY CONCEPTS
Science Perspectives 10
ExamView® Test Bank
• Total internal reflection may occur when an incident ray is aimed at a medium with a
lower index of refraction.
Science Perspectives 10
Teacher eSource SUITE
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• Many optical devices make use of the refraction and reflection of light.
Science Perspectives 10
website www.nelson.com
/scienceperspectives/10
EVIDENCE OF LEARNING
Look for evidence that students can
• use the terms critical angle, total internal reflection, and retro-reflector correctly
• explain why and under what conditions total internal reflection occurs
• identify several uses for fibre-optic technology
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SCIENCE BACKGROUND
Total Internal Reflection
• Total internal reflection is a logical
consequence of refraction. As the
incident angle of a slow medium
increases, the refracted angle in the
faster medium (which bends away
from the normal) begins to approach
90°. When it reaches 90°, the refracted
angle runs along the border of the two
media. This situation defines the critical
angle. Any incident angle greater than
this critical angle results in a refracted
angle greater than 90° that does not
leave the medium.
• As one would expect, substances that
are commonly known to sparkle have
high refractive indices and, therefore,
low critical angles. For example, rubies,
garnets, and sapphires all have a
refractive index of about 1.76. Zircon,
commonly used to make artificial
diamonds, has a refractive index of
1.96. Diamonds have a refractive index
of 2.42. The sparkling effect is caused
by light entering the diamond and
bouncing around several times before
escaping through the surface.
▼
• Fibre optic substances need to have a
very low critical angle and a very high
refractive index. Silicon, the substance
used most commonly in fibre optic
cables, has a refractive index of 4.01.
• Note that the critical angle for any
medium depends on the medium it
borders on. Water, for example, will
exhibit a different critical angle when
it has a boundary with air than it does
with ethanol.
• Retro-reflectors are optical devices
that return light that hit them in the
same direction as it came from. It is
important to note that the emergent
light ray does not have to follow the
same path as the incident light ray;
it must only be parallel to it. A retroreflector can be a single object, such
as a cube of glass with one corner
cut off, or a combination of objects,
such as the triangular prisms in the
periscope shown on page 529 of the
Student Book. Retro-reflectors are
used in many safety devices, including
bike reflectors and reflective strips on
clothing.
TEACHING NOTES
Engage
• Ask students to say what comes to mind when you say the word diamond.
Students will probably mention hard, valuable, and cuts glass. They will
probably also mention sparkle or shine. Ask students if they can recall hearing
or seeing commercials for jewellery in which this feature of diamonds was
emphasized. Explain that the brilliance of a diamond is a result of total
internal reflection, which they will learn about in this section.
Explore and Explain
• Introduce the chapter section by explaining to students the difference
between light travelling into a medium with a higher index of refraction and
light travelling into a medium with a lower index of refraction. Explain to
students what happens in Figure 1 and Figure 2 on page 526 of the Student
Book, and then make sure they understand Figure 3 on page 527.
• Point out that the critical angle for a water-air boundary is 48.8°. In
Figure 3(a), ask, Is the incident angle greater or less than 48.8°? Students should
be able to estimate that the angle is considerably less than the critical angle,
so the refracted ray exits the medium. For (b), ask the question again: Is the
incident angle greater or less than 48.8°? Now point out that the incident angle
is exactly 48.8°, so the refracted ray stays on the surface of the water. Finally,
for (c), repeat the process: Is the incident angle greater or less than 48.8°? Now
point out that the angle is greater than the critical angle, so the light stays
within the water medium. Finally, relate what the fish sees in (c): Does the fish
see the bird? (No, because light rays in the water reflect back into the water.)
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Learning Tip
Understanding Diagrams
After students have
drawn their diagrams and
shared with each other,
have them refine their
drawings and put them in
their notebook for future
reference. Encourage
them to use labels to
clarify what they have
drawn.
Chapter 12 The Refraction of Light
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Reading Tip
Evaluating
Encourage students
to make use of the
captions that accompany
illustrations even if they
think they know what the
illustration is intended
to show. Captions often
contain information that is
not stated elsewhere, and
so present an opportunity
for additional learning.
• Use the diagrams below to help students understand why a diamond sparkles
more than a substance like water. Copy the diagrams on the board for
students to see. Ask students what happens when light is reflected at the
surface of a rectangular container of water. With help from the diagram,
students should recognize that for light in water, total internal reflection
within a rectangular container (left diagram) will bounce once internally.
However, the angle of the second bounce will be less than the critical angle.
This results in the second refracted angle leaving the medium.
water
Reading Tip
Evaluating
Remind students to
evaluate everything they
read for bias. Failure
of a writer to present
both sides of an issue
may indicate the writer
is biased toward the
topic. A reader should
take possible bias into
consideration in evaluating
the reliability of a text.
Unit Task B ookmark
Students should think
about whether their device
would benefit from total
internal reflection. Be
sure they consider fibre
optics and retro-reflectors,
as well as other, original
applications.
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• Ask students to consider what happens with a diamond or other sparkly
medium. Ask them to explain how the situation is different from light in
water. Students should be able to use the diagram to help them explain that
with a medium that sparkles (second diagram), the angle of the first bounce
produces a second bounce that is still greater than the critical angle. Now the
second bounce also stays within the boundary of the medium. Explain that
diamond cutters create facets that take advantage of this effect and cause the
light to bounce multiple times before it exits the diamond.
• Students may already be familiar with novelty devices that demonstrate total
internal reflection. These devices, such as the one shown in Figure 6, consist
of bundles of optical fibres emerging from a solid base. The base usually
contains light sources of several different colours, which means the exposed
tips of the fibre optic cable are different colours. Explain that while this
application of total internal reflection is for entertainment purposes, there
are far more important applications, including data transmission on fibre
optic cables, lighting of instrument panels in automobiles, and in medical
examination devices (endoscope).
• Explain to students that prisms are very useful for their ability to internally
reflect light. Call students’ attention to Figure 8(a) on page 529 and quickly
review what they should have learned in previous sections. Ask, Why isn’t
the light ray refracted or reflected when it enters the prism? (It enters the prism
perpendicular to the left-hand face, which means it is parallel to the normal
of that face. The angle of incidence is 0°.) Why does the ray reflect inside of the
prism? (The angle is 45°, which is greater than the critical angle.) Why isn’t it
refracted or reflected when it leaves the prism? (Its angle of incidence is 0° with
respect to the bottom face.) Repeat the questions for Figure 8(b).
• Explain that triangular prisms can be used to make periscopes and binoculars,
each of which relies on the prisms to change the direction of light.
• Explain that retro-reflectors are used to return light in the same direction
from which it came. In this case, the same direction means that the incident
ray and the emergent ray are parallel.
• Remind students that they read about the LR3 in the Engage in Science
section on page 512 of the Student Book. Explain that the LR3 is an example
of a corner cube retro-reflector composed of 100 corner cube retro-reflectors.
The LR3 has allowed scientists to measure the distance from Earth to the
Moon, accurate to within a few millimeters.
• Retro-reflectors are used in a variety of safety devices, particularly road signs
and other road-related items (bike reflectors, reflective devices on clothing
and helmets).
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sparkly medium
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Extend and Assess
• To assess students’ understanding of total internal reflection, ask, Why can’t
total internal reflection occur when light goes from a fast medium to a slow
medium? (The light is bent toward the normal, so no matter what the incident
angle is, the light will exit the faster medium.)
• To ensure that students appreciate the applications of internal reflection,
ask, What are some ways that optical fibres are used? (Sample answers: to carry
telephone, computer, and television signals; to put tiny bits of light into scale
models; to illuminate instrument panels; to take pictures inside the human
body.)
• Have students draw a diagram showing what happens when a light ray
enters a triangular prism. (Students’ diagrams should look very much like
the diagram in either Figure 8(a) or Figure 8(b).) Ask, What other devices
use triangular prisms in this way? (Sample answers: cameras, periscopes,
binoculars) You may also wish to draw some triangular prisms on the
board and ask students to draw in the rays. Draw the prisms in a variety of
orientations.
• Direct students’ attention to the photo of the Laser Ranging Retro-Reflector
in the Engage in Science feature (page 512 of the Student Book). Say, The
face of the device is made up of a grid of many small elements. Each element
reflects a laser beam in the direction it came from. How does it accomplish this? (a
corner cube retro reflector)
• Have students complete the Check Your Learning questions on page 531 of
the Student Book.
CHECK
YOUR LEARNING
Suggested Answers
1. The speed of light in the medium containing the incident light ray must be slower than in the second medium. The
angle of incidence must exceed the critical angle.
2. Light has to travel more slowly in the first medium because as it speeds up, it bends away from the normal
(ray 1). If it bends far enough away from the normal, total internal reflection occurs (ray 2). If light were to travel
more slowly in the second medium, it would bend toward the normal (rays 3 and 4), which is in the direction
opposite to what would allow total internal reflection.
faster medium
slower medium
slower medium
faster medium
ray 2
ray 4
ray 1
3. (a) no
(c) yes
(b) yes
(d) no
ray 3
4. Prisms transmit more light than mirrors, and, unlike mirrors, their reflective coatings do not deteriorate over time.
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5. A smaller critical angle results in more total internal reflection because any angle of incidence that exceeds the
critical angle produces total internal reflection.
6. Sample answer: Flexible posts with retro-reflectors on them could be placed along the edges of the road to outline
curves. Retro-reflectors could also be laid on the centre line of the road to show traffic lanes.
7. Sample answers: Diamonds are cut so that much of the light that enters the sides undergoes total internal
reflection and exits at the top, making the diamond sparkle. Optical fibre cables use total internal reflection to carry
communications encoded in light rays. Triangular prisms use total internal reflection to redirect light in periscopes
and binoculars.
8. Total internal reflection would be possible in (b) and (c)—in both (b) and (c) the refracted ray bends away from the
normal; diagram (c) is actually just diagram (b) rotated by 90o.
9. Total internal reflection is only possible in examples (b) and (c) since these are the only diagrams in which the
refracted ray bends away from the normal. If the angles of incidence were increased, then total internal reflection
would occur in medium B in diagram (b), and in medium A in diagram (c).
DIFFERENTIATED INSTRUCTION
• Go over Figure 3 in a step-by-step manner. This review will be of particular
benefit to visual/spatial learners.
– Draw Figure 3 (a) on the board.
– Point out that the critical angle for a water–air boundary is 48.8°. Then
have students measure out an angle of 48.8° on a sheet of paper and make a
fixed cut-out angle that they can use for comparison.
– Have students use their angle cut-out to measure the incident angle. Ask, Is
the incident angle greater or less than 48.8°? (less) Will total internal reflection
occur? Explain. (No, because the incident angle is less than 48.8°.)
– Repeat the process for diagram (b). Is the incident angle greater or less than
48.8°? (same) Will total internal reflection occur? Explain. (No, the incident
angle is equal to 48.8°, so neither reflection nor refraction would occur.)
– Repeat the process for diagram (c). Is the incident angle greater or less than
48.8°? (same) Will total internal reflection occur? Why or why not? (Yes,
because the angle is greater than 48.8°.)
• Have bodily/kinesthetic learners repeat the steps for Figure 3 above using
their extended arms to estimate angles on the diagrams. Bodily/kinesthetic
learners may also benefit from using hand motions to explain how the prisms
reflect light rays in Figure 9 on page 529.
ENGLISH LANGUAGE LEARNERS
• Write the word internal on the board. Have a volunteer define the word (on
the inside). Then ask, What is the opposite of this word? Before answering the
question, discuss the concept of opposites using examples: black/white, up/
down, big/small, happy/sad. Then identify the opposite of internal (external).
Repeat with other terms from the chapter section: reflect/absorb, giant/tiny,
slow/fast, exit/enter, greater/less, off/on, visible/invisible. Have students use the
words in sentences.
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