Download The Laws of Reflection

11.6
Time
45–60 min
Vocabulary
• specular reflection
• diffuse reflection
Skills
Performing
Observing
Evaluating
Equipment and Materials
The Laws of Reflection
OVERALL EXPECTATIONS
• demonstrate scientific investigation skills in the four areas of skills
• evaluate the effectiveness of technological devices and procedures to make use
of light, and assess their social benefits
• 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
per group:
• flashlight (or an unshielded
ray box)
• sheet of aluminum foil
(about 30 cm 3 30 cm)
• ruler
• protractor
• piece of paper
• pencil
• two plane mirrors (optional)
• ray box (optional)
• formulate scientific questions about observed relationships, ideas, problems,
and/or issues, make predictions, and/or formulate hypotheses to focus
inquiries or research
• conduct inquiries, controlling some variables, adapting or extending
procedures as required, and using standard equipment and materials safely,
accurately, and effectively, to collect observations and data
Other Program Resources
• analyze a technological device that uses properties of light and explain how it
has enhanced society
Skills Handbook 3. Scientific
Inquiry Skills
Science Perspectives 10
website www.nelson.com
/scienceperspectives/10
Scientific Investigation Skills
Relating Science to Technology, Society, and the Environment
Developing Skills of Investigation and Communication
• use appropriate terminology related to light and optics
Understanding Basic Concepts
• explain the conditions required for partial reflection/refraction and for total
internal reflection in lenses, and describe the reflection/refraction using
labelled ray diagrams
KEY CONCEPTS
• When light is reflected off a flat, shiny surface, the image is equal in size to the
object and the same distance from the surface.
• Images in flat mirrors are located at the point where the backward extensions of
reflected rays intersect.
EVIDENCE OF LEARNING
Look for evidence that students can
• describe and explain the behaviour of light when it is reflected off a plane mirror
• understand the relationship between the angle of incidence and the angle of
reflection when light is reflected off a plane mirror
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▼
SCIENCE BACKGROUND
Laws of Reflection
• The second law of reflection states
that the incident ray, the reflected ray,
and the normal all lie on the same
plane. A plane is an imaginary twodimensional flat surface that extends
outward in all directions. In practical
situations, planes are often limited in
area, as Figure 1 on page 484 of the
Student Book shows. Mathematically,
planes are infinite in all directions.
• Note that any two rays with a point
in common (such as the point of
incidence) always exist on the same
plane. Two such rays can actually
be used to define a plane, so it is
not significant that the incident and
reflected ray lie on the same plane.
The fact that a third line, the normal,
also lies on this plane is significant
and shows why the second law of
reflection defines a special relationship.
• Although we often think of reflection
as only those situations where an
image is formed, recall that light must
reflect off objects for them to be seen.
All objects we can see reflect light.
This type of reflection, called diffuse
reflection, occurs from the reflection
of light off an irregular surface.
• The laws of reflection apply whether
light strikes a smooth surface or a
rough surface. Although no image
is formed when light reflects from a
rough surface that does not indicate
that the light behaves differently when
it strikes a rough surface.
• Dyslexia is a learning disability in which
an individual has difficulty reading print.
Dyslexia may be caused by head trauma,
may be hereditary, or may be caused by
developmental problems in the brain.
There is no cure for dyslexia; the method
of treatment is dependent upon the
cause for an individual. The condition
can sometimes be improved through
the use of filters or glasses that reduce
the glare of light reflected from paper.
Related Resources
Rosner, Marc. The
Scientific American Book
of Great Science Fair
Projects. Wiley, 2008.
VanCleave, Janice. Janice
VanCleave’s A1 Projects
in Physics: Winning
Experiments for Science
Fairs and Extra Credit.
Wiley, 2003.
Science Perspectives 10
ExamView® Test Bank
Science Perspectives 10
Teacher eSource SUITE
Upgrade
Science Perspectives 10
website www.nelson.com
/scienceperspectives/10
TEACHING NOTES
Engage
• Before going into depth about the laws of reflection, ask students to describe
what they already know about reflection. Prompts may include drawing a
diagram on the board of an incoming ray and asking, What can you tell me about
the angle at which this ray will reflect? (The angles of the incoming and outgoing
rays are equal.) What is its relationship (if any) to the incoming ray? Record
students’ responses on the board. If students mention information related to
either of the two laws of reflection, circle that response on the board. Explain
that students will be learning the laws of reflection for light in this section.
Explore and Explain
• Refer students to Figure 1 to begin discussing the laws of reflection. You may
wish to re-create the diagrams on the board. Ask, Which law is illustrated
in the left portion of the figure? (The angle of incidence equals the angle of
reflection.) Then ask, Which law is illustrated in the right portion of the figure?
(The incident ray, the reflected ray, and the normal all lie on the same plane.)
• Give students some time to experiment with different types of reflection by
having them complete the Try This: Reflecting Light activity.
T RY THIS
REFLECTING LIGHT
Skills
• Observing, Evaluating
Equipment and Materials (per group): flashlight (or
an unshielded ray box); sheet of aluminum foil (about
30 cm 3 30 cm)
Purpose
• Students will compare reflection from a smooth
surface to reflection from a rough surface.
Student Safety
• Remind students not to pull on the cord when they
unplug the ray box, have them pull on the plug itself.
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Chapter 11 The Production and Reflection of Light
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Notes
• Have students work in pairs.
• Remind students not to smooth out the crumpled
piece of foil after they have opened it.
Suggested Answers
A. The shape of the beam on the ceiling after
reflecting from the flattened foil was a single oval,
Reading Tip
Using Text Layout
Point out the capitalized
subheadings as described
in the Reading Tip on
page 484 of the Student
Book. Have students
identify and record the main
idea of each subsection of
text as it is read. Encourage
students to add these main
ideas to their study notes
for the lesson.
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B. The flattened foil acted like a mirror and produced
an image on the ceiling of the round reflector in
the flashlight. The crumpled foil, on the other hand,
acted like many small mirrors sending the light from
the flashlight in many different directions.
• Go over the material about specular and diffuse reflection. Have students
compare and contrast Figure 2 and Figure 5 on page 485 of the Student
Book. Explain that the laws of reflection apply on every kind of surface. Ask,
Do the light rays in Figure 2 obey the laws of reflection? How do you know? (Yes.
The angle of incidence and the angle of reflection are equal and the incident
ray, the reflected ray, and the normal all lie on the same plane.) Do the light
rays in Figure 5 obey the laws of reflection? How do you know? (Yes. The angle
of incidence and the angle of reflection are equal, and the incident ray, the
reflected ray, and the normal all lie on the same plane.)
• Differentiate between the two kinds of reflection by asking, If the light rays in
both Figure 2 and Figure 5 follow the laws of reflection, what causes the difference
in the reflections produced? (In Figure 2, the reflected rays are all parallel to
one another because the angles of incidence are all identical. In Figure 5, the
incident rays have different angles of incidence due to the uneven surface,
resulting in different angles of reflection. The resulting reflected rays are no
longer parallel and cannot produce a recognizable image.)
• The discussion of diffuse reflection provides a good opportunity to ask
students why angles are measured with respect to the normal rather than with
respect to the reflecting surface. For a flat surface (specular reflection), it does
not matter how the angles are measured. However, in order to measure angles
relative to the surface for diffuse reflection, a tangent to the surface at the point
of contact would have to be drawn. It is easier to just draw a normal instead.
• Some students may have difficulty understanding that each light ray in
Figure 5 has equal angles of incidence and reflection. Draw an uneven
surface similar to the one in Figure 5 on the board and have students copy
it; however, rather than random squiggles, use straight lines that form angles,
making it easier for students to copy and then to analyze. Then draw a single
incoming incident ray. Use a metre stick to find the normal for this ray. (You
may need to repeat the definition of the normal here: it is a line perpendicular
to the reflecting surface.) Then use the metre stick to show the reflected ray.
Repeat for other parallel incident rays, and then have students examine the
reflected rays. As students examine each ray individually, they will soon grasp
that each individual ray does obey the laws of reflection, but the uneven
surface results in reflected rays that are not parallel.
• Engage students further by having them complete Try This: Retro-Reflectors
(next page). This activity will demonstrate to students how a simple principle
can be applied to make extremely useful devices.
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but the oval was scattered into widely spread
patches of light after the foil was crumpled.
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T RY THIS
RETRO-REFLECTORS
Skills
• Performing, Observing, Evaluating
Purpose
• Students will use the laws of reflection to make a
diagram of a ray of light in a retro-reflector.
Equipment and Materials (per group): ruler;
protractor; piece of paper; pencil; two plane mirrors
and a ray box (optional)
Student Safety
• If a ray box is used, remind students not to pull on
the cord when they unplug the ray box; have them
pull on the plug itself.
Notes
• A retro-reflector is an object that causes light to
be directed back to its source. The principles of
retro-reflection are applied in reflective clothing,
such as the safety strips on jogging shoes and other
clothing designed to make the wearer visible to
those driving automobiles at night. Cats’ eyes seem
to emit light due to retro-reflection that occurs
because of the structure of the eye.
• Have students work in pairs for this activity.
• As students are working, walk through the class to
make sure that their drawings are correct. If you notice
errors in use of the laws of reflection, discuss with
students their error and encourage them to erase and
redraw.
Suggested Answers
A. The ray that emerged was parallel to the incident
ray but travelled in the opposite direction.
B. The second trial produced the same result.
C. A retro-reflector causes an incident ray to return in
the direction from which it came.
• Discuss with students the learning disability dyslexia. Explain the role that
reflection plays in causing problems for seeing text correctly. Discuss how
filters and coloured glasses can be used to reduce glare and make reading
easier for people with dyslexia.
Unit Task Bookmark
Many optical devices apply
the laws of reflection.
Have students brainstorm
how they could use what
they have learned about
reflection and plane
mirrors in this section as
they complete the Unit E
Task: Building an Optical
Device. Ask volunteers to
share their thoughts with
the class.
Extend and Assess
• Divide the class into four groups. Assign one law of reflection to the first
group and the other law of reflection to the second group. Assign specular
reflection to the third group and diffuse reflection to the fourth group. Have
each group prepare a poster presentation about their assigned topic. The
posters should include common or everyday examples of the topic of the
poster. Ask each group to share its completed poster with the class. Allow
students to examine all of the posters and ask them to write down any
questions they have about the posters or the material from the section. Have
a class discussion to clear up any topics or concepts about which students
have misunderstandings.
• Have students complete the Check Your Learning questions on page 486 of
the Student Book.
CHECK
YOUR LEARNING
Suggested Answers
1. The diagram should show that the incident ray comes in
at the same angle the reflected ray goes out, and the
normal is perpendicular to the surface at the point of
reflection; the two rays and the normal should lie on the
same plane.
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normal
reflected ray
incident ray
Chapter 11 The Production and Reflection of Light
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2. The angle of incidence equals the angle of reflection. The incident ray, the reflected ray, and the normal all lie on
the same plane.
3. (a) Smooth, shiny surfaces produce specular reflections, whereas irregular (rough) or dull surfaces produce diffuse
reflections. Specular reflection produces a reflected image, but diffuse reflection does not produce a reflected
image.
(b) Sample answers: Specular reflections: water on a street reflecting car headlights, seeing a mirror-like reflection
in a window pane or the blade of a kitchen butter knife. Diffuse reflections: light reflecting from a carpet, a dry
street, or a brick wall.
4. (a) The walls should exhibit diffuse reflection; otherwise, the room would be like an amusement park hall of
mirrors, with mirrors showing distracting reflections everywhere.
(b) Sample answer: I would use matte paint because it tends to produce diffuse reflection.
5. (a) 32o
(b) 47o
(c) 60o
6. Sample answer: Any room with mirrors, such as a bedroom or bathroom, has specular reflections from them.
Kitchens often have shiny pots and pans that also produce specular reflections. Bedspreads in bedrooms produce
diffuse reflections, as do carpets and matte-painted walls in all rooms.
DIFFERENTIATED INSTRUCTION
• Challenge students to find out more about reflection and eyesight. As
introduced in this section, reflection may play a role in the reading difficulties
associated with dyslexia. Many individuals without dyslexia are also troubled
by glare (reflection), especially when driving at night. Have students learn
about the antireflective coatings used on eyeglasses and share what they learn
about how these coatings work with the class using a presentation method of
their choice.
• Have students summarize the laws of reflection in the format of their choice.
Verbal/linguistic and mathematical/logical learners may choose to write a
summary, musical/rhythmical learners can create a rap or rhyme, visual/
spatial learners can create diagrams, and interpersonal learners may prefer to
collaborate to present an oral definition.
ENGLISH LANGUAGE LEARNERS
• Write the following words on the board with underlines as indicated: reflect,
reflected, reflecting, reflection. Have students locate each of these terms in
the text in this section. Explain how each underlined word part changes the
meaning of the term reflect.
– Point out that adding -ed forms the past tense of the word (Light rays
reflected off the mirror).
– Adding -ing indicates an ongoing action (The rays are reflecting right now).
– The –ion ending is used to turn the word into a noun (I see a reflection).
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