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National Institute of EducationPhysics Lesson Plan on Refraction
Cai Simin
054354C23
PGDE (Sec) Group 4
Sub-group 3
QCP 521 Physics II
27th October 2005
0
Subject : Physics 5052
Topic : Light (13)
Sub-Topic : Refraction of Light (13.2)
Class : Secondary 3 Express
Ability : Average to High
Duration : 70 minutes
Learning Aids and Resources
1. Learning Environment
Lesson is to be conducted in the Computer Laboratory. There should preferably
be at least one computer to two students.
A Projector that presents the computer screen on the board.
A Whiteboard for the teacher to conduct the lesson.
2. Visual Aids
Pictures related to refraction are shown to the students to trigger their interest.
A glass cube with an embedded image is passed around for students to examine.
Ray diagrams are projected to reinforce students’ understanding.
3. Information Technology
The usage of MS PowerPoint to present pictures and highlight important concepts
and statements in the topic.
A video demonstration of a toy car travelling on different surfaces to illustrate the
concept that speed changes in different medium.
A Java applet is used for the Hands-on Activity.
MS Excel is used to tabulate the results from the applet.
4. Worksheets
Notes: Students are to fill in the blanks with key words, diagrams or qualitative
working in the notes while following the lesson. This document summarises and
supplements the content in the textbook.
Worksheet: Homework to be assigned at the end of the lesson.
1
5. References
The students should have with them their textbook, Physics Insights by Loo Wan
Yong, Loo Kwok Wai and See Toh Weng Fong.
Pre-requisite Knowledge and Skills
Students should already have the following knowledge or skills prior to the lesson:
1. apply speed = ,
2. apply Newton’s Laws of Motion,
3. recall the speed of light in vacuum, c = 3 x 108 ms-1,
4. apply the relationship velocity = frequency x wavelength,
5. describe that light travels in a straight line,
6. describe rays as the paths along which light energy travels,
7. explain that a person can see an object because the light rays coming from it enter
his/her eye,
8. define the terms used in reflection, including normal, angle of incidence and
angle of reflection,
9. apply the laws of reflection, demonstrating that the incident ray, the reflected ray
and the normal at the point of incidence all lie on the same plane,
10. draw ray diagrams showing reflection by a plane mirror,
11. recognising that alternate angles between parallel lines are equal,
12. recognising the angle that a light ray, that is extended from the centre of the
diameter of a medium that is shaped in a circle or a semicircle, makes at the
intersection at the boundary of 2 mediums is perpendicular to the boundary,
13. calculate that sin 90° = 0, and
14. proficient in entering data into MS Excel spreadsheet.
Specific Instructional Objectives
By the end of the lesson, students should be able to:
1. define the terms refraction, angle of incidence, angle of refraction and normal,
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2. describe that the incident ray, the refracted ray and the normal at the point of
incidence all lie in the same plane,
3. discuss the knowledge that the frequency of a light ray does not change when it is
refracted, but instead, its speed changes,
4. recall that a light ray will be refracted towards the normal in the more optically
dense medium and away from the normal in the less optically dense medium,
5. define refractive index of a medium as ,
6. recall the relationship ,
7. recall Snell’s Law, ,
8. state that the refractive index of air is almost 1, resulting in the relationship
, if the incident ray is in vacuum (or air),
9. describe that a normally incident light ray is undeviated,
10. apply the relationship between sin i and sin r to new situations or to solve related
problems,
11. state and apply the principle of reversibility of light,
12. complete the assignment given.
New Concepts and Terms
refraction, angle of incidence, angle of refraction, normal, refractive index,
, Snell’s law
Lesson Outline
1. Trigger interest in students by passing around objects and showing them pictures
of phenomena caused by refraction.
2. Explain to the students that refraction occurs when light rays travel through
different mediums. Define refraction.
3. Show the video demonstration that a toy car “experiences” changes direction by
itself when travelling on different surfaces, and describe to the students that it is the
3
change in speed that has caused the change, which is also the underlying principle of
refraction of light rays.
4. Allowing students to appreciate that light rays bend towards the normal when they
travel from air to another medium, suggesting that light travels at a slower speed
when it is in another medium.
5. Explaining to students that the speed of light in the medium is dependent on its
refractive index, which is the ratio of the speed of light in vacuum and in the medium
6. Display ray diagrams, stating to the students the normal, the angle of incidence
and angle of refraction, emphasising that they are on the same plane.
7. Work through the examples in the notes with the students to identify the normal,
the angle of incidence and angle of refraction.
8. Allow students to play with the Java Applet, and from the data obtained, induce
the knowledge that and more specifically, Snell’s law,
.
9. Work through remaining examples in the notes with the students to obtain
solutions using the knowledge obtained in the lesson.
10. Explaining to students that a normally incident light ray is undeviated, using
Snell’s Law.
11. Assign Worksheet A as homework.
12. Review the pictures which were shown at the start of the lesson, and ask students
to reconcile what they have learnt in the lesson by explaining the phenomena to
another classmate; they will realise that some pictures cannot be explained with the
knowledge they have so far, instead, more knowledge will be imparted to them the
following lesson.
Teaching / Learning Activities Materials Duration
Introduction
4
Tell the students that they have already learnt “Reflection
of Light” in the previous lessons, so for this lesson, they
will be moving on to learn the next sub-topic of Optics,
the “Refraction of Light”. At the same time, exchange the
word “reflection” for “refraction” on the slide, and add
that they are different. This is to cater to students who are
more auditory or more visual to recall the terms more
easily.
Pass 2 objects around the class: a spoon in a container
that is filled with water, and a glass cube with an
embedded image of a monkey (refer to glasscube.jpg).
Direct students to observe the size and the shape of the
spoon when it is submerged in water; and the number of
monkeys they can observe in the cube when they look at
it from different angles. They should be able to observe 1,
2 or 3 monkeys. This activity allows the students to make
the connection to everyday context such as the “bending
of a spoon”, and how fascinating some daily objects can
be if they are observed.
Present the slides in Refraction.ppt, asking the students to
observe the various photographs that will be screened
while the 2 objects are being passed around. Some of the
pictures were taken in laboratory, while the rest show the
phenomena of the refraction of light in everyday life,
which students may or may not have observed. This
activity will trigger the interest of the students as they
become impressed with the world around them through
the colourful pictures, which are consequences of science.
Slide 1
Spoon in
container,
Glass cube
1 min
1 min
5
Ask the students how many tortoises they think there are.
Remind students that the images on the screen are similar
to the spoon in water that is passed around. Draw the
students’ attention to the orange pencil, and observe there
are extra parts of the pencil in the water also. Be careful
of spillage especially when the lesson is held in a
computer laboratory.
Point out that this is considered an abstract piece of
photography, but all the photographer did was to align
two glasses of water with the bottle, and “distortion”
occurs, similar to the previous pictures shown.
Ask the students “Does light travel in a straight line?”
The students will probably answer yes, as they were
taught that property in Lower Secondary Science. Show
them Slide 4. The pictures show that the light rays have
bent, therefore the students will be in a state of cognitive
conflict. Ask them “Why have the light rays bent?”, and
to keep that question in mind.
Ask the students if they find the pictures familiar, as they
are occurrences of reflection. Point out to them that there
are no mirrors in any of the pictures.
Tell the students that the topic they are going to learn is
the underlying principle of the spectacles they are
wearing, as well as the magnifying glass they see in the
slide. Point out the tigers in the water droplet in the
Slide 2
Slide 3
Slide 4
Slide 5
Slide 6
Slide 7
3 min
6
foreground.
Observe there are seven visible colours in every picture.
State that the occurrence of rainbows is due to refraction
also.
Explain to the students that they will see all these pictures
again throughout the subsequent lessons, and they will be
able to explain why all the phenomena have occurred by
the end of the topic. That will effectively maintain their
interest in the topic, as they will be quite amazed at what
they have seen.
Slide 8
Lesson Development
What is refraction?
Recall the question posed previously: Does light travel in
a straight line? Prompt the students to think if they have
been learning the wrong thing. Show Slide 9 (a repeat of
Slide 5 with explanation), and lead the students to realise
that the light rays have bent only at the boundaries of air
and glass, and they continue to travel in a straight line
when there is no change in medium.
Introduce medium, giving the example in the slide, that
the two mediums present are air and glass. Refer to Slide
10 (a repeat of Slide 3) to give the example of air and
water as the mediums. Explain that refraction occurs only
when the light rays travel through different mediums.
Therefore, light still travels in a straight line, when there
is no change in medium.
Slide 9
Slide 9 & 10
1 min
1 min
7
Define refraction, “Refraction is the change in direction,
or bending, of light when it passes from one medium to
another.”
Why does refraction take place?
Refer to Slide 12. Recap with the students the concept of
“medium” by stating the two mediums present in each
example. Point out the similarities of the “bending” of
direction of motion among the examples and the glass
block.
Show the video demonstration car.mov, explaining to the
students that the demonstration will help them understand
why light rays bend when they enter a different medium.
After the demonstration, ask students if they notice any
similarities between the toy car and the light rays seen in
Slide 5. They should observe that the toy car changes its
direction when it started moving on the towel. Play the
video again and ask them to reconfirm the observation.
Explain to the students that the first wheel that reaches the
other surface experiences more friction than the other 3
wheels, therefore by Newton’s Laws, and that all 4
wheels are still rotating at the same frequency, the speed
of the 1st wheel is the smallest, and travelled the least
distance. This will result in the other 3 wheels travelling
more and therefore the entire car changes its direction of
motion as a torque is produced, until the 2nd wheel is on
the towel. Similarly, the continuous rotation of the 3rd
wheel and 4th wheel will result in more directional
Slide 11
Slide 12
car.mov
Slide 5
car.mov
Slides 13 & 14
1 min
1 min
2 min
8
change. Therefore, it can be concluded that the change in
speed of the wheels has resulted in the change in direction
of the toy car.
Refer to Slide 15. Observe the directional change of the
toy car which has its speed reduced on the simplified
diagram. Comparing the diagram with the 2nd diagram of
a light ray travelling across two mediums, help the
students induce that the bending of light during refraction
is due to the change of speed of light in the different
mediums. In particular, the speed of light has reduced
when it travels from air to glass.
Emphasise that the frequency of the light remains the
same. It is important to address this potential
misconception at this stage, highlight the toy car in the
video, that the factor that has caused the change the
direction is the speed, not the frequency.
Speed of Light
At this point of time, ask students, “What is the speed of
light?” Students will probably answer “speed of light = 3
x 108 ms-1”, as they are familiar with the value learnt in
Lower Secondary Science. Reiterate that the speed of
light in vacuum, c = 3 x 108 ms-1. However, when light
travels in another medium, its speed decreases.
Explain to the students that refraction takes place due to
the change in the speed of light as it enters another
medium of different optical density. Optical density is
also known as the refractive index, n.
Slides 15 & 16
Slide 17
1 min
1 min
1 min
9
Distribute the document, notes.doc, to the students, and
allow them to fill in the blanks (until Refractive Index) as
a summary of the lesson thus far.
Ask the students to respond with the words they have
filled in the blanks. Refer to Slide 18 and 19, and notes –
teacher copy.doc.
From this point on, students are to follow and fill in
relevant information in their notes.
Refractive Index
Explain to the students that since the refraction of light
occurs as it enters another medium of different optical
density, therefore, the speed of light in the medium is
dependent on its refractive index, which is the ratio of the
speed of light in vacuum and in the medium.
n =
Demonstrate to the students how to calculate the
refractive index of vacuum with the formula (nvacuum = 1).
Explain to the students that the refractive index of air, nair,
is approximately 1.00003, which is almost 1.
Also, show students that refractive index has no unit.
Refer to slide 16. Recall that the speed of light in glass is
less than the speed of light in air. Therefore, using the
definition of refractive index, the refractive index of glass
Notes.doc
Slides 18 & 19,
notes.doc,
Slide 20
Slide 16
4 min
1 min
1 min
1 min
10
is larger than 1. Extrapolate that the refractive index of
any other medium is always larger than 1.
Summarise that light bends at the boundary of two
mediums of different refractive index.
Ray Diagrams and 1 st Law of Refraction
Assist the students in recalling how to draw a reflected
ray on a plane mirror. On the whiteboard, draw a plane
mirror and an incident ray. Ask for a volunteer to come
up to the board to complete the ray diagram. Remind the
students that they have to label the normal, the angle or
incidence and the angle of reflection whenever they draw
ray diagrams to illustrate reflection.
Refer to Slide 21. Point out to the students that there is
also a normal, an incident ray and the angle of incidence
which is measured from the normal in refraction, just like
reflection. Difference is instead of being reflected, the
light ray will travel into the 2nd medium at a different
angle. Define the refracted ray and the angle of
refraction, which is measured from the normal.
Emphasise the 1st Law of Refraction, that similar to
reflection, the normal, the incident ray and the refracted
ray are on the same plane.
Based on the ray diagram on the board, question the
students whether the ray bends towards or away from the
normal when it travels in a medium with higher refractive
index. Reiterate and summarise for the students, that
Slide 21
1 min
1 min
2 min
1 min
11
when the speed of light decreases, the ray of light will be
refracted towards the normal; when the speed of light
increases, the ray of light will be refracted away from the
normal.
Refer to Worksheet.doc. Students should attempt
Question 1 and 2 on their own. Walk around to check on
the progress of the students, making sure that the
refracted ray is bent towards the normal, and that the
angle of incidence and angle of refraction are labelled
correctly. This activity is to allow students to know
instinctively the general direction the ray will be
travelling towards without doing any calculations.
Explain to the students that they had only been sketching
the ray diagrams previously in Questions 1 and 2.
However, there is a mathematical relationship between
the angle of incidence and angle of refraction for the
various mediums of different refractive indices, which
they will discover themselves in the following activity.
2 nd Law of Refraction (Snell’s Law)
Refer the students to data.xls and the Java Applet located
at. http://acept.la.asu.edu/PiN/act/refract/refract.shtml.
Demonstrate how to use the applet, and enter the data into
the template in the form of an excel spreadsheet. Instruct
the students to play around with as many values as they
can after they have completed the template within the
stipulated time of 20 minutes, and save the document for
their own reference. Walk round the classroom to observe
the students, making sure they are able to follow the
Worksheet.doc
data.xls,
Java Applet
3 min
20 min
12
instructions and are doing their work diligently.
Discuss the findings with the students. Ask them to
observe if there are any patterns they have observed. Lead
them to discover and more specifically,
Snell’s law, .
For a light ray passing from a vacuum into a given
medium, the constant ratio is the medium’s
refractive index.
Consolidate the 2 laws of refraction:
(1) The incident ray, the refracted ray and the normal
at the point of incidence all lie in the same plane.
(2) For two given media, the ratio is a constant,
where i is the angle of incidence and r is the angle
of refraction. (This is also known as Snell’s law.)
Normally incident rays
Using Snell’s Law, demonstrate to the students that for a
light ray which is perpendicular to its boundary, in other
words, the angle of incidence = 90°, the value of the angle
of refraction = 0°.
Thus, if a light ray enters another medium
perpendicularly to the boundary, no deviation of the ray is
observed even when there is a change of optical density.
Such a ray is said to be normally incident at the boundary.
data.xls 3 min
1 min
3 min
13
Consolidation
Work through the examples given in the notes with the
students to obtain solutions, using the knowledge
obtained in the lesson previously.
notes.doc 12 min
Closure
Review the pictures which were shown at the start of the
lesson, and ask students to reconcile what they have learnt
in the lesson by explaining the phenomena to another
classmate; they will realise that some pictures cannot be
explained with the knowledge they have so far, instead,
more knowledge will be imparted to them the following
lesson.
Assign remaining questions in the Worksheet as
homework. Students should start doing their homework if
there is still remaining time for the lesson. They should be
able to complete the 3 remaining questions very quickly if
they have understood the lesson.
Slides 1 to 7
Worksheet.doc
1 min
1 min
References
http://hyperphysics.phy-astr.gsu.edu/hbase/sound/refrac.html.
Loo, W. Y., Loo, K. W., & See Toh, W. F. (2000) Physics Insights. Pearson Education:
Longman.
Ministry of Education: GCE ‘O’ Level Physics Syllabus (5052).
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