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

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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.

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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

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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

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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

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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

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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

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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

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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

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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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