LESSON PLAN
Level
Semester
Subject
Topic
Time Allocation
Prepared by
Core Competency
Basic
Competency
Indicator
Objective
Content
: Secondary 2
:2
: Physics
: Total Internal Reflection
: 2 x 45’
: ....................
: attached
1. ....................
2. ....................
3.6 Describe the properties of light, images formation, as well as its application to
explain human vision, eye structure in animals, and working principle of
optical devices
4.5 Creating reports the results of investigations on the formation of images
 Explain critical angle and total internal reflection
 Identify the main ideas in total internal reflection and state the advantages of
its use in optical fibres
Students are able to:
 Explain critical angle and total internal reflection
 Identify the main ideas in total internal reflection and state the advantages of
its use in optical fibres
 Total internal reflection takes place only when light travels from an optically
denser to a less dense optical medium
 The critical angle c is the angle of incidence in the optically denser medium at
which the angle of refraction in the less dense medium is 90o
 The critical angle c is given by
Approach/
Learning Model
: Scientific/ Problem Based Learning
Assessment
: Written Test
Materials/Tools
Habits of
Character
:: Self discipline, independence, open minded, responsibility, risk taker, personal
Best
Opening
 Rehearse the students about previous discussion.
 Teacher asks students to stand up and walking around in the classroom, while giving
some questions:
 What is light?
 How does the light travel?
 What is luminous object?
 What is non-luminous object?
 Teacher receives any possible answer and please the students who can answer the
questions correctly to sit down while the other still keep walking
Main Activity
Time
10’
Time
Activity 1:
 Teacher shows a video of someone’s swimming (the video taken under water)
 Teacher gives a question:
 Why does the underwater reflection of the girl can be clearly seen above her head?
 Teacher receives any possible answer
 Teacher re-explain the answer:
 We take this phenomenon for granted when we go swimming. This phenomenon is
due to the total internal reflection of light. Total internal reflection can occur when
light passes from an optically denser to a less dense medium. To understand this
unique behavior of light, we need to first understand critical angle.
 Teacher asks the students to read the textbook page 238 – 240 about critical angle
(gives 10 minutes)
 Teacher asks students’ opinion about the content of the text they are read
 Teacher accepts any possible opinion
 Teacher discusses about critical angle by drawing a picture:
 Teacher explains (based on textbook p. 238 – 239):
 A ray of light is directed perpendicularly through the semicircular glass block at its
midpoint O. The light ray passes through without any deviation.
 The ray of light is then directed at an angle I at O. The light ray emerging from the
glass block is refracted away from the normal, as it is travelling from an optically
denser to a less dense medium. A small amount of light can be seen reflected back into
the glass block.
 As the incident angle is increased, the refracted ray is seen to bend further away from
the normal until the angle or refraction r becomes 90o. The refracted ray now travels
along the plane surface of the glass block. The angle of incidence in the glass block at
which the angle of refraction in air is 90o, is known as the critical angle c.
 From the explanation, teacher asks the students:
 What is critical angle?
 Teacher accepts any possible answer
 Teacher recaps the main point:
 Critical angle is defined as the angle of incidence in the optically denser medium for
which the angle of refraction in the less dense medium is 90o
 Teacher give a question to the students:
 What happens if the angle of incidence is greater than the critical angle?
 Teacher accept any possible answer
 Teacher explain about total internal reflection:
 If the angle of incidence in the glass block is increased beyond the critical angle, the
light ray will be reflected back into the glass block. In this case, no light gets
refracted through the glass-to-air boundary. This phenomenon is known as total
internal reflection. This explains why we are able to see underwater reflections
70’




Teacher recaps the main point:
 Total internal reflection takes place only when:
1. A ray of light travels from an optically denser to a less dense medium, and
2. The angle of incidence in the optically denser medium is greater than the critical
angle
Teacher give a question to the students:
 How can we determine the critical angle of a medium?
Teacher accept any possible answer
Teacher explain about total internal reflection:
 Knowing the refractive index n of an optical medium, we can calculate its critical
angle. Let us consider a light ray travelling from glass to air where,
<i = <c and <r = 90o
 However, as the ray is travelling from glass to air, we need to first reverse its
direction of travel using the principle of reversibility. We therefore have

Teacher recaps the main point:


Teacher explains the worked example 12.13 and 12.14 (see on the text book p. 240 –
241)
Closing
 Teacher asks the students to submit their mind map
 Teacher summarize the main points:
 Total internal reflection takes place only when light travels from an optically denser
to a less dense optical medium
 The critical angle c is the angle of incidence in the optically denser medium at which
the angle of refraction in the less dense medium is 90o
 The critical angle c is given by

Teacher gives homework for the students about total internal reflection



Resources
GCE ‘O’ Level Physics Matters by Charles Chow et al (Textbook)
Cambridge IGCSE Physics Coursebook by David Sang
Perfect Guide ‘O’ Level Physics by Ryan Bong
Time
10’
Bandar Lampung, June 2014
Acknowledged by,
Headmaster of …….
Prepared by,
Subject Teacher
……………………………..
…………………………………