ETV “Junior Secondary Science” Programme
Sequel of Refraction of Light
Teachers’ Notes
Target Audience
Secondary 1 - 3
Duration
20 minutes
Production Aim
This programme is a teaching resource produced especially for enriching the
classroom teaching of the Syllabus for Science (Secondary 1 – 3) prepared by
the Curriculum Development Council (1998).
The content of the programme serves to illustrate the following parts of the
Syllabus:
“Unit 15 – Light, Colours and Beyond
Topic 15.6 – Refraction and its practical uses and
Topic 15.7 – Optical fibres and total internal reflection
Key Points
1.
2.
3.
The refraction of light.
The practical uses of lenses in our daily life.
Total internal reflection and apply the knowledge in the understanding of
some physical phenomena.
Content Outline
The programme is divided into 3 segments:
1.
Lenses
In this segment, it introduces the history of the invention of optical instrument.
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All optical instruments, including the telescope and the microscope, are made
up of different lenses.
Lenses are usually polished and made from pieces of glass. Both surfaces of
each of these pieces of glass are curved. A lens thick in the middle and thin
on the edge is a convex lens, whereas one that is thin in the middle and thick
around the edge is a concave lens.
It demonstrates how to use a glass of water to act as a convex lens and uses it
to observe images formed by object at different distance.
It demonstrates how a parallel light beam passes through cylindrical lenses:
When a beam of parallel light emerges from a ray box and passes through a
cylindrical convex lens, the parallel beam is converged. As a convex lens can
convert a parallel beam into a converging beam, it is also called a converging
lens.
On the other hand, a beam of parallel light diverges after passing through a
cylindrical concave lens. So a concave lens is also called a diverging lens.
2.
Water Drop Microscope
It demonstrates how to use a water drop and a hand lens to make a microscope.
There are two basic components in a microscope. One is a convex lens
placed close to the object to be observed. This is called the objective. The
other is a convex lens close to the observer. This is called the eyepiece.
First the objective forms an inverted and enlarged real image. The image is
then further enlarged by the eyepiece which acts like a magnifying glass.
3.
Total Internal Reflection
It demonstrates the total internal reflection by using a semi-circular glass
block.
A light ray is emitted from a ray box. When it hits the curved surface of a
semi-circular glass block, it goes into the glass. After reaching the inner
surface of the glass, the incident ray is divided into two rays. One is reflected
into the glass, and the other is refracted into the air. If the angle of incidence
on the inner surface of the glass is increased, the light ray refracted into the
air becomes weaker, and the reflected light becomes stronger. A point is
eventually reached where the entire incident ray is reflected into the glass and
no refraction occurs. The surface of the glass is just like a mirror. This is
known as “total internal reflection”.
In this segment, various applications of total internal reflection are introduced.
It introduces optical fibres and its application.
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Suggestions for Utilization
The teacher is advised to show the programme in one lesson. The teacher may spend
5 to 10 minutes at the beginning of each lesson to lead students to discuss with
reference to the Preparation before viewing the programme part of the Suggested
Activities. The teacher may then show the programme. After the show, the teacher
may spend another 10 to 15 minutes to discuss with students with a view to
consolidating the concepts and methods illustrated in the programme. The teacher is
advised to refer to the Activities after viewing the programme part of Suggested
Activities.
Suggested Activities
(The following activities are suggested for teacher’s reference only. The teacher
may wish to use the activities according to students’ abilities, the learning
environment of the class, and the teaching time available.)
Preparation before viewing the programme
1.
The teacher can discuss with students various examples in daily life which are
related to the refraction of light.
Activities after viewing the programme
(1) The teacher can demonstrate the setting up of a water drop microscope.
(2) The teacher can supervise students to make a periscope.
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