Light II

TEACHERS’ GUIDE

SUBJECT : PHYSICS

TOPIC : LIGHT

SUB-TOPIC : Lenses and Optical Instruments

CLASS : Senior Three

CLASS SIZE : 50 Learners

TIME REQUIRED : Minimum: 120 – 240 minutes

INTRODUCTION

Man has always had interest in observing things in a more detailed manner. Lenses or magnifying glasses, as they are sometimes called, have been used to observe objects. For example in study under microscope lenses are used to magnify tiny organisms that may not be easily seen by the naked eye, and in telescope lenses are used to magnify distant objects like the stars and moon. People with poor eye sight use lenses to enable them see better, for example use of reading glasses to enlarge prints, watch repairers and handset cell phone repairers also use magnifying glasses.

CONTENT

In this unit, the content covered will include:

• Types of lenses and their optical properties.
• Passage of standard rays through a lens.
• Power of a lens.
• Construction of ray diagrams.
• Properties of images formed by lenses.
• Magnification
• Experiments to determine the focal lengths of thin convex lenses.
• Human eye and eye defects
• Projector
• Simple camera

LEARNING OBJECTIVES

By the end of this unit the learner should be able to

1. Identify the different types of lenses.
2. Define the optical properties of lenses.
3. Define the power of lenses.
4. Construct graphically images formed by lenses using standard rays.
5. Describe images formed by lenses.
6. Determine magnification of images formed by lenses.
7. Determine the focal length of thin converging lenses.
8. Draw the projector and describe how it works.
9. Draw the human eye and explain how it forms images.
10. Draw the lens camera and explain how it forms images.
11. Explain the use of lenses in correction of eye defects.

Job related life skills

By the end of this topic, learners are expected to have acquired the following Job related Skills:

1. Personal attributes – self confidence, time management, creativity/imaginative, recording skills, enthusiasm, imagination and self awareness.

2. Communication – observation & listening skills, reporting in writing.

3. Team work – task oriented leadership skills, group work.

4. Problem solving - information seeking, environmental protection & conservation, seeking for information (research).

5. Applicability of number - numeracy (as they compare crop yields in treated and untreated plots) Problem solving - information seeking, environmental conservation, Reflective thinking.

6. Information Skills – ability to work with and present numerical data using appropriate intermediate calculations, ability to identify information needs, observe and collect evidence, present findings appropriately using graphs, charts, pictures and reports.

LEARNERS’ ACTIVITIES

Activities 1 to 4

In these activities, the learners investigate the optical properties of lenses.

Material required for each group

Bi-convex lens, Plano-convex lens, converging meniscus, bi-concave lens, Plano-concave lens, and diverging meniscus

Activity 1

• Learners examine the physical features of the lenses provided
• Learners use the lenses to view different objects and fill in the table below

	Type of lenses
	Bi-convex	Plano-convex	Converging meniscus	Bi-concave	Plano-concave	Diverging meniscus
Effect on size of object

Activity 2

Lenses in Art

• Learners magnify and draw in fine detail of at least one of the following: leaf, insect, bark of a tree, human skin.
• Display their work in class
• Discuss the use of magnifying glasses in Art

In the plenary, learners discuss the economic importance of using magnifying glasses in Art.

Activity 3

Lenses in hygiene

• Learners use magnifying glasses to view some living organisms in dirty water and record what they see.

In plenary, learners discuss the biological and economic importance of boiling water for drinking using their observations as a basis.

Activity 4

Lenses in sight seeing

• Using two convex lenses and a manila paper, learners construct a telescope and use it to view different sites in the neighbourhoods.
• Learners try out other groups’ telescopes and compare them with theirs.

In the plenary, learners discuss the different uses of viewing instruments in tourism, war and road construction.

Activity 5

Discussion

Topic: Discuss how you would use your camera to earn a million shilling in one year. The discussion should include: planning, budgeting, execution of the plan, saving procedures etc.

Note to the teacher: You may invite a resource person to help the learners to hold a fruitful discussion. He/she should be guided on the kind of things he/she should talk /not talk about.

ACTIVITY 6

In this activity learners will carry out experiment to measure the focal length of converging lens

Material required

Plane mirror, convex lens (f=10 cm or 15cm or 20cm), screen with a hole and wire gauge placed on the hole, torch bulb, two dry cells, bulb holder, connecting wires, lens holder, metre rule.

Instructions

1. Learners divide into groups and each group elects a chairperson and a secretary.
2. In this experiment you will determine the focal length of the given convex lens using an illuminated object and a plane mirror.

3. Set up the apparatus as shown in diagram above.
4. Adjust the position of the lens until a sharp image of the object (wire gauge) is formed on the screen alongside the object.
5. Measure the distance between the lens and the image. This distance is equal to the focal length of the lens.

In preliminary discussion

1. Each group presents their results.
2. Learners brainstorm on the applications of lens in projector, camera, human eye and use of lenses to correct defects in the human eye.

Teacher’s Notes

1. A simple lens is usually a piece of glass bounded by spherical surfaces.

2. Principal axis of a lens is the line joining the centres of curvature of its surfaces.

3. The principal focus of a converging lens is that point on the principal axis to which all incident rays on the lens, originally parallel and close to the principal axis, converge after passing through the lens. The principal focus of a converging lens is real.

4. The principal focus of a diverging lens is that point on the principal axis to which all incident rays on the lens, originally parallel and close to the principal axis, appear to diverge from after passing through the lens. The principal focus of a diverging lens is virtual.

5. The optical centre of a lens is the centre of the lens. All rays passing through the optical centre of a thin lens are drawn straight not deviated.

Standard Rays

For Convex lens

Position of object	Draw the ray diagram to show the location of images	Nature of image	M=h’/h
Between the Principal focus and the optical centre of the lens
at the Principal focus
Between the Principal focus and distance twice the focal length from the optical centre
at a distance twice the focal length from the optical centre
at a distance greater than twice focal length from the optical centre
at infinity

For concave lens

Position of object	Draw, using ray diagram, the location of the image	Nature of image	M=h’/h
between the focal point and the optical centre of the lens
at the focal point
between the focal point and distance twice the focal length from the optical centre
at a distance twice the focal length from the optical centre
at a distance greater than twice focal length from the optical centre
at infinity

For convex lenses:

1. There are three important rays in the ray diagram construction to locate the position of the image in thin lenses.

1. Ray from the object parallel to the principal axis
2. Ray from the object passing through the optical centre
3. Ray from the object passing through the principal focus

2. When the object is placed between focal point and optical centre, the image formed is behind the object, virtual, erect and magnified.

3. When the object is placed between the focal point and distance twice the focal length from the optical centre, the image formed is beyond 2F, real, inverted and magnified.

4. When the object is placed at the focal point, the image formed is at infinity.

5. When the object is placed at a distance twice the focal length from the optical centre, the image formed at the point twice the focal length is real, inverted, and the same size as the object.

6. When the object is placed at a distance greater than twice the focal length from the optical centre, the image formed is between 2F and F, real, inverted and diminished.

7. When the object is placed at infinity, the image formed is at the focal point.

8. The size of the image varies according to the position of the object from the optical centre

9. The linear magnification of the image is the ratio of the height of the image to the height of the object. I.e. m = height of image /height of object.

10. Power of a lens is the reciprocal of its focal length in metres. Power = 1/focal length in metres

Projector

Light source – Carbon electric arc or quartz iodine lamp to produce high intensity source of light and is placed at the centre of a concave mirror.

Concave mirror – Reflects back light otherwise wasted at the back of the projector

Plano-convex lenses (Condenser)-Collects and concentrates (condenses) light that would otherwise spread out and get wasted and converge it through the slide onto the projector lens.

Projection lens – Focuses the magnified slide image on the screen. This is done by moving the lens to and fro.

Link pages

http://en.wikipedia.org/wiki/lens_(optics)

http://www.physicsclassroom.com/Class/refrn/

Exercise

1. Describe the nature of the image when the object is placed

1. Between the lens and the principal focus
2. At the principal focus

2. List two uses of lenses
3. Define linear magnification.
4. What is a virtual image?

Answers

1. (a) magnified, upright, virtual

(b) at infinity

2. As magnifying glasses, eye glasses, etc.

3. Magnification = height of image / height of object

4. A virtual image is one which cannot be formed on a screen. It is formed where virtual rays meet.

SAMPLE SCHEME OF WORK

SUBJECT: PHYSICS

NAME OF TEACHER……………………………………....SCHOOL……..............……………….. TERM…......… CLASS…........… NO. OF PERIODS PER WEEK……………… YEAR……...….. NO. OF STUDENTS…………………

WEEK	PERIOD	TOPIC	SUB - TOPIC	OBJECTIVES	METHODS	TEACHING AIDS	REFERENCES	Comments
1	3	Light	Lenses and optical instruments	By the end of this topic, learners should be able to; -Define optical properties of lenses -Graphically construct images -Describe images formed -Calculate magnification	-Illustrates the types of lenses -Demonstrates the passage of standard rays through a lens -Illustrates the power of a lens -Discusses the properties of images	-Metre rule -Ray slit lenses	-
2	3	Light	Lenses and optical instruments	-Carry out experiments to determine focal length of a convex lens	-Experiments to determine focal length of a convex lens	Bulbs, bulb holders, cells and cell holders, connecting wires, Screen Cardboard with mesh wire Lenses.
3	3	Light	Lenses and optical instruments	-State and describe use of lenses -Draw a projector and describe how it works -Draw the eye and camera and describe how these form images	- Discusses the uses of lenses -projector -human eye and camera	Charts (camera if possible)

Lesson Evaluation:

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PHYSICS

SAMPLE LESSON PLAN

DATE	CLASS	SUBJECT	NO.OF LEARNERS	DURATION	TIME
…./…./…	SENIOR THREE	PHYSICS	50	80 MINS	8.00 – 9.20am

TOPIC : LIGHT

SUB – TOPIC : Lenses

Objectives : By the end of the lesson, learners should be able to:

1. Identify types of lenses
2. Define optical properties of lenses
3. Graphically construct images
4. State the nature of images formed.

Methods : Guided discovery

Demonstrations / Experiments

Question and Answer

Explanations

Teaching/Learning aids: Ray slits

Bulb and Bulb holders

Cell and Cell holders

Connecting wire lenses and Rulers

Chalkboard

Markers

Pictures/Photos

References:

Time	Theme	Teacher’s Activities	Learners’ activities
5 minutes	Types of lenses	-Distributes different types of lenses to groups -Instructs learners to study the properties of lenses (Activity 1)	-Learners examine lenses -explore properties of lenses
25 minutes	Standard rays	-using ray slits demonstrates passage of light through lenses -illustrates optical properties	- Observe -Draw the passage of standard rays through lens -Define the optical properties of lenses
40 minutes	Images formed by lenses	-illustrates drawing of rays to obtain images -instructs learners to draw images of objects at different distances from the optical centre Note: Diagrams should be drawn to scale.	-Draw the ray diagrams -Describe images formed
10 minutes	Images formed by lenses	-Guides learners to explore the properties of images formed by lenses practically.	-verify the properties of images formed by lenses practically.

Lesson Evaluation

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