MGVSS Physics (Teachers): Propagation

Subject: Physics

Class: S1

Topic: Behaviour of light

Sub-topic: Propagation

Introduction:

This topic describes the behavior of light as it travels through a medium of uniform density, and when an opaque object is placed in its path. This leads to the formation of shadows and eclipses.

Knowledge of this aspect of light is applied in many situations of life, e.g. photography, security systems, driving mirrors, vision, eyeglasses, binoculars, etc.

Brief description of topic:

This unit describes the behaviour of light in a homogeneous medium and when an opaque object obstructs it. Knowledge of the rectilinear propagation of light would enable the learner to explain the occurrence of shadows, and eclipses. The learner is expected to use locally available materials to construct a pinhole camera and explain how it works.

Content: Definition of light, sources of light, speed of light, types of media, rectilinear

Propagation, rays and beams, eclipse and the pin-hole camera.

Teaching /l earning materials

Light bulbs of different sizes, opaque objects,

Cardboards with holes, flexible tube.

Methodology

Discussion, experimentation, group work.

Teacher’s guide

(A) Learning objectives, learners should be able to:

Demonstrate experimentally the rectilinear propagation of light.
Explain the formation of shadows and eclipses
Construct and describe the working of a pin-hole camera

(B) Teacher’s notes

Light is a form of energy (ask learners to give different uses of light)
Sources of light include; artificial e.g. light bulbs, fires, moon. and Natural e.g. sun, light flies.

Types of materials

Opaque – don’t allow light to pass through them.
Transparent – allow little light to pass through them
Translucent – allow little light to pass through them e.g. frosted glass.

[Allow learners to give examples of their own]

The speed of light in air is approximately 300000000ms-¹, learners should express this in standard form and in km/h.

Teacher performs experiment of show the rectilinear propagation of light using (a) cardboards and (b) a flexible tube.

Rays are paths of light; beams are a collection of rays, parallel beams, convergent beams, and divergent beams.

(C) Experiments to show the rectilinear propagation of light.

Method 1

Begin with the holes out of line and ask some few students to observe at E and tell class about their observations.
Using a string through the holes to ensure they are in a straight line and repeat procedure (i) above then let the learners draw the conclusion.

Method 2

With the tube in various configurations keep on asking the question ‘can you see the light? The observation leading to the right conclusion will come at the end.

Shadows

Formation of shadows with sharp boundaries is evidence that light travels in a straight line.

Experiments to study how shadows are formed.

Small source with a large obstacle

Large source with large obstacle

Large source with a small obstacle.

ECLIPSES

During an eclipse, the sun, earth and moon are in a straight line. The word eclipse means hiding.

Eclipse of the sun

The moon lies between the sun and earth

Eclipse of the moon

The earth lies between the sun and moon the earth casts a shadow on the moon.

The teacher should assemble a pin-hole camera and explain how it works in class.

A pin-hole camera

Tasks with the pin-hole camera

Tasks answers

(a) Describe the image	(a) The image is real, inverted
(b) In what ways can the image be magnified?	(b) Decreasing object distance or Decreasing the length of the camera.
(c) What is the effect of enlarging the pin-hole?	(c) The image becomes brighter and blurred (i.e. without sharp boundaries).
(d) Explain your observation in (c) above	(d) A large hole may be considered as consisting of many pin-holes each one producing its own image. When several images overlap, a blurred image is obtained - An enlarged hole allows more light to pass making the image brighter.

EXERCISE

What is light?
List

2 natural sources of light
2 artificial sources of light

Mention three uses of light
The speed of light is 300000000ms-¹

Express this in

Standard form
Km/s

Using a labelled diagram describe an experiment to show that light travels in a straight line.
Define the following; (a) rays (b) beam

Copy and complete the following

During an eclipse of the moon the

In a pin-hole camera what is the effect of

Explain your answer in (c) above

A tree 15m tall is viewed using a pin hole camera 30cm long. The distance of the tree from the camera is 90m.
1. Describe the appearance of the image
2. Calculate the size and magnification of the image.

ANSWERS:

1. Light is a form of energy

2. (i) Natural sources of light-sun, fireflies

(ii) Artificial sources of Light-electricity bulb, fireplace

3. (i) enable us to see

(ii) Used in photography

(iii) Used by solar cells

4. (i) 3 x 10 ms-¹

(ii) 300,000 km s-¹

5. [See experiment]

6. (a) Ray –direction or path taken by light

Beam – a collection of rays

(ii) convergent beam

(iii) Parallel beam

8. A

9. (a) image is enlarged

(b) image is diminished

Explanation of (c)

Larger hole allows more light pass .a large hole acts as a collection of pin-holes each one producing its own image. when all the images overlap, blurred image (without sharp boundaries is obtained.

10) a) the image is real, inverted and diminished.

b) x/10 = 0.3/90

x = 0.033m

Or 3.3cm

Magnification = 0.033/90 = 0.0004

SCHEME OF WORK

School ……class……. Teacher……..term…….year……

Periods	Topic	Sub-topic	Objectives	Content	Methodology	T/L Aids	Ref	Remarks
4	behaviour of light	Propagation of light	By the end of lesson the learner demonstrates the rectilinear propagation of light, -Explains the formation of eclipses	- Definition of light, sources of light. Speed of light, Rectilinear Propagation Rays and beams Eclipses	Discussion Experimentation	Bulbs Cardboards Flexible tube	New complete junior physics by A. Atkinson And H.sinuff Pg 156 ()
2	Behaviour of light	Propagation of light	Constructs and explains the working of a pin hole- camera	Pin hole camera	Group work Experiment	Cardboards Glue Tracing paper Razor blades	New complete junior physics by A. Atkinson And H.sinuff Pg 160

LESSON PLAN:

SUBJECT: PHYSICS

CLASS: S1

TOPIC: Behaviour of light

SUBTOPIC: propagation of light

Time: 80 minutes

Objectives: by the end of the lesson the learners should be able to:

Demonstrate experimentally the rectilinear propagation of light.

Teaching aids: candle, cardboards, flexible tube, string, lighter.

Method: Discussion, experimentation

Reference: New complete junior physics by Atkinson and H.sinuff, pg 156.

Time	Teacher’s activity	Pupil’s activity
20min	-Ask pupils to define light, to mention sources of light and categorize them. -Discusses the speed of light and writes it on blackboard, asks pupils to express it in km/s and in standard form. - Ask pupils the uses of light - Discuss transparent, opaque and translucent materials and ask pupils to give examples.	Attempt to reply and should give the correct response as: -Light is a form of energy, place sources of light in categories of natural and artificial sources. - Discuss speed of light with teacher - Convert speed of light to km/s and also in standard form. Give uses of light -Mention examples types of media.
25min	Teacher sets up the following experiment Begins with the holes out of line and ask a few pupils to observe, then arranges the holes in a straight line and the same pupils repeat the observation. - The teacher then asks the pupils for their deduction. -Teacher reinforces the conclusion by repeating experiment using a flexible tube.	- Observe and draw deductions.
15min	Defines a ray, beam Defines parallel, convergent and divergent beams and asks pupils to mention examples.	Give examples
15min	Give exercise and supervises (see exercise)	Do exercise

REFERENCES:

Atkinson. A and Sinuff. H New complete Junior physics (1983) Longman, Nairobi.

MGVSS Physics (Teachers)

Tuesday, June 30, 2015

Propagation