Light I

Unit: 5

Subject: Physics

Class: S.1

Topic: Behaviour of Light.

Sub-topic: Reflection of light from plane surfaces.

Introduction:

At home we use mirrors in our wardrobes and bathrooms. We often carry them in our hand bags, plane mirrors have many uses in our daily lives. We can use either one mirror or a combination of mirrors to make useful and interesting devices. Children like playing with mirrors. They see their images in them but can’t touch them.

Time: Min: 200minutes. Maximum 240minutes.

Brief description of subtopic.

This subtopic explores the properties of images obtained using a single plane mirror and a combination of plane mirrors. The laws of reflection are used to provide an explanation for the observations made. Equipped with these principles one is able to devise optical devices using a plane mirror or a combination of plane mirrors.

Content: Laws of reflection, regular and irregular reflection, properties of images in plane mirrors, simple periscope, mirrors inclined to each other, the kaleidoscope.

Learning objectives: learners should be able to;

1. State the laws of reflection.
2. Experimentally verify the laws of reflection.
3. Describe the images formed by plane mirrors.
4. Construct a simple periscope and kaleidoscope.

Teacher’s Notes.

Reflection in Plane Mirrors

AO = Incident ray

OB = Reflected ray

ON = normal to mirror

i = angle of incidence

r = angle of reflection.

Laws of reflection:

1. The incident ray, reflected ray and normal to surface at the point of incidence all lie in the same plane.
2. The angle of incidence is equal to the angle of reflection.

Regular reflection Irregular (Diffuse) reflection.

- Abeam of light reflected from a shiny smooth surface is very bright. This makes it difficult to recognize details of the reflecting surface.

- Light is reflected irregularly (diffusely) from most objects around us. This enables us to see and recognise detail on the reflecting objects.

Properties of images in plane mirrors, they are;

1. Upright (i.e. the same way up as the object)
2. Virtual (i.e. cannot form on a screen)
3. Same size as object
4. Laterally inverted (i.e. the left of the object looks to be the right of the image)
5. Distance of the object from the mirror is equal to the distance of the image from the mirror.
6. A line joining a point on the object to its reflection on the image is perpendicular to the mirror.

Formation of an image in a plane mirror – How the eye sees the image.

An observer views an object when rays of light from the object are reflected from the mirror to the eye (s) of the observer.

• Draw ray OB
• Construct a normal at B
• Draw the reflected ray BD, use a protractor to ensure i and r are equal.
• Extrapolate DB behind the mirror.
• Repeat the procedure with another ray OC.
• The two extrapolated rays meet at 1 where the image appears to be.
• Join O to 1
• Measure OA and A1, what do you find? Answer OA = A1
• Measure the angle between O1 and the mirror. Answer angle = 90⁰

EXPERIMENTS:

Experiment to verify the laws of reflection.

(a) Using no-parallax method.

Apparatus: soft board, optical pins, plane mirror, protractor and a plane sheet of

paper.

Procedure:

1. Draw a line to mark the silvered surface of the mirror on the sheet of paper.
2. Mark a point X on the line.
3. Draw a normal at X.
4. Using a protractor mark off an angle ray of incidence, e.g. 20⁰ and draw the incident.
5. Stand the mirror vertically on the line representing the silvered surface.

6. Tuck pins P₁ and P₂ in the incident ray.

Note: P₁ and P₂ should stand vertically and far a part.

7. Position your eye at E to view the reflections of P₁ and P_2.
8. Tuck in P₃ such that P₁, P₂, and P₃ appear to be in a straight line.

Move your eye side ways to ensure there is no relative movement (no-parallax) between P_{1 ,}P_{2 and} P3 .

9. Repeat procedure (ix) above with another pin P₄.
10. Join P₃ and P₄. It should cross the silvered line at X.
11. Measure the angle of reflection, r.

Repeat procedures (iv) to (xii) for angles I = 30⁰, 40⁰, 50⁰, 60⁰, 70⁰

Tabulate your results in the table below.

i0	r0
20 30 40 50 60 70

What do you observe? (Generally, the angle of incidence is equal to the angle of reflection)

(b) Using a ray of light.

Apparatus: plane mirror, protractor, empty tin with a narrow slit, Bulb,

Cell holder, Cells, connecting wires.

[Hint: this experiment is best done in a dark room.]

1. Draw a line to represent the silvered surface.
2. Using a protractor construct a normal at X.
3. Position the protractor in such a way that the centre of the protractor is at X.
4. Direct a ray of light at X.
5. Read i and r.
6. Repeat procedures (iv) and (v) for various angles of incidence each time noting the angle of reflection.
7. Tabulate your results in the table below

i0	r0

What do you notice? Answer: The angle of incidence is equal to the angle of reflection.

3. To count the number of images in two mirrors inclined to each other.

1. Arrange two plane mirrors at right angles to each other and stand a small object between them.
2. Count the number of images.
3. Repeat procedures (i) and (ii) for angles of 120⁰, 100⁰, 80⁰, 60⁰, 50⁰, 40⁰, and fill your results in the table below:

Angle	Number of images.
1200 1000 800 600 500 400

Question: How many images will be visible when the angle is 110⁰, 70⁰, 30⁰, 20⁰, 10⁰, and 5⁰ and x⁰?

Answer 2, 4, 35, 71 and 360 - 1

Respectively.

Explanation:

Consider 2 plane mirrors inclined to each other at 60⁰.

M₁ is the reflection of M₁ in M₂

M₂ is the reflection of M₂ in M₁

M₂ and M₁ also act as mirrors.

M₁ is the reflection of M₁ in M₂.

M₂ is the reflection of M₂ in M₁.

These two also act as mirrors.

The combination of mirrors produces images A, B, C, D, and E.

In the diagram above you can count six sectors i.e. 360⁰ = 6 and 5 images.

60⁰

Therefore the number of images is given by:

Number of images = 360⁰ - 1

Angle between mirrors.

ANNULAR ECLIPSE

There are times when the distance of the moon from the earth is such that the tip of the umbra fails to reach the earth’s surface. From the earth, the sun appears as a circular ring. This is called annular eclipse.

Object The Kaleidoscope.

• Cut several discs of wood.
• Using a protractor mark out shaded sector AOB, with angle AOB = 30⁰
• Carefully cut out the shaded sector.
• Using wood glue fix plane mirrors along OA and OB.
• Repeat the procedure by changing the angle to 20⁰, 40⁰ 50⁰, 60⁰, 90⁰, 120⁰ using cardboard.
• Place one of the discs you made above at the bottom of the tube.
• You can now drop flowers or other small coloured objects into the tube.

For each one you drop into the tube, apparently many others are “produced”. Using discs with mirrors at different angles can enjoy this effect.

Parallel mirrors.

Now position the mirrors parallel to each other and count the number of images. How many can you get? Answer: very many (infinity)

Illustration.

I₁ = reflection of O in mirror 1

I _{1 2} = Reflection in Mirror 1 followed by reflection in mirror 2.

I _{1 2 1} = Reflection in mirror 1 followed by reflection in mirror 2, followed by reflection in mirror 1.

e.t.c

Question: mention one practical application of parallel mirrors.

Answer: in saloons customers are able to view both the front and back of their heads.

Project: construction of a simple periscope.

Materials: 2 plane mirrors,

Cardboard paper,

Glue,

Protractor.

Procedure: -

- Using glue and cardboard, construct 3 tubular sections each about 50cm long.

- Ensure that the edges of the tube are normal to each other.

- Using glue fix two plane mirrors in the tubes as in diagram.

- Make sure the mirrors each make angle of 45⁰ with the sides of the tube.

- Now join the tubes and fix them firmly.

Note: the periscope will not work properly if the mirrors are not parallel to each other.

You can now use your periscope to view different scenes.

- Describe the image you see.

- Mention some uses to which your periscope can be put.

[Answer (i) people at the back of crowd may use periscopes to see over the heads of people in front of them.

(ii) Sailors in submarine water often use them to see objects on the surface.

(iii) Can be used to view objects around edges of building.]

Diagram showing how a periscope works.

Mirror 1 forms image I₁

Mirror 2 views I₁ and forms image I₂

The observer sees I_2.

To draw this diagram accurately you should note the following:

1. The two mirrors must be parallel to each other.
2. A line joining I, O, I₂ is perpendicular both mirrors.
3. OI = OI₂
4. You can now use the positions of I₁ and I₂ to complete the diagram.

EXERCISE.

1. State the laws of reflection.
2. (i) Distinguish between regular and irregular reflection.

2. Fill in the table below.-

	Regular	Irregular.
Advantage
Disadvantage

3 (i) plane mirror.

Copy the diagram above and show the image.

(ii)

A girl run towards a plane mirror in the direction shown. Copy the diagram show the direction of the image.

4. A plane mirror is hanging on a vertical wall; a boy 1.6m tall wants to see both his feet and top of his head in the mirror when he is standing erect.

Assuming his eyes are 15cm below the top of his head find

1. The minimum height of the mirror.
2. Its distance from the floor.

5.

In the diagram above two pupils A and B stands behind B along a straight line joining AB to the mirror, if AB is perpendicular to the mirror, Find the distance between B and the image of A.

Answers.

1. The incident ray, reflected ray and normal to surface at point of reflection lie on the

same surface.

The angle of incidence is equal to the angle of reflection.

2. (i) Regular reflection: This is when a parallel beam is incident on a regular surface

producing a parallel-reflected beam.

Irregular reflection: This is when a parallel beam is incident on an irregular surface

producing a non-parallel reflected beam.

(ii)

	Regular	Irregular.
Advantage	Produces a bright beam of reflected light.	Enables us to recognise details on the reflecting surface.
Disadvantage	The reflected beam is very bright. It is difficult to recognise detail on the reflecting surface.	The reflected beam is dull.

3 (i)

(ii)

4.

Length of mirror required = AB

AB = 0.725 + 0.075 = 0.8m

Distance from the floor = 0.725m.

5. Diagram.

SCHEME OF WORK

School…… Class: S.1 Term……… Year…………….

Subject Physics.

Week ending on.	Periods	Topic	Sub-topic	Objectives	Content	Methodology	T/L Aids	Reference	Comment.
	2	Behaviour of light.	Reflection of light from plane surfaces.	By the end of the lesson, the learner; - States the laws of reflection. - Describes images in plane mirrors. - Draws a ray diagram to show how the eye sees the image.	Laws of reflection Regular and diffuse reflection. Properties of images. Formation of an image (How the eye sees the image).	Discussion	-Plane mirrors -Protractor - Rulers	New complete Junior Physics,by A.Atkinson and H.Sinuff . Ordinary level physics by A.F. Abbott.
	3	Behaviour of light.	Reflection of light from plane surfaces.	Experimentally - Verifies the laws of reflection. - counts the number of images in mirrors inclined to each other. - Draws a ray diagram to show how a periscope works.	Experiments - To verify the laws of reflection. - To count number of images in mirrors inclined to each other. - The kaleidoscope - The periscope	- Class experiments - Project	- Plane mirrors. - Optical - Pins - Protractors - Tin with slit. - Bulb, bulb holder. - Cells, cell holder. -Connecting wires. - Soft board.	- - do --	The theory of the Kaleidoscope and periscope should be done in class. The learners should construct them as projects outside class.

LESSON PLAN.

Subject: Physics

Topic: Behaviour of light.

Sub Topic: Reflection of light from a plane mirror.

Time: 80mins

Objectives;

By the end of the lesson the learner should be able to:

1. State the laws of reflection.
2. Describe the image formed in a plane mirror.
3. Draw a ray diagram to show how the eye sees the image.

Teaching aids: Plane mirrors, protractor Rulers.

Method: Discussion.

Reference: - New complete Junior physics by A. Atkinson and H. Sinuff.

• Ordinary level physics by A.F Abbott.

Time	Content	Teacher’s Activity	Pupils Activity
5min	Review	Teachers review lesson on propagation of light by asking questions on.	Answer questions asked.
10min	Definition of reflection. Laws of reflection properties of images.	Teacher asks leading questions to lead pupils into defining reflection and writes down correct answers on chalkboard. Teacher draws the diagram on reflection on black board And defines terminology and explains the laws of reflection. The teacher distributes some mirrors and ask pupils to state the properties of images as he writes the correct ones on black board.	Try to define reflection. Copy definition of reflection. Copy diagram in exercise books and the accompanying terminology and the laws of reflection. State the properties of images and copy them.
20min	How the eye sees the image.	Using a black board ruler and protractor accurately draws the diagram. On how the eye sees the image carefully explaining each of the steps necessary to produce an accurate diagram. Teacher goes round class supervising. Writes on blackboard accompanying notes.	Copy diagram in exercise books. Copy the notes.
5min		Summarises the lesson by asking leading questions.	Answer questions
Self evaluation: