Teachers' guide - elate by ajizai


									Unit:     5

Subject: Physics

Class:    S.1

Topic:        Behaviour of Light.

Sub-topic: Reflection of light from plane surfaces.

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;
    i.   State the laws of reflection.
   ii.   Experimentally verify the laws of reflection.
  iii.   Describe the images formed by plane mirrors.
   iv.   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;
    i.        Upright (i.e. the same way up as the object)
    ii.       Virtual   (i.e. cannot form on a screen)

   iii.      Same size as object
   iv.       Laterally inverted (i.e. the left of the object looks to be the right of the
   v.        Distance of the object from the mirror is equal to the distance of the image
             from the mirror.
   vi.       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

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

   i)      Draw a line to mark the silvered surface of the mirror on the sheet of
   ii)     Mark a point X on the line.
   iii)    Draw a normal at X.
   iv)     Using a protractor mark off an angle ray of incidence, e.g. 200 and draw
           the incident.
   v)      Stand the mirror vertically on the line representing the silvered surface.

   vi)     Tuck pins P1 and P2 in the incident ray.
           Note: P1 and P2 should stand vertically and far a part.
   vii)    Position your eye at E to view the reflections of P1 and P2.
   viii)   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 P1 ,P2 and P3 .
   ix)     Repeat procedure (ix) above with another pin P4.
   x)      Join P3 and P4. It should cross the silvered line at X.
   xi)     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

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.]

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

                   i0               r0

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

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

   (i)         Arrange two plane mirrors at right angles to each other and stand a small
               object between them.
   (ii)        Count the number of images.
   (iii)       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

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



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.

Therefore the number of images is given by:

Number of images = 3600 - 1
                       Angle between mirrors.


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
-   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)


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.
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,

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 I2.
To draw this diagram accurately you should note the following:
   (i)     The two mirrors must be parallel to each other.
   (ii)    A line joining I, O, I2 is perpendicular both mirrors.
   (iii)   OI = OI2
   (iv)    You can now use the positions of I1 and I2 to complete the diagram.

           1. State the laws of reflection.
           2. (i) Distinguish between regular and irregular reflection.
               (ii)     Fill in the table below.-

                                   Regular             Irregular.

3 (i) plane mirror.

Copy the diagram above and show the image.

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
       (i)    The minimum height of the mirror.
       (ii)   Its distance from the floor.


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.

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.

                                        Regular                          Irregular.
                 Advantage              Produces a bright beam of        Enables us to recognise
                                        reflected light.                 details on the reflecting

                 Disadvantage            The reflected beam is very   The reflected beam is
                                         bright. It is difficult to   dull.
                                         recognise detail on the
                                         reflecting surface.

3 (i)



Length of mirror required = AB
     AB = 0.725 + 0.075 = 0.8m
     Distance from the floor = 0.725m.

5. Diagram.


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