LIGHT, SHADOWS AND REFLECTIONS

  LIGHT, SHADOWS AND REFLECTIONS

              Chapter 11 Class VI (C.B.S.E./NCERT)              

 

Key Learning Points-Introduction, Sources of light, Luminous and Non-luminous objects, Transparent, Translucent and Opaque Objects, Straight pathway of Light, Shadow, Differences between shadow and image, A Pinhole Camera, Mirror and Reflection of Light

Learning Objectives- At the end of this lesson the students will be able to-

1. To understand about luminous and non-luminous objects

2. To know about transparent, translucent and opaque objects

3. To understand shadow formation conditions,

4. To differentiate between shadow and image

5. To have skills to prepare pinhole camera and use a pinhole camera

6. To understand straight pathway of light

7. To know refection of light and re-reflection by mirrors.

Introduction- Light plays a very crucial role in our daily life. Light is a form of energy that gives sensation of vision to our eyes. Except the time of sleeping we require light to do almost every work. Light is produced by light emitting sources called luminous objects. Light can completely pass through transparent objects, partially pass through translucent objects and do not pass through opaque objects. Shadow is formed on screen by opaque objects when light falls on it. Shadows are dark coloured, smaller or larger than the object. In a plane mirror, image of an object is laterally inverted, same colour, size and formed in front of object in mirror. A Pinhole camera is used to indirectly see the bright objects like the Sun. Light always travels in straight pathway. Mirrors reflect the light falling on them.      

Sources of Light- The light is produced by natural as well as artificial (man-made) objects. e.g.- The Sun, The Stars, Fireflies or glow-worms (or Jugnu) are some of natural sources of light. e. g.-The glowing electric bulb, the burning candles, burning matchsticks are examples of artificial sources of light.

Luminous and Non-luminous objects-The objects which give out light of their own are called luminous objects. e.g.- The Sun, The Stars, Fireflies or glow-worms, The glowing electric bulb, the burning candles, burning matchsticks etc. The brightness of a luminous object decreases as we move away from it.

                                                            

The objects which do not produce light of their own are called non-luminous objects. e.g.- The planets, moon, books, bench, desk, etc.

Transparent, Translucent and Opaque Objects- Those objects through which light can pass completely are called transparent objects. e.g.- Air, Clean Water, Clear glass, spectacles etc.

Those objects through which light can partially pass are called translucent objects. e.g.- Muddy water, smoky glass, oiled paper, milk etc.

Those objects through which light cannot pass are called opaque objects. e.g.- Stone, book, Wooden furniture, Rubber ball etc.   

Straight pathway of Light- The light travels always in linear or rectilinear or straight pathway. e. g. The light coming out of a torch, motor car, The Sun etc. The Sunlight propagates from the Sun to the Earth. But, its direction changes due to air. The direction of light hardly changes in air for short distances. 

Shadow-The dark coloured patch formed on the screen opposite to the light by an opaque object is called shadow. For the formation of a shadow, a source of light, an opaque object and a screen are required. Shadow is formed by an opaque object when light falls on it. Its size and shape change according to direction of light or orientation of object. It can be large or small than the object. During morning and evening shadow is larger. It is smaller during noon. The colour of shadow is dark irrespective of the colour of object. Shadows are not formed in dark i.e. in absence of light. The shadows are not uniformly dark. Central inner part is darker than outer surrounding part. The darker part of shadow is called umbra and less dark part is called penumbra. There is no change of left part of object to right part of image and right part of object to left part of image.

In nature, solar eclipse and lunar eclipse are examples of shadow formation.

A Pinhole Camera- It is a simple device to indirectly see the bright objects. It works on the principle of straight pathway of light. The sunlight passing through the leaves of a large tree produces numerous circular images of the sun on the ground. It is due to natural pinhole camera formed by the gaps of leaves.

Construction- The requirements for constructing a pinhole camera are- Two rectangular boxes of wood or cardboard, one lager than the other box, butter paper, glue or cello-tape, needle and black colour. The small box was painted black from outside. A rectangular cut was made in the center of one of the side of small box. Over this cut, a piece of butter paper was pasted covering the rectangular hole. The opposite side was used for viewing the image. This smaller box was inserted in a slightly larger box having a tiny hole (pinhole) facing the butter paper of smaller box. This is a pin-hole camera.

Working- The pin-hole camera was used to view a burning candle. An inverted (upside down) image of burning candle was observed on the butter paper. It is based on straight pathway of light.

Precautions- The pinhole made in the lager box should not be wide otherwise clear image of object will not be formed. Do not look the very bright light sources like the Sun for longer time.  

        Mirror- Mirrors are reflective pieces of glass one side of which is silvered.

                     Mirrors can be – Plane mirrors and spherical mirrors

Reflection of Light- The change in direction of light due to any reflective surface like mirror is called refection. The reflected light can be re-reflected by keeping again and again by keeping mirrors the way.

There are two types of reflection of light- i. Regular reflection

                                                                    ii. Irregular or diffused reflection

Activity 1 - Classification of objects into transparent, translucent and opaque.

Requirements- Pencil, Rubber ball, Sheet of writing paper, textbook, plastic ruler, piece of wood, power glasses and torch.

Theory-Light can completely pass through transparent objects, partially pass through translucent objects and do not pass through opaque objects.

Procedure-

Objects	Light passes completely (Transparent)	Light passes partially (Translucent)	Light do not passes (Opaque)
Pencil			Opaque
Rubber ball			Opaque
Writing Paper		Translucent
Textbook			Opaque
Plastic Ruler		Translucent
Clear Water	Transparent
Power Glass	Transparent

Conclusion- On the basis of allowing light to pass or not, the objects are classified into transparent, translucent and opaque.

Activity 2-To demonstrate straight pathway of light

Requirements- Candle, match box, hollow pipe, bended hollow pipe or pipe prepared of paper

Theory- The light travels (or propagates) in a straight line. One can see the light of burning candle by hollow pipe but not by bended pipe.

                                                           

             Figure showing: observations of light through hollow and bended pipes.

Activity 3-Shadow formation by an opaque object

Requirements- A bright source of light like torch, an opaque object like a book or toy, a white cardboard sheet or wall.

Theory- The shadow is dark coloured optical image formed on the screen by an opaque object on opposite side of the source of light. For the formation of a shadow, a source of light, an opaque object and a screen are required. Shadow cannot be seen in dark i.e. in absence of light. Inner part of shadow is comparatively darker than outer fainted part. They are called umbra and penumbra respectively.

Procedure- When torch light falls on the book, the dark coloured shadow of it is formed on the screen like white sheet or wall. Its size is larger or smaller. Note the shape by orienting the rectangular textbook in different directions.

CLASS TEST 1    Time=30 Minutes   Marks= 4 x 5 =20

Q.1 Explain transparent, translucent and opaque objects with a suitable example of each.

Q.2 Differentiate between luminous and non-luminous objects.

Q.3 Prove by an activity that light travels in a straight pathway.

Q.4 What is shadow? What are necessary conditions for formation of a shadow?

CLASS TEST 2    Time=30 Minutes   Marks= 4 x 5 =20

Q.1 What is a pinhole camera? What is it’s use?

Q.2 What is reflection of light? How reflected light can be re-reflected?

Q.3 When our shadow is largest and smallest during the day.

Q.4 Write any three differences between an image and a shadow.

GROUP PROJECTS

1. Prepare a periscope and write its construction and working.

2. Prepare a Kaleidoscope and write its construction and working.

3. Prepare a Pinhole camera and write its construction and working.

SOLVED EXERCISE QUESTIONS

        

Q.1 Rearrange the boxes given below to make a sentence that helps us understand opaque objects.

OWS  AKE  OPAQ  UEO  BJEC  TSM  SHAD

Ans- OPAQ UEO BJEC TSM AKE SHAD OW

Q.2 Classify the objects or materials given below as opaque, transparent or translucent and luminous or non-luminous:

Air, water, a piece of rock, a sheet of aluminium, a mirror, a wooden board, a sheet of polythene, a CD, smoke, a sheet of plane glass, fog, a piece of red hot iron, an umbrella, a lighted fluorescent tube, a wall, a sheet of carbon paper, the flame of a gas burner, a sheet of cardboard, a lighted torch, a sheet of cellophane, a wire mesh, kerosene stove, sun, firefly, moon. 

Ans-

Object	Opaque/transparent/Translucent	Luminous/Non-luminous
Air	Transparent	Non-luminous
Water	Transparent	Non-luminous
A piece of rock	Opaque	Non-luminous
A sheet of aluminium	Opaque	Non-luminous
A mirror	Opaque	Non-luminous
A wooden board	Opaque	Non-luminous
A sheet of polythene	Translucent	Non-luminous
A CD	Opaque	Non-luminous
Smoke	Translucent	Non-luminous
Fog	Translucent	Non-luminous
A piece of red hot iron	Opaque	Luminous
An umbrella	Opaque	Non-luminous
A lighted fluorescent tube	Opaque	Luminous
A wall	Opaque	Non-luminous
A sheet of carbon paper	Opaque	Non-luminous
The flame of a gas burner	Translucent	Luminous
A sheet of cardboard	Opaque	Non-luminous
A lighted torch	Opaque	Luminous
A sheet of cellophane	Translucent	Non-luminous
A wire mesh	Translucent	Non-luminous
Kerosene stove	Opaque	Luminous
Sun	Opaque	Luminous
Firefly	Opaque	Luminous
Moon	Opaque	Non-luminous

Q.3 Can you think of creating a shape that would give a circular shadow if held in one way and a rectangular shadow if held in other way?

Ans- A cylinder will produce a circular as well as rectangular shadow if its position is changed against light. When light falls on it from the top, circular shadow is formed on the screen. But, when light falls on it from the side, rectangular shadow is formed on the screen.                                          

 

 

 

              

Q.4 In a completely dark room, if you hold up a mirror in front of you, will you see a reflection of yourself in the mirror?

Ans- In a dark room I will not be able to see my reflection i.e. image in the mirror. It is because light is essential for formation of an image in the mirror.

 Multiple Choice Questions

1.  In which of the following the speed of light is fastest?

                 (a) Air           (b) Vaccum       (c) Glass           (d) Water

             2. The object which does not emit light is called-

    (a) Opaque object                  (b) Translucent object   

    (c) Luminous object               (d) Non-luminous object

3. Which of the following is a luminous object?

                 (a) The Earth     (b) The Moon     (c) Burning candle    (d) Science Textbook

4. The Moon and all planets of Solar system are –

                 (a) Transparent      (b) Opaque      (c) Luminous       (d) Non-luminous

            5. Which of the following is necessary for formation of shadow?

                (a) A source of light     (b) An opaque object      (c) A Screen     (d) All of these

            6. Which of the following will not form a rectangular shadow?

                (a) A ball     (b) A ruler scale    (c) A circular steel tumbler   (d) An ice-cream cone  

             7. What is essential for formation of a shadow?

                (a) Opaque object         (b) Transparent object      (c) Air      (d) Mirror

             8. Bujhoo and Paheli observed the shadow of a tree at 9:00 A.M., 12:00 P.M. and   3:00 P.M. Which of the following statement is correct about their observation about the shape and size of the shadow of tree?

     (a) The shape changes but size remains same 

     (b)The size changes but the shape remains same   

     (c) Both shape and size change

     (d) Neither shape nor size change

  9.  We see any object because of…………

                  (a) Air                     (b) water                  (c) Light                       (d) Soil

            10. Which is not essential in formation of a shadow?

                   (a) A transparent glass                 (b) An opaque object   

                   (c) A Screen                                     (d) A source of light

            11. Which of the following is a/are luminous object?

                   (a) A glow worm       (b) The Sun       (c) A glowing torch     (d) All of these

              12. What is / are essential for formation of shadow?

                    (a) Screen         (b) Light               (c) Opaque object             (d) All of these

              13. Periscope is based on which principal of light-

                    (a) Refraction of light                            (b) Reflection of light 

                    (c) Diffraction of light                            (d) Interference of light

             14. Beautiful patterns are formed in an optical device because of

                    refection and re-reflection. This device is-                    

           

                    (a)  Periscope     (b) Spectroscope       (c) Kaleidoscope      (d) Telescope

              15. Pin-hole camera has –

                     (a) Very small hole                                   (b) Translucent screen   

                     (c) Opening for eyes                                 (d) All of these

                             VERY SHORT ANSWER TYPE QUESTIONS

              1. Give two examples each of natural and artificial sources of light.

              2. Give two examples each of luminous and non-luminous objects.

              3. What is the use of pinhole camera?

              4. What is shadow?

              5. What is the colour of shadow?

 

                SHORT ANSWER TYPE QUESTIONS

               1. Define luminous and non-luminous objects.

               2. When shadow of a tree is larger and smaller than the tree?

               3. What is mirror? Write the two characteristics of images formed by a mirror?

               4. Why the direction of light changes when light falls on plane mirror?

               5. The leaves of crown of a tree form numerous circular patches on ground. Why?

                LONG ANSWER TYPE QUESTIONS

       1. What is shadow? What are necessary conditions for formation of a shadow?

       2. Explain the working of Pinhole camera with a simple diagram of it.

       3. Sort out the objects given below into-

                          (a) Natural sources and artificial sources of light

                          (b) Luminous and Non-luminous objects

          The Earth, Apple, Glowing bulb, The Sun, School bag, Book, Football, Torch,   Fireflies, Lighter, Mobile Phone, Ink Pen, and Crayons

 

PISA TYPE/ HOTS TYPE QUESTIONS-    Time = 15 minutes   Total marks =1 x 5=5

 

 

                           Fig.1                                                             Fig.2

Look at the two above figures. They are reflections from the reflective surfaces. Black coloured rays show the incident rays while the blue coloured rays show the reflected rays. The reflection of light on the plane (or smooth or regular or even) reflective surface follows the laws of reflection of light but this law fails on the diffused (or irregular surface or rough or uneven) surface. Based on this, answer the following questions-

(a) Figure 1 shows which type of reflection of light?

(b) Figure 1 shows which type of reflection of light?

(c) In which figure the laws of reflection of light fail?

(d) In which figure the laws of reflection of light are followed?

(e) In nature generally which refection of light from objects can be correlated with which figure?