LIGHT, SHADOWS AND REFLECTIONS
                        Light, Shadows and Reflections

1.        Source of light : An object which emits light, is called a source of light. For Example : Sun, torch etc.

2.       
Luminous objects :These are the objects which emit their own light.

3.       
Non-luminous objects :These are the objects which do not emit light of their own. Such a body becomes visible when light falls on it. For example, the moon, the planets etc.

4.       
Pinhole camera :It is a device which forms a photograph-like image of a bright object on a screen.

5.       
Transparent objects :Those objects which allow all the light to pass through them are called transparent objects.

6.       
Opaque objects :If an object completely stops all the light falling on it, it is an opaque object.

7.       
Translucent objects :Some objects allow only a part of light falling on them to pass through, not all of it, such objects are called translucent objects. For example : a single thin sheet of paper.

8.       
Ray of light : A straight thin beam of light from a source to an object is called a ray of light.

9.       
Obstacle : An object which comes to the path of the light is called an obstacle.

10.    
Shadow :Opaque objects do not allow light to pass through them and cast dark patches behind them. These dark patches are called shadows.

 

11.     Formation of a shadow
(i)            All the opaque objects seem to form a dark shadow of their own.
(ii)          We need a source of light, an opaque object in the way, and a screen to see a shadow.
(iii)         Screen. This is a screen on which the shadow is formed. It may be a butter paper or simply ground.
(iv)         Shadow give us some information about shapes of objects.
(v)          The colour of the opaque object does not affect the colour of the shadow.
(vi)         All the space behind the opaque object, into some distance behind it seems to be filled with the shadow.


12.    
Image formed by a pinhole camera
(i)            Images formed by a pinhole camera are upside down. Here is a picture showing the ray paths of light, from an object far away to a pinhole, and then to screen.

 
 
(ii)          When sunlight passes through the leaves of a tree, the gaps between the leaves act as the pinholes. These natural pinholes cast nice round images of the sun.

13.    
Rectilinear propagation : Light travels in a straight line. It is called rectilinear propagation of light.

14.    
Mirror : A smooth shining surface, which rebounds the light back in same or in different direction is called a mirror.

15.    
Reflection of light :When a ray of light falls on a smooth and polished surface, whole of light is return back in the same medium. It is called reflection.
 

16.    
Image formation by a plane mirror :We are able to see images through a mirror. Image formed by a mirror (flat) has following features :
(i)            Reflected image retains the colour of the object.
(ii)          Image is erected but laterally inverted.

17.    
Lateral inversion : Right side of the object appears as left side in the image formed by a plane mirror. For example, if we show our right hand, image in the mirror will show its left hand.

 

18.    
In a mirror, if you see another person, surely the other person can also see you in that mirror.
 




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


 
OWSAKEOPAQUEOBJECTSMSHAD


Ans.
 

OPAQUEOBJECTSMAKESHADOWS

 
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.

(a)  Transparent objects : Air, water, a sheet of plane glass.
(b)  Translucent objects :A sheet of polythene, smoke, fog, a sheet of cellophane, a wire mesh.
(c)  Opaque objects :A piece of rock, a sheet of aluminium, a mirror, a wooden board, a CD, an umbrella, a wall, a sheet of carbon paper, a sheet of cardboard.
(d)  Luminous objects : A piece of red hot iron, a lighted fluorescent tube, the flame of a gas burner, a lighted torch, kerosene stove, sun, firefly.
(e)  Non-luminous objects : Include all (a) Transparent objects (b) Translucent objects (c) opaque objects and moon.

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 another way?


Ans.
Yes, there are many things which give a circular shadow if held in one way and a rectangular shadow if held in another way. For example :  a cylinder, a circular disc, etc.


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.
No. In a completely dark room no image will be formed, because image by a mirror is formed due to reflection of light from the object and since there will be no light in a completely dark room, no image will be formed.
 

ADDITIONAL IMPORTANT QUESTIONS

Q.1. Name sources of light that are not hot.


Ans.
Tubelight, jugnu.

Q.2. List four natural sources of light.


Ans.
(i) Sun,                          (ii) Stars
        (iii) Fire                            (iv) Jugnu (Firefly)

Q.3. Write down four man-made sources of light :


Ans.
(i) Candle                        (ii) Oil lamp
         (iii) Electric bulb              (iv) Torch

Q.4. Does the fire emit light?


Ans.
Yes, fire emits light.

Q.5. Define luminous body.


Ans.
The objects which emit light of their own are called luminous bodies e.g., the sun, the stars etc.

Q.6. What are translucent objects?


Ans.
Those objects which allow only a small part of the light rays to pass through them are called translucent objects. These are the objects through which one cannot see properly.

Q.7. What are transparent objects?


Ans.
Those substances which allow light rays to pass through them completely are called transparent objects. One can see clearly through these objects.

Q.8. Define opaque objects with example.


Ans.
Those materials which do not allow light to pass through them, are called opaque. Example : wood, stone etc.

Q.9. Classify the following into transparent, opaque and translucent objects :

        Wax, spectacles, a heap of salt, a stone, dense smoke, wood, skin, balloon rubber,
        membrane of a table, blood and milk.

Ans. Transparent objects are :
Spectacles, and membrane of table.

Opaque object is :
A stone, wood, a heap of salt, dense smoke.

Translucent objects are :
Blood, milk, wax, skin, balloon, rubber.

Q.10. Close your eyes while looking toward bright sunshine. Now cover the eyes with your palm still keeping them closed. Do you notice any difference? On the basis of this experience, state whether your eyelids are transparent, translucent or opaque.


Ans.
On the basis of the experience we observe our eyelids are opaque.

Q.11. Sometimes, you are able to see the Sun or the Moon behind the clouds. What can you say about the ability of such clouds to transmit light?


Ans.
We may say that cloud behaves as transparent object.

Q.12. How will you convert a transparent glass sheet into a translucent sheet? Suggest any two ways to do it.


Ans.
(i)  By smearing a thin layer of oil we may convert a transparent glass sheet into a translucent sheet.
(ii) By covering one side by butter paper.

Q.13. Define reflection of light.


Ans.
When a beam of light strikes with a smooth surface, the whole light is return back into the same medium, it is called reflection of light.

Q.14. Define Pinhole camera.


Ans.
It is a device which forms a photograph, like image of a bright object on a screen.

Q.15. Define screen.


Ans.
Screen is a butter paper on which images are formed.

Q.16. Give five examples of luminous objects.


Ans.
Sun, stars, electric bulb, burning candle, firefly etc.

Q.17. On what factor does the proportion of light that enter an object depend?


Ans.
Optical nature of the object.

Q.18. How are moon and planets visible to us through they are not luminous?


Ans.
Because they reflect light from the sun.

Q.19. What is an obstacle?


Ans.
An object which comes to the path of light is called an obstacle.

Q.20. Give examples of two substances through which light does not pass?


Ans.
Wood and bricks are substances through which light does not pass.

Q.21. Whether a shadow is two dimensional or three dimensional?


Ans.
Shadow is not just the two dimenstional outline that you see on the ground. All the space behind the opaque object up to some distance behind it, seems to be filled with the shadow. (See Fig. 11.2).

Q.22. What we need in order to see a shadow?


Ans.
We need (i) a source of light,
         (ii) an opaque object in the way of light and
         (iii) a screen

Q.23. Define Lateral Inversion.


Ans.
Phenomenon of changing left to right and right to left by the mirror, while forming images is called lateral inversion.

Q.24. What happens when light falls on a mirror?


Ans.
Mirror is silvered on one side, so it does not allow the light to pass through it. It reflects almost whole of the light falling on it.

Q.25. Define ‘Mirror’.


Ans.
A smooth shining surface, which rebounds the light back in same or in different direction is called a mirror.

Q.26. Why is silver glass used as a mirror?


Ans.
The silvered glass has a smooth surface and the smoothness helps in forming clear image. Silvering makes it shiny and the shiny surface helps in reducing the absorption.

Q.27. Imagine a situation where air is opaque. Write down what would happen in your notebook.


Ans.
If air were opaque, then we could not see the things in the room or even very close to us.

Q.28. While standing before a plane mirror, if you move your  right hand, which hand does your image move?


Ans.
If we move our right hand, our image will move left hand. It is because in a plane mirror our “left appears right” and “right appears left”. This is called lateral inversion. Hence we can say that the plane mirror forms laterally inverted images.

Q.29. Can you think of a situation where we can see the path of the light?


Ans.
In dark room we can see with torch light which goes straight. Similarly, dust particles become visible when light enters the room through fine hole.
All these examples indicate that light travels in straight line.

Q.30. Give two examples of substances through which light passes easily.


Ans.
Glass, water, etc. are substances through which light passes. The substances through which light can pass are called transparent substances.

Q.31. Under what special conditions, light does not travel in a straight line?


Ans.
If the size of the object in the path of light is very small (0.0001 cm/less) the light bends at the edge of the object. Thus in such case, light does not travel in
straight line.

Q.32. Does the direction of shadow change during the day? Does the length of shadow also change from season to season?


Ans.
Yes, the direction of shadow changes as the sun changes its position during the day. The length of the shadow also changes from season to season.

Q.33. What change in the path of light takes place when light falls on a shiny surface? What is this called?


Ans.
The light comes back in the same plane, when light falls on a shiny surface.This is known as reflection of light.

Q.34. How many reflected rays can there be for a given single incident ray falling on a plane mirror?

Ans.
For one incident ray, there is only one reflected ray.

Q.35. If we are sitting in a room, we cannot see people sitting in the next room. Why is this so?


Ans.
If we are sitting in a room, we cannot see people sitting in the next room because of opaque wall made of bricks between the two rooms.

Q.36. Is the glass used in spectacles transparent or opaque? What would happen if it were not so?


Ans.
The glass used in spectacles in transparent. If it were not transparent, then we would not be able to see through spectacles.

Q.37. Why do objects in a room become visible even if sunlight does not enter it?


Ans.
The objects in a room become visible even if sunlight does not enter it, due to the scattering of light. The air around the objects allows this scattered light to
pass through it and we can see the objects.

Q.38. What is reflection?


Ans.
When a ray of light falls on a smooth and shiny surface, the whole of light is sent back in the same medium. It is called reflection. Mirrors do not allow even a small amount of medium light to pass through them. Mirrors show regular and complete reflection.

Q.39. How does light travel from one point to the other?


Ans.
Light from a source of light spreads in all directions and reaches from one point to another point.

Q.40. Why is the image formed in a pinhole camera inverted?


Ans.
In a pinhole camera image formed is inverted because the object is between radius of curvature and focus.

Q.41. State any one difference between stansparent and translucent materials.


Ans.
In translucent materials only a small part of the light rays may be transmitted. Whereas in transparent materials light is transmitted almost completely.

Q.42. What is scattering?


Ans.
When light falls on a rough and average shining surface, it is turned back in different directions. It is called scattering. The light rays are sent back in different directions due to uneven surface.

Q.43. What does an object do the light that fall on it?


Ans.
It may allow the light to pass through as a glass sheet does. It may completely cur off the light from passing through by absorbing light, as the opaque bodies do. The object may scatter the light incident on it.

Q.44. Why do we need a shiny surface for reflection?


Ans.
The extent of reflection depends upon the shine and smoothness of the surface. More is the shine and smoothness of the surface, more will be the reflection. That is why, mirrors reflect most of the light falling on it. Hence, for reflection, shiny surfaces are required.

Q.45. What are the characteristics of image formed by plane mirror?


Ans.
Characteristics of images formed by plane mirror are :
(i)            Plane mirror forms erect image.
(ii)          Image formed is of the same size as the object.
(iii)         The distance of image from the mirror is equal to the distance of object from the mirror.
(iv)         It is virtual, it can’t be obtained on a screen.
(v)          Image is laterally inverted.

Q.46. What is shadow? How is it formed? How does the colour of an opaque object affects the colour of the shadow?


Ans.
A shadow is a dark outline or image cast by an opaque object that blocks light coming from a source of light. It is formed when light hits the opaque object which does not let the light pass through. Everywhere else around the opaque object, the light continues in a straight path until it bounces off the ground or wall behind the object. The wall or ground behind the Opaque object is the screen. On this screen is a dark patch, or shadow, with the same outline as the object surrounded by light. The colour of the opaque object does  not affect the colour of the shadow that we see.

Q.47. Give the set up of an experiment to show the rectilinear propagation of light.


Ans. We need :
Three pieces of cardboard of the same shape and size, a thick nail, a candle, matches, a knitting needle or a similar long thin stick.
Take the three pieces of cardboard. Place them one on top of another and make a hole in the middle of all three using a thick nail.
Stand the three cards up on the table, each a short distance away from the next. You can keep them upright by placing them between books, notebooks, blocks of wood, etc.

Take a candle which is of the same height as the holes in the cards. Light the candle and place it in front of the cards .
Adjust the cards until you can see the flame of the candle from the other side.

We see that the light of candle is visible only when the holes on cards lie in a straight line and if we disturb any of card then the light of candle disappears.

Thus on this observation we can say that “light propagates in a straight line.”

Q.48. What are the differences between a shadow of a flower and its reflected image?


Ans.

ShadowImage
 
1.    A shadow gives only a rough idea of the shape of a flower.
 
2.    It has no colours.
 
3.    Formed by the absence of light due to blockage by an opaque object.
1.    Image gives exact shape.
 
 
2.    It tells about the colour of flower.
 
3.    Formed due to reflection of light.
 
Q.49. Here capital letters of English Alphabet are given. Encircle the letters which will not show lateral inversion on facing a plane mirror.
                    A       B        C        D         E          F          G          H           I            J
                   K        M        N       O         P          Q          R         S            T           U
                   V        W        X       Y         Z
Ans.        A        B      C        D         E          F          G          H            I            J
                K        M        N       O         P          Q          R         S            T           U
                V        W        X       Y         Z
 
 

OBJECTIVE TYPE QUESTIONS

Q.1. Match the following items given in Column A with that in Column B :

 
 
Column AColumn B
 
(a)  Transparent
 
(b)  Opaque
 
(c)  Translucent
 
(d)  Luminous body
 
(e)  Shadow
 
(f)   Image
 
(g)  Reflection
 
(h)  Lateral Inversion
(i)    Region of absence of light.
 
(ii)  Scattering back of the light by shining surface.
 
(iii) Object through which one can see clearly.
 
(iv) Object through which one can not see at all.
 
(v)  Formed due to reflection by mirrors.
 
(vi) Object through which we cannot see clearly
 
(vii) Phenomenon of changing left to right.
 
(viii) Produces light of its own.
 
 
Q.2. Fill in the blanks with appropriate words :
(i)            An object or material could be opaque, transparent, translucent or ………. .
(ii)          This is a …………… on which the shadow is formed.
(iii)         A ……………. Is a dark outline of an opaque object that blocks light coming from a source.
(iv)         Image formed by a pinhole camera is ……….. .
(v)          Image formed by a plane mirror is ………… .
(vi)         In a plane mirror our left hand looks like right hand. This phenomenon is called …………… .
(vii)       …………….. objects cast no shadow.
(viii)      Opaque objects cast …………. Shadows in morning and evening while …………. shadow at noon.
(ix)          Light travels in a ……….. .
(x)          Shadow is ……………. .


Q.3. State whether the statements given below are True or False :

(i)          A torch bulb is a luminous object.
(ii)        Light travels in a straight line.
(iii)       Image formed by a plane mirror is inverted.
(iv)       Light gets reflected when strikes a shiny surface.
(v)        Transport substances reflect all the light falling on them.
(vi)       All the shining bodies in sky have their own light.
(vii)     Jugnu (Firefly) is a living luminous body.
(viii)    When screen is far away from the objects the shadow will have no penumbra.
(ix)       A reflected image gives us more information about the object than a shadow.
(x)        In a mirror, if A is able to see B, B also see A.
(xi)       Objects around us seem to have different ways in which they interact with light.

Q.4. Choose the correct option in the following questions :


(i)         
Light travels in

(a)  Straight line                                (b) curved line
(c) zig zag line                                     (d) randomly

(ii)       
When an opaque object comes in the path of light it forms
(a)  An image with colours                 (b) shadow
(c) black and white image                 (d) depends on the colour of the light

(iii)      
Which type of objects allow light to pass through them?
(a)  Translucent                                    (b) Opaque
(c) Transparent                                    (d) Penumbra

(iv)      
Which is an example of a translucent object?
(a)  A thin sheet of paper                    (b) A thin glass slab
(c) A thin iron sheet                            (d) All of these

(v)     
Bouncing back of light from shining surfaces is called
(a)  Reflection                                        (b) Refraction
(c) Bending                                            (d) Dispersion

(vi)    
What is lateral inversion?
(a)  Image becomes inverted
(b)  Image bends laterall
(c)  Right of the object appears left of the image
(d)  All of these happen

(vii)  
Which letter of english alphabet will not show lateral inversion?
(a)  I, O, U                                                (b) N, Z, X
(c) I, X, E                                                 (d) A, E, I

(viii) 
Which is a device to image the sun?
(a)  Plane mirror                                     (b) Pinhole camera
(c) A straight pipe                                 (d) Glass slab

(ix)    
Which of the following is a cold source of light?
(a)  Firefly                                               (b) Tube light
(c) The sun                                            (d) Electric bulb

(x)   
Out of these which one is not a man made luminous body.
(a)  Electric bulb                                      (b) Burning candle
(c) Firefly                                                 (d) Oil lamp
 

ANSWERS TO OBJECTIVE TYPE QUESTIONS
Q.1.Match the items in Column A with Column B :
Column AColumn B
 
(a)  Transparent
 
(b)  Opaque
 
(c)  Translucent
 
(d)  Luminous body
 
(e)  Shadow
 
(f)   Image
 
(g)  Reflection
 
(h)  Lateral Inversion
(iii) Object through which one can see clearly
 
(iv) Object through which one can not see at all.
 
(vi) Object through which we cannot see clearly
 
(viii) Produces light of its own.
 
(i)     Region of absence of light.
 
(v) Formed due to reflection by mirrors.
 
(ii) Scattering back of the light by shining surface.
 
(vii) Phenomenon of changing left to right.
 
 
Q.2. Fill in the blanks :
        (i) luminous                           (ii) screen                        (iii) shadow
        (iv) upside down                   (v) erect                           (vi) lateral inversion
        (vii) transparent                    (viii) big, small                  (ix) straight line
         (x) Three dimensional
Q.3. True/ False :
         (i) True             (ii) True            (iii) False            (iv) True           (v) False
         (vi) False          (vii) True          (viii) False          (ix) True           (x) True
         (xi) True.
Q.4. Choose the correct option :
   (i)      (a) A shadow is formed when an opaque object comes in the way of light.
   (ii)     (b) A shadow is formed when an opaque object comes in the way of light.
(iii)       (c) Opaque objects do not allow the light to pass through them while translucent object allow only a part of light to pass through them.
(iv)        (a) Glass slab is transparent and iron sheet is opaque.
(v)        (a) When a ray of light fall on a smooth and polished surface, whole of light is return back in the same medium, It is called reflection of light.
(vi)       (c) Right side of the object appears as left side in the image formed by a plane mirror.
(vii)      (a) Z and  E will show lateral inversion.
(viii)    (b) Sun can be imaged by using a pinhole camera.
(ix)       (a) Firefly (Jugnu) emit short bursts of light due to photochemical reaction, which take place in its body temperature.
(x)        (c) Firefly (Jugnu) is a natural luminous body.