You Must Know
1. Light is a form of energy that produces the sensation of sight in our eyes.
2. A source of light can be either natural or artificial (man-made).
3. Objects can be classified as ‘transparent’, ‘translucent’ and ‘opaque’ objects.
4. Light always (ordinarily) propagates along a straight path. This property is called ‘Rectilinear Propagation of Light’.
5. Shadows are formed when the path of light gets blocked by some opaque object.
6. Solar eclipse, and Lunar eclipse, are two examples of formation of shadows in nature.
7. Pinhole camera produces an inverted and diminished image of an object.
8. When light falls on a surface, it is observed to change its direction of propagation. This phenomenon is called ‘Reflection of light!
9. When an object is placed in front of a plane mirror, the left side of the object appears as the right side of the image and vice-versa. This property is called lateral inversion.
10. The image, formed by a plane mirror, is virtual, erect, of the same size as the object and is laterally inverted.
11. There are several differences between the (i) shadow of an object (ii) its image, as formed by a plane mirror.
Something To Know
A. Fill in the blanks.
1. An object that allows light to pass through it (almost) completely, is known as a transparent object.
2. A shadow is observed, on a screen, when an opaque object comes in, between it and a source of light.
3. Shadows formed are always black irrespective of the colour of the object.
4. A Solar eclipse is a natural phenomenon that sometimes occurs on a new moon day.
5. When light is reflected from a plane mirror, it changes its direction propagation.
B. Match the following:
1. Natural sources of light (e) sun, stars and firefly.
2. CFL, LED and tube light (d) man-made sources of light
3. Rectilinear propagation of light (a) light moves along a straight path
4. Eclipse (c) shadow cast by a heavenly body
5. Interchange of the ‘left’ and ‘right’ sides in the image (b) lateral inversion
C. Tick (✔) the correct option.
1. Objects, like the sun, which emit light of their own, are called-
luminous objects
absorbers of light
non-luminous objects.
reflectors of light
Answer – luminous objects
2. An object, through which we can see partially, but not very clearly, is—
a rubber ball
a sheet of plane glass
a sheet of tracing paper
a compact disk
Answer – a sheet of tracing paper
3. The ‘something’, which can change its shape and size, and may appear and disappear anywhere/anytime, is known as a-
image
beam of light
shadow
reflector of light
Answer – shadow
4. When sun (during early morning) is behind an object, the size of its shadow in comparison to that of the object would be—
smaller
larger
equal
almost zero
Answer – larger
5. The image, formed by a pinhole camera, is-
erect and diminished
inverted and diminished.
inverted and enlarged
Answer – inverted and diminished.
D. Answers the following questions in brief.
erect and enlarged
1. Classify the materials given below as opaque, transparent or translucent.
air, smoke, fog, aluminium sheet, wooden board, cello tape, polythene sheet, lime juice, wax, rubber, tyre and milk.
Answer – Opaque: aluminium, wooden board, wax, rubber tyre and milk.
Transparent: Air, cello tape, polythene sheet.
Translucent: smoke, lime juice.
2. State the type of path that light ordinarily follows when going from one point. to another?
Answer – Straight line.
3. Do fishes cast shadows in water?
Answer – Since water is transparent, it does not act like screen. So, fishes do not cast shadows in water.
4. On a sunny day, two identical bedsheets, of red and green colour, are hanging on a clothes line. What would be the colour of the shadows of these two bed sheets?
Answer – Black.
5. State the relative positions, of the sun, moon and the earth, during a lunar eclipse.
Answer – During a lunar eclipse, the earth is between the sun and the moon.
E. Answer the following questions.
1. Cover the face of a torch with a transparent coloured paper. Send its beam through an opaque object. What do you observe?
Answer – In this case, irrespective of the colour of the light beam, the shadow is of black colour.
2. With the help of suitable diagrams, show that the size of shadow formed, depends upon the relative position of the source and the opaque object.
Answer –
3. Raghav set up the following arrangement in the science laboratory. He put (i) a steel glass (ii) a transparent glass, at the position P, one by one. Would a shadow be formed in each case? Give reason for your answer.
Answer – No shadow will be formed in case of transparent glass but a shadow will be formed in case of a steel glass. Steel glass is opaque and hence its shadow is formed.
4. With the help of a diagram, show the formation of an image by a pinhole camera.
Answer –
5. Give the meaning of the term ‘Lateral inversion. Suggest how it may be used to send a ‘coded (secret)’ message.
Answer – Interchange of the left and right sides in the image is called lateral inversion. It can be used to send secret messages by using inverted letters in words or by making signals by hands.
Value Based Question
When Sumiti got admission to her new school, her class teacher advised her to follow the example of her cousin sister, Karmatha, a senior student in the same school. The teacher told her that Karmatha had, all along, been a hard working, sincere, regular, well behaved and disciplined student. She also told her that Karmatha’s nature was unlike that of the shadow of an object. This was because the shadow often changes its size and appearance, with a change in the source of light to which it was due.
1. Which, according to you, are the two values, displayed by Karmatha, that Sumiti must follow?
Answer – Dedication to a cause and firmness in one’s resolution.
2. Does the size of the shadow, of a given object, change even when there is no change in the source of light? Give one example in support of your answer.
Answer – When there is no change in the source of light, the size of the shadow does not change. This can be seen by the size of the shadow made by a candle which is at a fixed position.
3. Try to add as many entries, as possible, to the table given below.
Answer –
| S.No. | Name of the entry’ | Its Cause | Does it change or not? |
| 1. | Shadow of an object. | Source of light | It often changes its size. |
| 2. | Direction of a freely suspended magnet | Earth’s magnetism | Does not change |
| 3. | Shape of water | Shape of container | It often changes |
| 4. | Shape of a solid | Shape of container | It does not change |
Something To Do
1. On a sunny day, go out on to the playground and set up a metre scale (metallic, or wooden) vertically. At different times of the day, go out and measure the length of shadow of the metre scale with another meter scale. Make a bar chart showing the length of the shadow at different times. Can it work as a sundial?
Try visiting a place where there is a sundial.
Answer – Do it yourself. But note –
Shadow is longer during morning and evening and smaller during noon.
2. Make shadow puppets. Use your puppets and your imagination, to set up a short play that makes use of the characteristics of light and shadows. If possible, stage the play.
Answer – Do it yourself.
3. Transparent objects can also cause shadows. Think it over. Identify such cases, and the conditions, for shadow formation.
Answer – To understand this, take a glass sheet and keep it in sun. Try to locate the faint shadow formed by it.
4. There is an interesting ‘Pinhole camera’ in nature. Try finding out its details.
Answer – The canopy of a tree works like a pinhole camera. Try to view the sun through a thick foliage of tree.