Class 6 Science Chapter 8 Important Questions

The non-luminous objects can be seen only when light coming from a luminous object (like the sun, an electric bulb or tube-light or torch, candle etc.) falls on them. This light is reflected by non-luminous object in all directions. When this reflected light enters our eyes, we can see the non-luminous objects.

The moon is a non-luminous object which does not have its own light. But the sun is a luminous source of light. Actually, we can see the moon because it reflects sunlight into our eyes. In generally, when sunlight falls on the surface of moon, then some of the sunlight is reflected by the moon towards the earth. And to us it appears as if the light is being given out by the moon itself. But in reality, moonlight is the sunlight which is reflected from the surface of moon. Thus, moon behaves as a cold, non-luminous mirror. It is a reflector of sunlight.

All the objects outside the room (direct sunlight) scatter the sun light falling on them in all the directions. Even the air scatters some sunlight. It is this scattered sunlight which enters the room and helps us to see the things lying in the room which are not in direct sunlight. In the other words, when the reflected light coming from the table or other object, the light enters our eyes, then we are able to see the table or certain object.

Those materials which allow all the light to pass through them are called transparent materials. We can see clearly through the transparent materials. The materials like glass, polythene, water, air, groundnut oil are transparent materials. The transparent objects allow all the light to pass through them because they do not absorb or reflect any light. If we are able to see through an object clearly it is allowing all the light to pass through it and hence it is a transparent object.

Those materials which allow only some of the light to pass through them are called translucent materials. We cannot see clearly through a translucent material. The materials like butter paper, oiled paper, tissue paper, ground glass or frosted glass, sheet of white cotton, muddy water and clouds are transparent materials. The translucent objects absorb or reflect a good part of the light falling on them and allow only some of the light to pass through them.
If we can see through an object but not clearly, it is allowing the light to pass through it only partially and hence it is a translucent object.

Those materials which do not allow any light to pass through them are called opaque materials. We cannot see anything through an opaque object. The materials like cardboard, door, window, brick, wall, stone, book, wood, metal sheet etc., all are opaque materials. The opaque objects do not allow light to pass through them because they absorb or reflect all the light falling on them.
If we cannot see through an object at all, it is not allowing any light to pass through it and hence it is an opaque object.

When an object is placed in front of a source of light, it produces a shade or dark area behind it or opposite side. The “shade” cast by an object is called its “shadow”. Shadows are formed when light is stopped by an object. An opaque object stops the light completely, so and opaque object casts a dark shadow behind it. A translucent object stops the light partially, so a translucent object casts of week shadow or less dark shadow. A transparent object does not stop any light from passing through it, so a transparent object does not cast any shadow behind it.

The shadow of an object has the following characteristics:
i)The shadow of an object is erect that means same side up as the object.
ii)The shadow of an object is real that means it can be formed on a screen.
iii) Irrespective of the colour of the object, the shadow is always black.
iv) The shadow can be smaller than the object, equal to the object or bigger than the object.

In the evening, go out in an open ground with a few friends. Take a torch and a large sheet of cardboard with you. Hold the torch close to the ground and shine it upwards so that its light falls on your friend’s face. You now have a source of light that is falling on an opaque object. If there were no trees, building or any other object behind your friend. This does not mean that there is no shadow. After all, the light from the torch is not able to pass through his body to the other side. Now, ask another friend to hold the cardboard sheet behind your friend. Thus, the shadow can be seen only on a screen. The ground, walls of a room, a building, or other such surfaces act as a screen for the shadows you observe in everyday life. Shadows give us some information about shapes of objects. Sometimes, shadows can also mislead us about the shape of the object.

The bird of obstructs the sunlight to form shadow on the nearby ground. But when the same bird is flying high up in the air, even then the bird of obstructs the sunlight falling on it but its shadow is not seen on the ground because the ground is very, very far below the bird.

An electric bulb and the candle form sharp shadows because they are comparatively small source of light. A fluorescent tube light does not form sharp shadows because it is a quite large source of light. Since a fluorescent tube light is very long, therefore some of the light form it always reaches behind the opaque objects leading to the formation of very faint shadows. In fact, the shadows are formed by fluorescent tube lights are so faint that it is usually very difficult to see them.

The pinhole camera consists of a closed box having a small pinhole in the front and the translucent screen at the back. The translucent screen at the back side of the pinhole camera boxes made of butter paper (also called in tracing paper). The butter paper acts as a screen to receive the image of the object. The pinhole camera is usually used to view the images of various objects like trees and building etc. The pinhole camera works on the principle that light travels in straight lines to the screen.

There is an interesting pin hole camera in nature. Sometimes, when we pass under a tree covered with large number of leaves, we notice small patches of sun light under it. These circular images are, in fact, pin hole images of the sun. The gaps between the leaves, act as the pin holes. These gaps are all kinds of irregular shapes, but, we can see circular images of the sun.

In science, an object which reflects light well is called a mirror. A plane mirror is a thin, flat and smooth sheet of glass having a shiny coating of silver metal or aluminum metal on one side. The silver coating is protected by a layer of red paint. Remember, the reflection of light in a plane mirror takes place at the silvered surface inside it. A mirror works because it reflects light. A mirror reflects most of the light falling on it in a definite direction.

The image which can be obtained on a screen is called a real image. The image formed on a cinema screen is an example of real images. A real image is formed when light rays coming from an object actually meet at a point after reflection from the mirror. A real image can be formed on a screen because light rays actually pass through it.

There are different characteristics of image formed by a plane mirror:
i)The image in a plane mirror is erect.
ii)The image in a plane mirror is of the same image as the object that means that image is neither enlarge not diminished.
iii)The image in a plane mirror is laterally inverted or sideways reversed with respect to the object.
iv)The image formed in a plane mirror is at the same distance behind the mirror as the object is in front of it.
v)The image formed in a plane mirror is virtual or unreal that means it cannot be obtained on a screen.
There are different use of plane mirrors:
i)Plane mirrors are fixed on the walls of certain shops, like jewellery shops, shopping malls etc.
ii)Plane mirror used at home to view our-selves.
iii)Plane mirror used in making periscope.

Shadow is just an absence of light at that place. Shadow is not affected by the colours of the object because the amount of light preventing to pass will remain same.
The shadow is affected by the shape of the object only. If the object is large, the shadow will be large and vice-versa.

We can see the objects through air clearly, means we can observe all the objects in their original shapes and sizes without any disturbance, it means the environment around us which is just air, must be transparent. But when thick smoke, thick clouds etc. are presents in the air, it does not remain transparent.

We can see the objects through air clearly, means we can observe all the objects in their original shapes and sizes without any disturbance, it means the environment around us which is just air, must be transparent. But when thick smoke, thick clouds etc. are presents in the air, it does not remain transparent.

The shadow of an object cannot be caught if the screen and the object are very far from the each other. This is the same case. Here, also the shadow of the football cannot be seen on the ground (which acts as a screen) when it is kicked high in the air. This happens because if we take the object away from the screen, the shadow becomes smaller and smaller in size and a time comes when it totally disappears from the screen.