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Subjects /Class 6 / Science.6 / Light, Shadows and Reflections

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INTRODUCTION
27 Jun 2022

  • Light is a form of energy which helps us in seeing objects through our eyes.
  • When light falls on an object, some of the light gets reflected. The reflected light comes to our eyes and that way we are able to see an object.
  • Thus, we cannot se things without light and it helps us see objects.
  • Some examples of things that have their own light are:
    • Torch, Fireflies, Sun, Candle and glow worm etc.

Sources of Light

Luminous Objects:

  • The Objects that produce their own light are called luminous objects. Thus, objects like Sun that give out or emit light of their own are called luminous objects.
    • For example: Sun, Fire, Torch
  • When light from a luminous object (like Sun, a torch or an electric light) falls on objects and then travels towards our eye, we are able to see the objects.
    • For example: Chair does not produce its own light, so to see the chair in the dark room, we need a torch to light up the room and help us see the chair.

Non-Luminous Object:

  • The objects which don't emit their own light are called non-luminous objects. To see such objects we need the help of luminous objects.
    • For example: Moon, Trees, Shoe

Transparent Opaque and Translucent Objects

There are three types of objects, namely:

  • Transparent
  • Translucent
  • Opaque

Transparent Objects:

  • These objects allow light to go through them in straight line completely. we are able to see through these objects clearly.
  • For example: Water, Air etc.

Translucent Objects:

  • These objects allow light to pass through them partially. the object isn't clearly seen through these objects.
  • For example: Butter paper, frosted glass, tissue paper

Opaque Objects:

  • These objects don't allow light to pass through them. we cannot see the objects through these objects. Opaque objects block the light completely.
  • For example: Door, Trees, Wall etc.


Characteristics of Light

  • Light is a form of energy.
  • Light travels in a straight line.
  • Light may be reflected. It will be bent. The speed of light is 3 \( \times \)108 m/sec.
  • It may be polarized.

Formation of Shadows

When you hold an opaque object like a cricket ball in the sunlight, you may see a dark black coloured shape on the ground. This dark patch is the shadow of the cricket ball.

Sometimes you can identify the object by looking at its shadow. However, sometimes shadow may mislead you. For example, you can make the shadow of a dog face on a wall using your fingers.

Do you think that a shadow could be formed in a dark room without any source of light?

The answer is no. We need a source of light and an opaque object and a screen to see a shadow.

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.

When an opaque object is placed in path of light, a dark portion is made on the alternative side of object on the screen.

This dark portion is shadow.


Properties of Shadow



  • It is dark in color.
  • Transparent objects don't make shadow.
  • The object must be opaque or translucent and the shadow is formed opposite to the source of light.
  • It gives the information about the shape of object. Sometimes, shadows can also mislead us about the shape of the object.
  • Size of shadow may be increase or decrease by change in distance between object and screen are changed.

Explain a Pin-Hole Camera

How to make a pin-hole camera?

  • Take two boxes of cardboard such that one can slide into another with no gap in between them.
  • Cut open one side of each box.
  • On the opposite face of the larger box, make a small hole in the middle.
  • In the smaller box, cut out from the middle a square with a side of about 5 to 6 cm.
  • Cover this open square in the box with tracing paper (translucent screen).
  • Slide the smaller box inside the larger one with the hole, in such a way that the side with the tracing paper is inside.
  • Your pin hole camera is ready to use.

A pin-hole camera is simply a box with an awfully tiny hole on one amongst its sides. When light falls on the hole, and an inverted image is made on the side opposite to the hole. The human eye acts very much like a pin-hole camera.

Sometimes, when we pass under a tree covered with large number of leaves, we notice small patches of sunlight under it. These circular images are, in fact, pinhole images of the Sun. Thus, it acts as a natural pinhole camera.

How the images seem like pin-hole camera?

Image formed by a pinhole camera is inverted, coloured and little in size.


Rectilinear Propagation of light

Light travels in the straight line. It is called the rectilinear propagation of light.

Ray of Light:

  • It is a path along which light travels in the given direction.

Beam of light:

  • Bunch of rays of light travel in the given direction forms the beam of light.

Mirror and its Types

Mirror is a highly polished surface which reflects the light falling on it. A mirror changes the direction of light that falls on it. It is a polished surface which may make the image of an object.

We all use mirrors at home. When we look into mirror, we see our own face inside the mirror. This is the reflection of our face into the mirror.

Sometimes we see the reflections of trees, buildings and other objects in the water of a pond or a lake. The pond or lake also acts as mirror.

Types of Mirror are:

Plane Mirror:

  • Image formed is of a similar size.

Concave Mirror:

  • Reflecting surface is bent in.

Convex Mirror:

  • Reflecting surface is bulged out.

Define Reflection of Light?

The process of returning the light to an equivalent medium after striking a surface.

NCERT SOLUTIONS

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


Solution:

The given boxes can be rearranged to form ‘Opaque Objects Make Shadows’ as shown below


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 aluminum, 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, the moon.

Solution:

Opaque:  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.

Transparent: Air, water, a sheet of plane glass.

Translucent: A sheet of polythene, smoke, fog, a sheet of cellophane, a wire mesh.

Luminous: A piece of red hot iron, a lighted fluorescent tube, the flame of a gas burner, a lighted torch, sun, firefly, kerosene stove.

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, an umbrella, a wall, a sheet of carbon paper, a sheet of cardboard, a sheet of cellophane, a wire mesh, moon.

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?

Solution:

Yes,

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?

Solution:

It is not possible to see a reflection of yourself in the mirror  in a fully dark room because we need light source to reflect our face on the mirror.