The Nature of Light and Vision

Light is a form of energy that helps us see the objects around us. In a completely dark room, we are unable to see anything. However, when we light a candle or turn on a torch, the light from these sources allows us to see the objects present in the room. This demonstrates that without light, things cannot be seen.

Luminous and Non-Luminous Objects

Objects can be classified into two categories based on whether they produce their own light.

• Luminous Objects: These are objects that give out or emit light of their own. Their light allows us to see them and also to see other objects around them.

  Examples: The Sun, a torch bulb, a lit candle, and an electric light.

• Non-Luminous Objects: These are objects that do not produce their own light. We can see them only when light from a luminous object falls on them and is then reflected (bounces off) towards our eyes. The vast majority of objects we see are non-luminous.

  Examples: A chair, a book, a shoe, a painting, and the Moon (which we see because it reflects sunlight).

Transparent, Opaque, and Translucent Objects

Based on how they interact with light, materials can be grouped into three categories. The category an object belongs to depends on whether it allows light to pass through it completely, partially, or not at all.

Classification of Objects

1. Transparent Objects: These are objects that allow light to pass through them completely. We can see clearly through transparent objects.

   Examples: Clear glass, water, air, some plastics.

2. Opaque Objects: These are objects that do not allow any light to pass through them at all. We cannot see through opaque objects.

   Examples: Wood, a wall, a book, a pencil, a rubber ball, metals.

3. Translucent Objects: These are objects through which we can see, but not very clearly. They allow light to pass through them only partially, scattering it in the process.

   Examples: Tracing paper, frosted glass, butter paper, some plastics.

Observation Table

Shadows and Their Formation

A shadow is a dark patch or area formed behind an opaque object when it blocks the path of light. The shadow is formed on the side of the object that is away from the light source.

Conditions Required for Shadow Formation

To form a shadow, three things are necessary:

1. A source of light (like the Sun or a torch).
2. An opaque object to obstruct the path of light.
3. A screen or surface behind the object for the shadow to be formed on (like the ground, a wall, or a sheet of cardboard).

Without any of these three components, a shadow cannot be seen. In a completely dark room, there is no shadow because there is no light. Similarly, in a room with only a light source, there is no shadow if there is no opaque object to block the light or a screen for the shadow to fall upon.

Properties of a Shadow

• A shadow is always dark: The colour of a shadow is always dark, regardless of the colour of the opaque object. The shadows of a red rose and a yellow rose will look the same.
• A shadow shows only the outline: It provides information about the shape or outline of the object, but not the details or the colour. Sometimes, shadows can be misleading, as we can create hand shadows that look like different animals.
• The size and shape of a shadow can change: The size of a shadow depends on the distance between the light source, the object, and the screen. The shape of the shadow can also change if the object is turned or the angle of the light source changes.

The Pinhole Camera and Rectilinear Propagation of Light

A pinhole camera is a simple device that works on the principle of how light travels. It can be made easily and helps in understanding the nature of light and image formation.

Constructing a Simple Pinhole Camera

A simple pinhole camera can be made using two cardboard boxes, one slightly smaller than the other so it can slide inside.

1. Take the larger box and cut open one side completely. On the opposite face, make a tiny, neat hole in the middle (the pinhole).
2. Take the smaller box and cut open one side. On the opposite face, cut out a square from the middle and cover it with tracing paper. This will be the screen.
3. Slide the smaller box into the larger one, with the tracing paper screen on the inside.

To use the camera, look through the open end of the smaller box at a brightly lit object. Slide the inner box forward or backward until a clear image is formed on the tracing paper screen.

Characteristics of a Pinhole Image

The image formed by a pinhole camera has distinct characteristics that differentiate it from a shadow:

• The image is upside down (inverted).
• The image shows the colours of the object.
• The image is a projection of the object, not just its dark outline.

An interesting example of a natural pinhole camera is seen when we pass under a tree with dense leaves. The small gaps between the leaves act as irregular pinholes, and they project small, circular images of the Sun on the ground.

Principle: Light Travels in a Straight Line

The formation of shadows and the working of a pinhole camera are both possible only because light travels in a straight line. This property is known as the rectilinear propagation of light.

An activity to prove this involves looking at a candle flame through a straight pipe and then through a bent pipe. The flame is visible only through the straight pipe because the path from the candle to the eye is straight. When the pipe is bent, the straight path of light is blocked, and the flame cannot be seen.

Mirrors and Reflection

We all use mirrors to see ourselves. The image we see in a mirror is called a reflection. A mirror is a smooth, polished surface that can reflect light in a regular way.

Reflection of Light

Reflection is the process by which a mirror changes the direction of light that falls on it. When a beam of light strikes a mirror, it bounces off in a different direction.

An activity can demonstrate this: in a dark room, if you shine a torch beam onto a mirror held by a friend, you will see a patch of light appear somewhere else in the room. By changing the angle of the mirror or the torch, you can change the direction of the reflected patch of light. This shows that a mirror changes the direction of light.

Visualizing Reflection

The path of light can be visualized in an activity using a comb and a mirror on a dark sheet of paper.

Method: Fix a comb on one side of a large sheet and a mirror on the other. Shine a beam of light from a torch through the comb's teeth. The narrow beams of light passing through the comb travel as straight lines, strike the mirror, and then bounce off (reflect) as straight lines in a different direction.

This activity provides a clear visual idea of how light travels in straight lines and gets reflected from a mirror, creating a distinct pattern.

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

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