ICSE Class 7 Physics Chapter 5 Light The Basic Idea of Shadow

Light - The Basic Idea of Shadow

Syllabus

1. Sources of light (sun, fire, electric bulb and fluorescent tubes, heated bodies) - luminous and non-luminous bodies - how we see the Moon - bioluminescence (brief mention).

2. Transparent, translucent and opaque bodies/materials - examples.

3. Rectilinear propagation of light - pinhole camera - variations caused by changing distance between screen and pinhole - simple ray diagrams to show formation of image.

4. Shadows - umbra, penumbra - eclipses of the sun and the moon - simple ray diagrams.

Classifying a set of objects/materials as luminous/non-luminous; transparent/translucent/opaque (E).

Construction of a simple pinhole camera (E).

Light

Imagine yourself entering a room which is dark. Will you be able to see the objects inside the room? No.

Suppose the light is switched on, now will you be able to see the objects inside the room? Yes. Why is it so?

An object is visible to us when the light falling on the object bounces towards our eyes. The light falls on the object, reflects and then reaches our eyes. This makes the object visible to us.

We see the beauty of nature around us, coloured pictures in cinema halls, on television, in books and magazines, etc. This is all due to light. Light is, therefore, a vital and useful factor not only to our environment but also to our life.

Light is a form of energy which helps us to see objects. In absence of light, we cannot see anything. Although light itself cannot be seen but in the presence of light, other objects can be seen.

Light is thus defined as from of an external physical cause that affects our eye to produce the sensation of vision.

Light is a form of energy like sound. But unlike sound, light can travel through space, i.e., it does not require a medium to travel. That is how we receive light from the sun.

Light always travels in a straight line path in the form of rays. It is considered to be the fastest moving energy.

Teacher's Note

When you watch a movie in a cinema, the projector beam travels in straight lines through the dark air, which is why you can see the light rays in dusty air.

Sources Of Light

In the daytime, we see things due to the light from the Sun. At night, we use an electric light, a torch or a candle. The Sun, an electric light, a torch and a burning candle are some examples of light source. The light from a source of light spreads in all direction. When this light enters our eyes, we are able to see the objects. We see a source of light or an object when light from it reaches our eyes. So, we may think that every object that we see is a source of light. But, it is not so. The light from an object, such as a burning candle or match stick is its own but the light emitted by the moon is not its own rather it is reflecting the light received by it from the sun. The light emitted by a source helps us to see other object.

An object which is capable of giving out light is called a source of light.

The sources of light can be divided into two categories: (i) natural and (ii) man-made or artificial sources of light.

Natural Sources Of Light

The sun is the biggest and the most important natural source of light. It is very bright. It is about 150 million kilometres away from the earth, still it makes the day absolutely bright for us. Stars other than the sun are also ever natural sources of light. Some stars are brighter than the sun but they are too far away from us, as compared to the sun; we therefore do not realise the light given out by the stars, so they appear to be tiny trembling spots.

Bioluminescence

A few typical plants and animals emit light, for example, jugnu (or glow worm) and firefly. This emission of light given out by a living organism is called bioluminescence. This results from a chemical reaction in which chemical energy is directly converted into light. In this process, very little heat is evolved. Bioluminescence objects such as jugnu, etc. are considered as natural sources of light.

The emission of light by a living organism as a result of some chemical energy getting converted into light energy is called bioluminescence.

Teacher's Note

Fireflies in gardens at night are a natural example of bioluminescence, where living organisms produce light without heat, unlike electric bulbs.

Artificial Sources Of Light

During night time, there is no sunlight, so we cannot see things in its absence. But to perform various activities at night, man has made some artificial sources of light. For example, a burning candle, an oil lamp, a bulb, a tube-light, a torch, etc., are all man-made, so they are known as artificial sources of light.

Fig. 5.2 Some artificial sources of light

Scattering Of Light

Why it becomes bright all around us when the sun rises?

There are many gas and dust particles floating in the air. When the sun rays fall on these particles, they absorb and re-emit some light in all directions. This is called the scattering of light. Due to scattering of sunlight, our surrounding becomes bright.

Remember that a light source spreads an equal amount of light in all the possible directions. As you move away from the source, you will observe that the brightness starts decreasing. Let us try to understand it from the following activity.

Teacher's Note

On a foggy morning, the sun appears dimmer because dust and water particles in the air scatter the light in many directions instead of letting it travel straight to your eyes.

Intext Questions

1. Name any four man-made sources of light.

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

3. Does the flame of a gas stove emit light?

4. Name few living things which emit light.

Intensity And Unit Of Light

We see that every source of light has its own brightness. The measurement of brightness produced by a light-source is called its luminous intensity of the light source. Earlier, we used to measure the brightness of light in comparison with the brightness of a candle and we used the unit called candle power. For example, the luminous intensity of a bulb was considered to be about 100 candles power i.e., a bulb gives the same brightness as 100 candles will give it together.

The modern internationally accepted unit for brightness or intensity is called lumen. One candle power is equal to 1256 lumen.

Speed of light is very high i.e., nearly 3,00,000 km/s, or 3 × 10^8 m s-1. Light takes about 8 minutes to travel from the Sun to the Earth.

Hot And Cold Sources Of Light

The sources of light which emit light and heat simultaneously are called hot sources of light.

The sources of light which emit only light and no heat are called cold sources of light.

The bodies which emit light when heated to very high temperature are called incandescent bodies.

Consider the light given out by a firefly (Jugnu) and the tip of an incence stick. The brightness of their light is almost the same. But is the temperature of the two sources also the same? No. Similarly you can touch a tube-light which is giving out light but you cannot touch a glowing bulb. If you touch the tip of an agarbatti or a glowing bulb, you will burn your hands. But, if you touch a firefly or a tube-light, your hands are safe.

Luminous And Non-Luminous Bodies

The sun, stars, bulb, candle, etc., which emit their own light are called luminous bodies, while those which do not have their own light like the moon, a chair, a table, etc., are called non-luminous bodies. However, if light falls on non-luminous bodies, they become visible to us. Look around and try to list some more examples of luminous and non-luminous bodies in your notebook.

Examples of luminous bodies: Hot filament in an electric bulb, burning candle, oil lamp, torch, a lantern, etc.

Examples of non-luminous bodies: Earth, all planets, pen, book, paper, stones, wood, bricks, living organisms, etc.

Luminous bodies may be natural or man-made. For example, the sun, stars and firefly (jugnu) are natural luminous bodies. A glowing electric bulb, a burning candle or oil lamp are man-made luminous bodies.

Any non-luminous object can be made luminous by heating it. If we heat an iron wire over a gas flame, it becomes red hot and after sometime it will start emitting light. When the temperature is about 800°C, it is able to emit out its own light.

All of you must have seen moon at night giving out cool milky white light. Also some nearby planet like venus appearing like a bright start during early evening. Why do we call them to be non-luminous although light is coming out of them?

The simple reason is that, they do not produce their own light. They appear bright as they reflect the light they receive from the sun.

Viewing The Moon

We see the beautiful moon shining at night. We know moon is a non-luminous body i.e., it has no light of its own. Then how does it shine? The simple answer is that the moon receives light from the sun and the sunlight after bouncing (or reflecting) from the moon reaches us on the earth. Thus, we see the moon shining. Figure 5.5 shows how the moon is visible to an observer on the earth.

Fig. 5.5 The moon is a non-luminous body

Teacher's Note

When you see the moon at night, you are actually seeing reflected sunlight, just like how you see yourself in a mirror because light bounces off its surface.

Optical Medium: Any Medium Through Which Light Can Pass Partially Or Completely Is Called An Optical Medium.

(i) Homogeneous medium: It is an optical medium which has uniform density throughout.

(ii) Heterogeneous medium: An optical medium which has different density at different points is called heterogeneous medium.

Glass, silver, diamond, distilled water, pure alcohol, etc. are homogeneous medium whereas air, fog, mist, etc. are heterogeneous medium.

Transparent, Translucent And Opaque Bodies (Medium)

When light or a ray of light falls on an object, three things can happen (Fig. 5.6):

(i) it may pass through the object almost completely;

(ii) only a part of it may pass through the object.

(iii) it may completely be obstructed by the object.

Fig. 5.6

On this basis, we can classify materials into following three categories.

(i) Transparent Medium

A medium through which light can pass completely is called a transparent medium and the property due to which the light can pass through the medium is called the transmitting property of the medium. One can see through such mediums clearly. Glass and water are examples of transparent media. Sometimes, the transparency of a given material may decrease due to environmental factors, for example, fog or suspended impurities decrease the transparency of air and water.

Uses of transparent materials

Transparent materials, specially glass is used for many purposes.

1. In making window panes and doors

2. In spectacles

3. In binoculars

4. In making microscopes and telescopes, etc.

(ii) Translucent Medium

The medium through which light can pass partially is called a translucent or semi-transparent medium. Such materials absorb some light and allow the remaining light to pass through. One can just make out a rough sketch of an object kept behind the translucent material.

For example, tracing paper, waxed paper, grounded glass, etc.

Uses of translucent materials

Windows and doors are made of translucent materials so that only a small amount of light can enter into the room.

(iii) Opaque Medium

Objects like wood, black paper, metals, etc. do not allow light to transmit through them at all. Such medium which does not allow any light to pass through is known as opaque medium. When light falls on opaque materials, most of it is either absorbed or reflected. Thus, one cannot see through an opaque material and also cannot make even a rough sketch of the object kept behind it.

Uses of opaque materials

Opaque materials are used when we do not want to allow light to pass through it. Opaque objects when kept in the path of light, cast a shadow.

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