Energy

Your eyes help you see. But are eyes enough? Go into a dark room at night. Your eyes are open, but you see nothing! So we need something more. We need light.

We see a thing when light from it reaches our eyes. The Sun, a lamp and a candle flame give out their own light. A flower or a tree gives out no light. Light falls on it and bounces off. Some of that light travels into our eyes, and we see the flower.

The energy that helps us see things is called light energy.

Sources of light

In the daytime, sunlight lets us see. At night, or in a dark room, there is no light. Then we use an electric bulb, a torch, a candle, a kerosene lamp (tuki) or a burning splint.

A thing that gives out light is called a source of light. There are two kinds.

• Natural sources are made by nature: the Sun and the stars.
• Man-made sources are made by people: electric bulbs, torches, candles and kerosene lamps.

Luminous and non-luminous objects

A thing that gives out its own light is called a luminous object. The Sun, the stars, a candle flame and a lit bulb are luminous.

A thing that has no light of its own is called a non-luminous object. A stick, a glass, a book, a stone and you are non-luminous. We see them only when light falls on them.

Transparent, translucent and opaque objects

Can you see through a window? Can you see through a wall? Some things let light pass through them. Others don’t. Try it.

Here is what you found.

• A transparent object lets light pass through it easily. You can see clearly through it. Glass, clean water and air are transparent. They make almost no shadow.
• A translucent (or semi-transparent) object lets only a little light pass through. You see only a blur through it. Oiled paper, white plastic and thin white cloth are translucent. They make a faint, soft shadow.
• An opaque object lets no light pass through. You can’t see through it at all. Stone, clay, wood and thick black cloth are opaque. They make a dark shadow.

Color of light

Read this poem about a rainbow. Then talk about it.

Sunlight looks white. But white light is really a mix of seven colors: red, orange, yellow, green, blue, indigo and violet. To remember them in order, say VIBGYOR backwards: ROYGBIV.

When sunlight goes into a raindrop or a glass prism at a slant, it bends. Each color bends a different amount. Violet bends the most, red the least. So the colors fan out into a band. This splitting of white light into its seven colors is called the dispersion of light.

1. True or false?

2. Fill in the blanks

non-luminous, man-made, seven, natural, transparent, five

3. Choose the correct answer

4. Answer these questions

Strike a madal with your hand. Its skin vibrates: it shakes quickly back and forth. That vibration makes the sound. In the same way, the air inside a flute vibrates when you blow it. A guitar string vibrates when you pluck it.

Sound is a kind of energy made by a vibrating thing. Nothing makes a sound all by itself. First we must give it energy: we hit it, beat it, blow into it, pluck it or shake it. Then it vibrates, and we hear a sound.

Sources of sound

A thing that makes a sound is called a source of sound. People, birds, animals, televisions, radios, musical instruments, bells and alarm clocks are all sources of sound.

Propagation of sound

A dog barks behind a wall. You can’t see it, but you hear it. How did the sound get to your ears? The barking dog makes the air next to it vibrate. That air makes the air next to it vibrate, and so on, all the way to your ears. Sound travels like this. We say it propagates: it is passed on, or transmitted, from one bit of air to the next.

Sound needs something to travel through. That something is called a medium. Sound can travel through:

• a gas, like air (your friend’s voice across the room);
• a liquid, like water (the spoons in the bucket);
• a solid, like a thread or a wall (the string telephone).

The sound makes the bits of the medium vibrate, one after another. That is how it travels.

Where there is nothing at all, not even air, there is a vacuum. A vacuum has nothing to vibrate. So sound can’t travel through a vacuum. That is why people can’t talk to each other in a vacuum just by speaking.

Loud, soft, shrill and deep sounds

Sounds can be loud or soft. They can be shrill (high, like a whistle or a bird’s chirp) or deep (low, like a big drum or thunder). What decides this?

• Loud or soft depends on how much force makes the sound. Hit a madal hard, and its skin makes big vibrations: a loud sound. Tap it gently, and the vibrations are small: a soft sound. When you whisper, the sound is soft. When you shout, it is loud.
• Shrill or deep depends on what the thing is like and how fast it vibrates. Metal things, and thin, short, tight strings, vibrate very fast and give a shrill sound. Wood, plastic, and thick, long, slack strings vibrate more slowly and give a deep sound.

Noise

A loud, unpleasant sound is called noise. When there is a loud noise, we often cover our ears. Noise is harmful in many ways.

• It distracts us. In a noisy class, it is hard to understand what is taught.
• It is hard to talk to each other in a noisy place.
• Staying in a noisy place for a long time weakens our hearing.
• Loud noise can cause headaches, anger and worry, and other mental problems. It can also harm the flow of blood in our body.

Read this poem about noise. Then talk about it.

Too much noise is bad for our health. So we should follow good sound etiquette (good manners with sound):

• a.Play radios, televisions and musical instruments at a low volume.
• b.Build factories and loud machines far from where people live.
• c.Make cinema halls and dance halls soundproof, so their sound doesn’t get out.
• d.Don’t blow horns near schools and hospitals.
• e.Don’t blow horns for nothing, and follow the traffic signs.

1. True or false?

2. Fill in the blanks

sharper, good etiquette, vibrate, mental illness, sound, hoarse

3. Choose the correct answer

4. Answer these questions

Electric circuit

Read this poem about a torch. Then talk about the questions.

You switch on a lamp at home, and it lights at once. A fan, a TV or a rice cooker starts at once too. Every electrical device needs electricity to work.

Electricity flows through conducting wires. A switch lets it flow, or stops it, as we need.

The device that uses electricity, the source of electricity, the conducting wires and the switch together make a path. This path is called an electric circuit.

• When the switch is on, the path is complete. Electricity flows round it, and the bulb lights. This is a closed circuit.
• When the switch is off, there is a gap in the path. Electricity can’t flow, and the bulb stays dark. This is an open circuit.

The flow of electricity round a circuit is called an electric current.

Safety measures to be followed while using electricity

Electricity is very useful in our daily life. But if it is not used carefully, it can cause terrible accidents. It can give a dangerous electric shock, and it can start fires.

So, while using electricity, follow these safety measures:

• a.Never put your fingers, or anything else, into plugs and sockets.
• b.Cover a bare wire with insulating tape. Insulating tape is made of an insulator: a material that doesn’t let electricity through.
• c.Wear rubber gloves for electrical work. Rubber is an insulator.
• d.If anything electrical breaks down, call a trained electrician to repair it.
• e.Don’t use electrical things without the permission of an older person, or without one watching.
• f.During a power cut, a lightning storm or a fire, turn off the main switch.

1. True or false?

2. Fill in the blanks

insulator, electrical equipment, electric circuit, on, electricity, off

3. Choose the correct answer

4. Answer these questions