light n sound

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Light and sound

At the end of this chapter, you will be competent to...

Apply the various properties and behaviours of light for the welfare of

man.

Use principles of production and transmission of sound to producemusical instruments.

Investigate the scientific basis of the modern communication systems.

07


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7.1 Using properties of light to fulfil human needsLight has a wonderful ability to control our daily activities. During daytime we

become active while in the night we go to sleep. We have to conduct many activities in

the dark too. In such instances we have to use suitable light sources in order to get

light.

Objects that produce light are called luminous objects.

Objects that do not produce light are called non-luminous objects. Think of the

objects around us and see whether we have more luminous objects than non-luminous

objects.

Some examples of luminous objects and non-luminous objects around us aregiven below in Table 7.1.

We use sources of light to see objects around us. In order to see some objects,

only our eyes is not sufficient. Let us see why light is also necessary to see objects.

7.1.2 Necessity of light for seing

In a dark room we can not see

anything. If we are to see anyobjects in the dark it must be a

luminous body or else it must be lit

up. For this we need a source of

light. Light from the source falls on

the object and is reflected to our

eyes. Then we can see the object. Fig. 7.1

Table 7.1 Some luminous and non-luminous objects

Luminous objects

1. Sun

2. Lighted bulb

3. Lighted lamp

4. Glowing bulb

Non luminous objects

1. Moon

2. Stone

3. Paper

4. Planets

Think !

Why is the moon considered a non luminous object?


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Do you know?

There are animals that produce light. These are animals can be considered

as luminous.

Fireflies and glow worms are some examples. They produce light from a

place on the underside of their abdomen.

Certain types of fish that live deep in the ocean also produce light. Certain

kinds of mushrooms and some bacteria too produce light.

Fig. 7.2 - Firefly Fig. 7.3 - Mushrooms

Beams of light, rays of light and the path of light

You would have seen the beam of light

when a torch is lit. (Fig 7.4)

Beams can be more clearly seen when

there is dust or mist in the environment. Beams

of sunlight coming through trees is a common

sight on a misty morning. (Fig. 7.5)

Fig. 7.4 - A beam of light

Fig. 7.5 - Beams of sunlight

A beam of light is made up of several

rays of light. You can obtain narrow beams

of light needed for practical purposes by

placing a comb or piece of cardboard with

narrow slits on the path of a broad beam

of light. Let us do the following activity to

find the path of light.Fig. 7.6 - Narrow beams of sunlight


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7.1.3 Properties of light

You may recall how you had great fun by directing light falling on a mirror inseveral directions such as on walls. You may also have seen how shining metal sheets

are held around the field of action during the shooting of films or teledramas. (Fig 7.7)

This is done to re divert the direction of light on to the scene and light them up.

Here the function of the metal sheets, similar to the mirror is to reflect light. Let us learn

more about reflection.

Activity - 7.1To demonstrate light travels in straight lines.

Find three similar pieces of cardboard cards, candle, a pin and box of matches.

Place the three cards together and pierce all three at the same height using the

pin.

Place the cards at certain distances from each other so that the holes are in a

straight line.

Place a lighted candle in front of the last card. Look through the first card.Can you see the light from the candle?

Now displace the second card sideways and observe the flame through the

pinhole. Can you see it? According to your observations what conclusion

can you come to regarding the path of light? Flame is visible only when the

pinholes are in the same horizontal line.

1

2

3


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Reflection of light

A beam of light falls on a surface

and travels back in another direction. This

phenomenon is called reflection of light.

Shiny smooth surfaces such as mirrors

(Fig. 7.8) reflect light well. Example:

mirrors, stagnant water surfaces.

All polished surfaces do not reflect light to

the same extent. When a parallel beam of light

falls on a shiny surface, the beam is reflected

back and travels back as a parallel beam. This

is called regular reflection. (Fig 7.8)

When a beam of light strikes a rough

surface, reflection is not uniform as in regular reflection. Light gets scattered there.

This is called irregular reflection. (Fig. 7.9)

We use the phenomenon of reflection often in our daily life.

Examples :

Use of plane mirrors as looking glasses.

To multiply the numbers of objects in shops. (Fig. 7.10)

For looking at objects behind us, as in a barbers shop to see the back of the

head.

Fig. 7.7 - Shooting a teledrama

Fig. 7.8 - Regular reflection

plane mirror

Fig. 7.9 - Irregular reflection


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Kaleidoscope

Can you recall an instance when you

observed various beautiful patterns using a toy

kaleidoscope made out of three strips of mirrors?

These patterns in the kaleidoscope is due to

reflection of light. You too can make a

kaleidoscope. (Fig. 7.11)

Fig. 7.10 - A shop use of plane mirrors

black paper

Fig. 7.11 - Kaleidoscope

Assignment - 1

Find three strips of plane mirrors of equal size (6cm x 2cm), one half

sheet, a tissue paper and gum tape.

Fix the plane mirrors in the shape of a triangle as shown in the figure.

Wrap them in a paper and fix with gum tape. Cover one end with the

tissue paper and fix with gum tape.

Introduce bits of coloured material such as petals of flowers, colour

paper etc. and observe the different patterns formed. You may change

the patterns by turning the kaleidoscope.

Try to count the number of patterns made with one flower petal.

Periscope

This is an instrument used for looking at objects at a higher level from a lower

level. E.g. in submarines, bunkers. You too can make a periscope using two plane

mirrors. (Fig 7.12)


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Refraction of light

A glass full of water was placed on a sheet of printed paper spread on a table.

Amal, looking at it from above saw that the letters under the glass appeared large and

uplifted. The reason for this was a problem for Amal. On checking with the science

teacher he was told that it was a result of refraction of light. Let us find out what

refraction of light is. Refraction is another kind of behaviour of light.

When rays of light travelling through one medium are incident obliquely on thesurface of another transparent medium, the light rays bend. This bending of light or the

change in direction of light when it passes from one medium to another is called

refraction.

Let us identify what happens when light rays strike a glass block.

Fig. 7.12 - Periscope

Activity - 7. 2

Find a white sheet of paper, a glass

block, a plane mirror, piece of

cardboard with a slit to enable to obtain

a narrow beam of light, a drawing pin.

Place the white paper on the table and

keep the glass block on the paper.

Direct a narrow beam of light on to the

glass block. A laser beam can be used

for this purpose.

Observe the path of the narrow beam of light.


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The way in which light rays get re-

fracted when it travels from air to galss

and from glass to air can be observed

clearly.

A photograph of the path of a

beam of light through a glass block is

given in Fig 7.13.

When a beam of light goes into

water from air, and when it leaves the

water and goes back into air it

undergoes refraction of this nature. This can be observed by directing a beam of light

through a glass filled with water.

Now let us find out what happens when a beam of light is sent through a prism.

When a narrow beam of sunlight

is sent through a prism, it can be

observed that the beam gets separated

in to different colours. (Seven colours)

(Fig. 7.14)

These set of seven colours

obtained on the screen is called the

spectrum. The seven colours of the

spectrum are as shown in the diagram.

In the spectrum the seven colours are red, orange, yellow, green, blue, indigo and

violet. Therefore, we can conclude that white light is composed of seven colours.

Fig. 7.13 - Way in which a ray of light is refracted

Activity - 7. 3

Project a narrow beem of light to the water in a transparent plastic vessel.

Observe the path of the light beam.

Activity - 7. 4

Find a plane mirror and a prism (60 x 60 x 60).

Project a narrow beam of sunlight to the rectangular surface of the prism.

Allow the beam to fall on a wall or a screen.

What can you observe?

Fig. 7.14 -

A beam of light seperated into seven colours


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Do you know?

It was Sir Issac Newton who discovered that light consists of seven different

colours for the first time.

Some phenomena connected with refraction

Certain phenomena caused by refraction of light are experienced in daily life.

Some of them are listed below.

Mirage is a common phenomenon that can be seen on tarred roads on warm days.

Rainbow

Do you Know ?

Have you ever thought why during day time, objects have various colours. e.g.

red. It is because, all the other colours in white light are absorbed by that object and

only the red colour is reflected out. We can explain how other colours are seen in

this manner.

The surfaces which absorb all the seven colours in light are seen as black, and

surfaces which reflect all the seven colours in light are seen as white.


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7.2 Production of sound propogation andmusical instruments

Listen to the various sounds that are present around

you. You will hear all sorts of sounds. How is sound

produced?

Hang a small pith ball near a gong of a bell. Now ring

the bell and observe what happens to the pith ball.

You will be able to observe that the pith ball starts

vibrating. It is clear that the gong of the bell is vibrating. Ifyou touch a ringing bell with your hand, the ringing stops.

Why? Sound is produced by means of vibrations. When

you touch, vibrations stop. Therefore production of sound also stops. This shows that

sound is produced due to vibrations (Fig 7.15).

Frequency of sound

Frequency is the rate at which an object vibrates. It can

be expressed as the vibrations occurring in a unit time. If one

vibration occurs in one second then the frequency is 1 Hz.Allow the metal blade in Fig 7.16 to vibrate. Its Median

position is A. If it moves from A to B first, then again it goes to

C via A and comes back to A, then one vibration has taken

place.

If the number of vibrations occurring in one second is

100, the frequency is said to be 100 Hz.

Limits of hearing

Do we hear all the sounds? We hear only a range of frequencies, i.e. from20 Hz to 20000 Hz. We do not hear the sounds of frequencies below 20 Hz or

above 20000 Hz.

Sound produced by a source of sound is propogated (transmitted) to distant places

from the origin of sound. To do so, there should be a medium.

The sounds we hear are transmitted through air. Air is not the only medium through

which sound travels. Sound travels through other media such as water, wood, metals,

etc.

Fig 7.15

Fig 7.16


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Fig. 7.18 - Instruments which produce sound by vibration of strings

Fig 7.17

bell jar

to the vacuum

pump

7.2.2 Propogation of sound

Arrange the apparatus as shown in the diagram.

An electric bell is kept inside a bell jar containing air.

Air in the jar can be removed by using a vacuum

pump.

Switch on the bell. The sound can be heard

from out side. Now remove air using the vacuum

pump. As air is removed, the sound of the electric

bell gets weaker and weaker finally it is not heardany more. The reason is that now there is no medium

(air) for sound waves to travel. This shows that a

medium is necessary for sound to travel.

7.2.3 Sources of sound

Instruments that produce sound are called sources of sound. Musical instruments

are sources of sound that we come across all the time. Some musical instruments

produce sound by vibration of strings, some by vibration of membranes while some

produce sound by vibration of air columns.

Producing sound by vibrating strings

In musical instruments such as violin, guitar. sitar and mandolin sound is

produced by the vibration of strings. (vibrating strings) These instruments have a box

made up of thin wood called the sound box. Strings are fixed on the sound box.

A bridge is also fixed on the box to prevent the strings touching the box. Sound is

produced by stroking or plucking at these strings.


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Producing sound by vibrating membranes

Some examples for these instruments are drums, tabla, dolak, rabana, symbols.

These instruments have stretched membranes. They are played by tapping or

striking the membranes.

Producing sound by vibrating air columns

You are familiar with whistles made by tender coconut leaves. When you blow the

sound is produced by vibration of the air column inside the whistle. Flute, piano,

clarinet, trumpet aresome other examples for such instruments. In these instrumentssound is produced by vibrating air columns.

Fig. 7.19 - Instruments which produce sound by vibration of membranes

Fig. 7.20 - Instruments which produce sound by vibration of air columns


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7.3 Methods of communication.

Various methods of communication are observed among the animals in our envi-

ronment. The singing of birds that you hear early morning as you get up is also a mode

of communication. These birds cry out in the morning to indicate their territory to the

other birds, of their kind.

Observe the behaviour of ants carefully. You will notice that two ants when they

meet keep their mouths close together and communicate with each other. By observing

the behaviour of dogs, you may see that they too have a way of communicating with

each other.

Man being a social animal, too had the necessity to communicate with each

other from distant past. Also, they needed to spread their messages as far as possible.

At the beginning, man conveyed his ideas by shouting out the message as loud as

possible to be heard by distant people. This kind of communication method which

prevailed in the past is of a very simple type. Let us think of some simple methods of

communication in the past.

Fig 7.21 - A man shouting out loud, and a man lighting a bon - fire.

Tieing up the message to the arrow and releasing the arrow.

Lighting up bon - fires on tops of mountains.

Sending the message by messengers on horse - back.

Using birds such as pigeons to carry the message

Making noises by drums.

Using coloured flags to convey messages.

Using light signals to convey messages.

Assignment - 2

Collect information about the different methods of communication that were

used in the past and prepare a booklet.


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From a primitive state, communication technology has progressed to great extents

so that now we can send messages to any part of the world within seconds. Let us

inquire about this progress step by step.

Telegraph

With the production of electricity, scientists attempted to use it

for purposes of sending messages. In this respect telegraph was an

extreamely succesful attempt.

Dr. Samuel Morse who was a professor in New York University

was the first person to discover that messages would be transmitted

along wires in the year 1835.Morse developed a machine where markings could be made on

a strip of paper placed at a distance, by processing a switch on

and off an electric circuit. Morse also developed The Morse signal code where

dots and lines were used to represent the letters of the English alphabet and numbers

from 1-9.

Fig 7. 23 - Morse code Telegraph circuit

This method where electric wires or cables are used to convey messages is calledthe telegraph method. Here a dot is made by pressing the switch of a circuit at once

and releasing it, and a line is made by keeping it for a short time. This message is made

to make an impression on a moving strip of paper. Later it is necessary to translate this

into words. In expressing the Morse signals in words they are sounded as follows:

DIT

DAH

Fig 7. 22 -

Samuel Morse


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Example

1. SUN

DIT DIT DIT DIT DIT DAH DAH DIT

2. GOD

DAH DAH DIT DAH DAH DAH DAH DIT DIT

3. A popular Morse signal is the SOS signal issued by ships in distress.

S O S

DIT DIT DIT DAH DAH DAH DIT DIT DIT

A Strange incident

An American motorist noticed that the brake lights of the vehicle which was in

front of him was switching on and off , continousely and in a particular rhythm. He was

aware of the Morse signal and identified that it was a SOS message. He realised that

the driver of the vehicle in front was in some distress and he informed the police. The

police officer stopped the vehicle, and noticed that a thief was holding a pistol at the

driver and was taking him to some destination by force.

Assignment - 3

Write the word MORSE CODE using the Morse signals given in the

Fig 7.23.

Fig 7. 24 - Braille alphabet Fig 7. 25 - Reading Braille Letters

Do you know?

The Braille alphabet was introduced by Louis Brail to facilitate communication

among people with visual disabilities. The English Braille system is given below.


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Since the driver in the car behind knew of the Morse code he could save a grave

situation.

Even in our country, at the beginning of the Postal Department the Morse Code

was used to convey telegraph messages. At a certain period of time the telegraph

method was popular throughout the world. The disadvantages of this method was that

a translator was needed for converting the message to Morse method, and to convert

the message conveyed to the language in use.

To overcome these problems, in 1876, Alexander Graham Bell invented the tel-

ephone where the message was conveyed by voice.

TelephoneIn the telephone, a microphone and an

earphone are enclosed in the same machine. The

sound waves (Voice) received by the microphone

causes increases or decreases in the current which

are transmitted through the wire to the telephone of

the receiver. Here the changes in the current is

converted to sound. From recent time cord - less

telephones or cell phones are being used.

Fax machine

The telephone is a very useful machine but there are occasions where messages

cannot be communicated accurately by this method. At

times due to disturbances in the place of origin or at the

recieving end the message may not be clear. Further the

accent of the speaker may also have an effect.

These problems are overcome by sending written

messages from one place to another, using the fax machine.

This was invented by a German (Arthur Korn) in 1907.

The word FACIMILE is written in a shortened form as

FAX.

Fig 7. 26 - Telephone

Do you know ?During lightning it is dangerous to use telephones with wires. If lightning

strikes the telephone wires, they will get transmitted along the wires and cause

damage to the instrument as well as the user of the telephone. Therefore at times of

lightning it is safer to disconnect your telephone.

Fig 7. 27- Fax machine


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A written message introduced into the fax machine is read line by line by the

scanner in the machine, and converted to electric signals. These are transmitted by

telephone lines to the recieving end, where it is retranslated into words, which is ob-

tained on paper. Hence the message received is very similar to the message sent.

Internet

Internet can be known as a hybrid of telephone and

computer.

Computer is a machine which can store a large amount of

information, and which can synthesise the data on orders ofthe user. A large amount of information can be stored in one

computer. There are methods by which data can be shared

between sevaral computers, that is by networking.

Think of the amount of data that could be obtained by networking the data in all

the computers in a country .

A network of about 50 million computers in about 150 countries are joined togerther

in the internet ie.NETWORK OF NETWORKS.

The symbol used to indicate internet on the computer screen is as shown in Fig

7.28. The letter e indicates infinity. The circle round it indicates that the entire

universe can be reached by the internet.

You can obtain all the information about any incidence that occured in any part of

the world from prehistoric times to a few seconds from now if you log on to world

wide web (www) connected to the internet.

We also can enter information into the internet. Many schools in Sri Lanka too

have initiated web files in the internet. Such a web page is indicated below.

Fig 7.28 -

Internet symbol

Assignment - 4

Prepare a small booklet on the topic The uses of internet?

Do you know?

There is a popular saying as follows:

If you did not live in a dark cave in a lonely island you should know about the

internet.


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Some Useful web sites are as follows:

Ministry of Education - www. moe.gov.lk

Education Publication Depatment - www. edupub.gov.lk

Examination Depatment - www. doenets.lk

National Institute Education - www. nie.lk

NASA Institute - www. nasa.gov

Home Page of a school web site

E- mail

Sending messages from one computer to another using the internet is called

E - mail is so efficient that compared to the speed of the e-mail, the postal services

can be considered as SNAIL - MAIL. In this type of communication, paper, enve-

lopes, stamps or a postal service is not necessary.

Here the letter is typed on the computer screen, and by pressing a single button

the massage can be transmitted to any part of the world.

Some useful E mail addresses are indicated below:

Poison Centre - [email protected]

Zoological gardens - [email protected]

Museaum - [email protected]

Assignment - 5Prepare a table to compare features of an e-mail with those of ordinary post.

Take the workforce of receivers/cost/speed into consideration.


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Exercises

1) Following are the statements given by a student about his experiences.

Answer the questions using those statements. Your answer should be based on

the properties and behaviour of light.

I. Though it is easy to look at a rough paper on which a beam of light is fallen. It

is difficult to look at a mirror on which a beam of light is fallen. Why is that?

II. Various shadows of figures can be

created by keeping hands across the

path of beam of a light. One of the

reasons for shadows is that light does

not go through the hands. Write another

reason.

III. The real depth of a river is higher than

the apparent depth.

a) What property of light causes

this?

b) State the connection between

the size of apparent size and the

real size of an object in the bot-

tom of a deep river.

AB indicates a ray of light com-

ing from an object in the bot-

tom of a river. What is the way

it comes to our eye BC, BD or

BE?

2) Sound travels through various media like air, water and metal.

A) i. Voice cannot be heard on the moon, as on the earth. Why?

ii. Though the sounds of a distant train cannot be heard through air it can be

heard by keeping ear on a bar of metal, which is placed so that ittouches the rail. Why?

iii. Why do the doors and windows of houses vibrate when thundering?

B) We cannot hear the sound of all the vibrating frequencies.

i. Define vibrating frequency?

ii. What is the range of frequencies of human?

iii. Name an animal which can exceed the human range of frequency.

apparent

state

B

A

E

D

C

real state

light n sound

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