Scribbling Waves
How do you draw a water wave?
Like this :
Let us find out what do we call the different parts of the wave :
The topmost points are called peaks and bottom most points are called falls.
Peaks are also called crests. Falls are also called troughs.
You can place a scale and measure distance between two near peaks.
Do you know what do we call this length? We call it a wavelength.
After you measured the distance between the near peaks, let us measure the distance between nearby falls.
Did you find the two distances same? Answer to this is YES.
Now, if we measure the distance between points where we have written A and B, we will again find this to be same.
So, we call Wavelength as distance between two near peaks or two near falls of a Wave.
Can you tell, which of the two waves drawn above have a larger wavelength?
Of course! The second wave has a longer wavelength than the first one.
Let us draw the portion of the wave between A and B.
Look at what the wave does.
The wave starts from A, and after traveling through one peak and one fall comes back to the same height at B.
We say that this wave has completed 1 oscillation.
Count the number of oscillations in the second wave picture of this page.
How many? Answer : 2 .
How many oscillations does the wave make in the first wave picture of this page?
This is a bit tricky. See if you obtain this answer: 2 and a half.
For you :
See how these waves travel in 1 second, then fill in the blanks .
Wave 1 completes …… oscillations in 1 second.
Wave 2 completes …… oscillations in 1 second.
Wave 3 completes ……. oscillations in 1 second.
We have another name for the number of oscillations a wave makes in a second.
We call it FREQUENCY OF THE WAVE.
The way we measure distance in meters , we measure frequency in Hertz.
So,
If a wave makes 3 oscillations in 1 second, we say its frequency is 3 Hertz.
If a wave makes 1000 oscillation in 1 second, we say its frequency is 1000 Hertz.
If a wave makes more number of oscillation in one second, you can see that it looks that it jiggles faster and its frequency is higher.
If a wave makes less number of oscillations in one second, you can see that it looks that it jiggles slower and its frequency is lower.
Thus ,
more number of oscillations = jiggles faster = higher frequency
less number of oscillations = jiggles slower = lower frequency
Which among the three waves you saw jiggle most?
WAVE 1 makes ….. oscillations in 1 second , jiggles ……… (most / more / least) and has frequency ….. Hertz.
WAVE 1 makes ….. oscillations in 1 second , jiggles ……… (most / more / least) and has frequency ….. Hertz.
WAVE 1 makes ….. oscillations in 1 second , jiggles ……… (most / more / least) and has frequency ….. Hertz.
You learned that you force air from your lungs through your vocal chords to your lips from where sound is produced.
You can force air from your lungs in different ways. Vocal chord will vibrate in a different manner in each of these ways and so will the air particles.
In some ways, they jiggle faster and in some ways they jiggle slower.
When they jiggle faster, we say that their “frequency” is high and when they jiggle slower, “frequency” is low.
Listen to these sounds :
Air vibrating with 100 oscillations per second ( 100 Hertz )
Air vibrating with 200 oscillations per second ( 200 Hertz )
Sometimes we can have sounds by mixing sounds of different frequencies. How do they sound ?
Like the following :
100 Hertz and 500 Hertz sounds played together
500 Hertz and 1000 Hertz sounds played together
A thunder sounds as VRHOOOOM, a very heavy sound. How is this different from a whistle you blow into very forcefully HURRRRRRRRRR?
The thunder sounds heavy , thick and low. The whistle sounds light, thin and high.
A sound which has more number of oscillations in one second has a higher frequency. Such a sound appears to us as light, thin and high when we hear it. We call such a sound HIgh Pitched.
A sound which has less number of oscillations in one second has a lower frequency. To our ears it appears as heavy, thick and low. We call it Low Pitched.
A thunder can have a frequency of 50 Hertz while frequency of a whistle can be 1000 Hertz.
Thus we find that the sound of thunder is low pitched and the sound of whistle is high pitched.
Other examples of Low Pitched sound : Growl of a lion, Noise of traffic.
Other examples of High Pitched sound : A baby crying, glass breaking .
For Teachers :
Small Science Teacher’s Book for Class IV tells you many ways in which you can interact with sound to recognize its frequency / pitch, loudness / intensity. Refer to Pages 105 – 111 in the chapter on Air.
A man’s voice and a woman’s voice sound different because they have different frequencies.
Listen to such different sounds played one after another
Finally answer the following :
Arrange the following waves in an order of increasing frequency.
Also arrange them in an order in which the one which jiggles most is at first and one which jiggles least is at last.
You learned what is a wavelength. Arrange the following in an order of increasing wavelength.
What are the kinds of wave we know about?
Light Wave, Sound Wave, Water Wave
All these waves have a wavelength and a frequency. Let us see how we can play with these.
What do you think you will get if you multiply a wave’ s frequency with a wavelength?
The answer is, you get the speed with which the wave is traveling!
Can you figure out how this is so?
Also ,
If you divide the speed of a wave by its frequency, you get its wavelength.
If you divide the speed of a wave by its wavelength,you get its frequency.
We measure frequency in Hertz
and
wavelength in meters / centimeters / micrometers or even angstroms
( 10000000000 angstroms make 1 meter. So small is an angstrom! )
Different colored light has different wavelengths, which we measure in thousands of angstroms.
Speed is measured in meters per second.
Our ears can only hear sounds of frequency from 20 Hertz to 20,000 Hertz.
Sounds which have higher frequency than 20 , 000 Hertz cannot be heard by us. They are called Ultrasonic. You may have heard of Ultrasonic Jet Planes or Dog-Whistles which produce ultrasonic sounds.
Dogs can hear frequencies from 40 Hertz to 60 , 000 Hertz , much higher than us.
Now we understand what do sound waves look like. You will find it fascinating how bats play with these waves to capture their food. They use Ultrasonic Sounds for this, which we cannot hear.
First we will introduce ourselves to bats , and only after that we will talk about how they “see with the help of sound waves “.
You should come back and read this page again when we talk about Echo Location.
So, first let us introduce ourselves to Bats.
Click : Introduction to Bats.
Text: Kumar Arunachal, June 2011
