Wave nature (Basic science)

WavesNature

Chaiporn PattanajakBanphue Pittayasan School

What are waves?

What are waves? A wave is a phenomenon in which

energy is transferred through vibration.

What are mechanical waves?

Mechanical waves are waves which require a medium.

A medium is a form of matter through which the wave travels (such as water, air, glass, etc.)

Waves such as light, x-rays, and other forms of radiation do not require a medium.

Transverse Waves

In a transverse wave the matter in the wave moves up and down at a right angle to the direction of the wave.

Transverse Waves are waves which travel in a

direction perpendicular to the direction of vibrations.

Direction of vibrations

Direction of wave

In a longitudinal wave the matter in the wave moves back and forth parallel to the direction of the wave.

Longitudinal Waves (compression waves)

are waves which travel in a direction parallel to the direction of vibrations.

Direction of vibration

Direction of wave

examples include sound wavessound waves

Longitudinal Waves (compression waves)

Characteristics of A Wave Crests and Troughs Amplitude Wavelength Frequency Period Speed

Crests and Trough Crests are high point of a wave Troughs are low point of a wave

displacement

distance

crest crest

troughtrough

Check your understood

distance

displacement

A

G

F

E

D

C

B

I

H

O J

1. Which points of crests ?

2. Which points of troughs ?

Ans. A , E , I

Ans. C , G

Amplitude (A) is the height of a crest or depth of a trough

measured from the normal undisturbed positions.

displacement

distance

crest crest

trough

Amplitude

Amplitude

is measured in metres (m)

Wavelength (λ) is the distance between two successive

crests or troughsdisplacement

distance

crestcrest crest

trough trough

Wavelength

Wavelength

is measured in metres (m)

Transverse waves

Longitudinal waves

wavelength

symbols of wavelength =symbols of wavelength = λ

λ

λ

Check your knowledge

distance

displacement

A

G

F

E

D

C

B

I

H

O J

Which pairs points are called wavelength? (true or false)

A ->C , B -> E , A -> E , D -> F , B -> F

frequency

or frequency is the number of waves that pass through a point in one second.

symbols of frequency =symbols of frequency = ff

f =f =n =n = number of wave , t = timenumber of wave , t = time

t

n

is the number of vibrations in one second

Unit is waves per second or Hertz (Hz).

Example

> 1 Hz = 1 wave per second.

st 1=

Hzond

wavef 1

sec1

1 ==

> 50 Hz is 50 vibrations in 1 second.

Test Yourselves Explain the meaning of

35 Hz, 100 Hz ,

Wave Period (T) It is the time it takes for one cycle to complete. It also is the reciprocal of the frequency.

AA

BB

CC

DD

OOAA

BB

CC

DD

AA

TT

a particle move around a particle move around

circlecircle.. (fig. I) (fig. I) 2

T

a period is a time move around circlea period is a time move around circle..

(fig. II)(fig. II)

A’A’A’A’

SI Unit For Period is measured in second (s)

Relationship between Period (T) and Frequency (f)

f = 100 Hz ⇒ T = 1/100 s f = 40 Hz ⇒ T = 1/40 s f = 8 Hz ⇒ T = 1/8 s

f = f Hz ⇒ sf1T =

T1f = Hz

frequency is relate period in equation :frequency is relate period in equation :

Speed Of a Wave is the distance moved in one second.

Wave speed is usually measured in meters/second, but may be measured using other distance units (such as centimeters per second).

Recall Speed of a motion

= distance ÷ time

t

sV =

Distance of wave =

=t

1

wavelength

frequency

where v = speed, s = distance and t = time

Wave Equation

Speed of a wave= wavelength ÷ period= wavelength x frequency

V = λf

where v = speed, f = frequency and λ = wavelength

ExampleA wave is traveling at a speed of 12 m/s and itsWavelength is 3 m. Calculate the wave frequency.

Solution1. This is what you know : speed (v) = 12 m/s , wavelength( ) = 3m.

2. This is you want to find : wave frequency ( f )

3. This is the equation you need to use :

4. Solve for f and then substitute the known value in the equation.

λ

fv λ=

λv

f =

Hzsxm

smf 4)/1(4

3

/12 ===

Example The wave in figure are

traveling across water. Each wave is 2m long, so that wavelength is 2m. One second later 3 wave have passed the flag. Find wave speed.

λ

SolutionSolution

1.1.This is what to This is what to knowknow

wavelength ( ) wavelength ( ) = 2m= 2m

Frequency (f)= Frequency (f)= = 3 Hz. = 3 Hz.

ond

wave

sec1

3

FromFrom

smHzxmv /6)3()2( ==fv λ=

Problem Problem

QuestionThe in A below are traveling across water.

a) Are the wave transverse or longitudinal?b) What is the wavelength of the wave?c) What is the amplitude of the wave?d) If 2 waves pass the flag every second, what is i) the

frequency ii) the period?e) Use the wave equation to calculate the speed of wave

in A?f) What is the wavelength of the wave in diagram B

bellow?g) If the wave in B have the same speed as those in A,

what is their frequency and period?

1. The diagram shows the outline of a water wave.What are the values of the amplitude and the wavelength?

amplitude / cm wavelength / cm

A 0.6 10 B 0.6 20 C 0.6 30 D 1.2 20

B

2. Which wave is longitudinal ?

A lightB radioC soundD water

C

D

3. The speed of 100 Hz wave was measured on four different days. The results are shown in the table. On which day the wave have the longest wavelength ?

day speed of wave

A 315 ms-1

B 320 ms-1

C 335 ms-1

D 340 ms-1

4. A source vibrates at frequency of 20 Hz and produces waves of wavelength 0.02 m.

At which speed do these waves travel out from the source ?

A 0.001 ms-1 B 0.021 ms-1 C 0.40 ms-1 D 20 ms-1 C

Hint:

v = λf

5. A VHF radio station broadcasts at a frequency of 60MHz (6.0 x 107 Hz). The speed of radio waves is 3.0 x 108ms-1.What is the wavelength of the waves broadcast by the station?

A 0.2 mB 0.5 mC 2.0 mD 5.0 m D

Hint:v = λf

Nov 1998

6. The diagram shows a coil spring along which a longitudinal wave is moving.(a) Mark on the diagram a distance equal to the wave length of the wave. [1]

(b) Label on the diagram with the letter C where the coils of the spring are compressed. [1]

Wave length

C

(continue in next slide)

C C

(c) Is the wave on the spring more like a sound wave or a light wave? [1]

(Cont. …) Q 6

Sound wave, because sound wave is a longitudinalwave but light wave is a transverse wave..

7. The diagram below shows a transverse wave travelling a rope.

(a) Mark on the diagram which is equal to (i) the wavelength, (ii) the amplitude of the wave. Label your answers. [2] (b) If the wavelength is 0.8m and the frequency is 2 Hz, what is the speed at which the wave moves along the rope ? [2]

wavelength

amplitude

v = λf = 0.8 x 2 = 1.6 ms-1

Nov 1996

8(a) Explain the difference between a transverse and a longitudinal wave. [2]

(b) State one example of each type of wave.TransverseLongitudinal [2]

Light Sound

Transverse wave is a wave that its direction of vibration is perpendicular to the direction of propagation. It can travel through vacuum. Longitudinal wave is a wave that its direction of vibration is parallel to the direction of propagation. It needs medium to travel.

9.(a) Explain the difference between transverse and longitudinal waves, referring in your answer to

the direction in which the waves travel. [2]

GCE ‘O’ LEVEL Nov 1995

Transverse waves are waves that their directionof vibration is perpendicular to their directionof propagation. Longitudinal waves are waves that theirdirection of vibration is parallel to theirdirection of propagation.

(continue on next slide)

9.(b) The speed of sound in water is 1500m/s. What is a wave length of a sound wave of frequency 250Hz travelling through water ? [2]

Since v = λftherefore 1500 = λ 250 λ = 1500 / 250 = 6.0 m

(Cont. …) Q. 9

10. The diagram shows the air pressure at different distances from a loudspeaker.

(continue on next slide)

(a) Mark carefully on the diagram: [2] (i) a distance which is the wavelength of the sound wave; (ii) the amplitude of the sound waves.

wavelength

amplitude

11 (b) If the wavelength of a sound wave of frequency 250 Hz is 1.4 m, calculate the speed of the wave. [2]

(Cont. …) Q. 11

since v = λftherefore, = 1.4 x 250 = 350 ms-1

11(a) With the aid of a diagram, explain what is meant by the frequency and wavelength of a wave. [5]

(b) What is a longitudinal wave ? [2]

λ

Frequency is thenumber of vibrationsper second.Wavelength is the distance from a point on a wave to its next corresponding point.It usually denote as λ ..

Longitudinal wave is a wave that its directionof vibration always parallel to the direction ofpropagation.

(continue in next slide)