6.3.1 Sound Wave

Posted on April 17, 2020 by user

Characteristic of Sound Wave

Sound wave is a series of compression and rarefaction of layers of air molecules repeatedly through space.
The forward and backward vibration of the air molecules in the direction of motion of a sound wave shows that sound is a longitudinal wave.

Propagation of Sound Wave

Sound wave is a mechanical wave that requires a medium for its propagation. Therefore sound wave cannot propagate in vacuum.
The medium for propagation can be solid, liquid or gas.
Sound waves propagate fastest in solid and slowest in gas.

Type of Sound Wave

Human ear is capable of hearing sound with frequency in the range of 20Hz – 20,000Hz, and the sound wave with frequency in this range is called an audio/Sonic wave.
Sound wave with frequency lower than 20Hz is called an Infrasonic Wave.
Sound wave with frequency higher than 20,000Hz is called an Ultrasonic wave.

Infrasonic	Audio/Sonic	Ultrasonic
<20Hz	20Hz – 20000Hz	>20 kHz

Speed of Sound Wave

Speed of sound wave in solids is greater than in liquids, which is greater than in gases.
Speed of sound in air is not affected by pressure, but is affected by the temperature.
As temperature increases, speed of sound in air (and other gases) is also increases.
Sound usually travels more slowly with greater altitude, due to reduced temperature.
Speed of sound can be calculated by the equation

6.2.5.4 Interference of Sound Wave

Posted on April 17, 2020 by user

Experiment

Figure below shows the apparatus set up to investigate the interference of sound wave.
Two distanced speakers are connected to the same audio generator.
A microphone is move in front of the 2 speakers from left to right.

Result:
Alternating loud and soft sounds are detected as the microphone is moved from left to right.

Formula

Wavelength of the sound wave is given by the equation

a = Distance between two loudspeakers
x = Distance between two successive loud sound.
D = Distance between the straight line path and the two loudspeakers.

Q & A

Explain briefly why alternating loud and soft sound is detected in front of the speakers?

The alternating loud and soft sounds are caused by interference of the sound wave.
The loud sound corresponds to the occurrence of constructive interference.
The soft sound corresponds to the occurrence of the destructive interference.

Q & A

Explain why the experiment is not suitable to be done in a lab.

Sound wave will be reflected by the wall and causes interference of reflected waves.

Q & A

How do we make sure that the sources of the sound wave are coherent?

The two loud speakers are sources of two coherent sound waves as they are connected to the same audio signal generator.

6.2.5.3 Interference of Light Wave

Posted on April 17, 2020 by user

Young’s Double Slits Experiment

Young's double-slit is made up of two slits ruled on a piece of slide and painted with 'aquadak'.
When monochromatic light passes through the Young's double-slit, diffraction of light occurs and two coherent sources (same amplitude, frequency and phase) are produced.
The two coherent sources will overlap and superpose to produce the effects of constructive and destructive interference.
Constructive interference generates bright fringes while destructive interference produces dark fringes.
Fringe size of bright fringes and dark fringes are about similar in size.
Requirement for the interference pattern to be observable:
1. Monochromatic light is used
2. The size of the slit need to be very small
3. The distance between the 2 slits must be small (About 0.5mm)
The distance between fringes increase as
1. light with longer wavelength is used
2. the 2 slits is closer
3. the distance between the double slit and the translucent screen is increased.
The wave length of light wave can be calculated by using the following equation: $λ = \frac{a x}{D}$

Compare to the pattern of single slit experiment

Young's Double Slits Experiment (Interference)

Single Slit Experiment (Diffraction)

Q & A

What is meant by a monochromatic light?

Monochromatic light is light with only one colour.

6.2.5.2 Interference Pattern

Posted on April 17, 2020 by user

Nodal Line and Anti-nodal Line

An anti-node is a point of maximum amplitude where constructive interference occurs. Whereas a node is point of minimum amplitude where destructive interference occurs.
The anti-nodal line joins all anti-node points. The nodal line joins all node points.

Formula for Interference

The distance between two antinode line, x, can be related to the wavelength, λ, by the follwing equation:

λ = Wavelength
a = Distance between the two wave sources
x = Distance between two successive anti-node lines or node lines
D = Distance from the wave sources to the plane where x is measured.

6.2.5.1 Principle of Superposition

Posted on April 17, 2020 by user

The principle of superposition states that where two or more waves meet, the total displacement at any point is the vector sum of the displacements that each individual wave would cause at that point.

(Meeting of two peaks)

(Meeting of a peak and a trough)

Coherent Wave

Two wave sources which are coherent have same frequency or have constant phase difference.

6.2.5 Interference

Posted on April 17, 2020 by user

The effect of superposition of two coherent waves is interference.
Interference is a phenomenon of wave caused by the superposition of 2 or more coherent waves.
The effect of interference can be studied using a ripple tank. Two dippers are used to produce two sources of coherent waves.
Figure 3 shows the interference pattern observed.
Interference can be constructive or destructive.

Constructive Interference and Anti-node

In the interference of 2 waves, there are places where the 2 waves are always in phase (same phase) and the superposition of the waves produces oscillation with maximum amplitude. This is called the constructive interference.
A place where constructive interference occurs is called the anti-node.

Destructive Interference and Node

In the interference of 2 waves, there are also places where the 2 waves are always anti-phase (phase difference = 180°) and the superposition of the waves produces oscillation with minimum amplitude. This is called the destructive interference.
A place where destructive interference occurs is called the node.

6.2.4.2 Diffraction of Sound Wave

Posted on April 17, 2020 by user

Experiment

Observation

The street cleaner can hear the sound of the radio even though it is behind an obstacle.

Explanation

The sound of the radio spreads around the corner of the wall due to diffraction of sound wave.

6.2.4.1 Diffraction of Light Wave

Posted on April 17, 2020 by user

Pin Hole Experiment

Light is diffracted when passes through a pin hole.
Diffraction pattern is made up of alternate bright and dark ring..

Single Slit Experiment

Light is diffracted if it passes through a narrow slit.
Diffraction pattern is made up of the bright bands and dark bands of different width.
The central band is wider and brighter. The dark and bright bands of narrower width are alternatively observed on the left and right side of the central bight band.
Diffraction pattern obtained will be clearer if the size of the slit decreases.
Conditions for diffraction to take place are:
1. Light source must be monochromatic. That is, the light must possess only one wavelength.
2. Slit size must be small enough as compare with the wavelength of light.

Factors affecting the pattern of diffraction

The factors that will affect the distance between the bright bands are

Size of the slit
Colour of the light
Distance of the screen from the slit

Experiment 1
Effect of the size of slit on diffraction pattern

Conclusion:
The smaller the size of the slit, the greater the effect of diffraction.

Experiment 2
Effect of the colour of light on diffraction pattern

Conclusion:
The longer the wavelength, the greater the effect of diffraction.
(Red light has longer wavelength while purple light has shortest wavelength.)

Experiment 3
Effect of the distance of the screen from the slit on diffraction pattern

Conclusion:
The further the screen from the slit, the further the distance between bright bands formed on the screen.

6.2.4 Diffraction of Waves

Posted on April 17, 2020 by user

Diffraction is the spreading of a wave when it travels through an opening or a small obstacle.

(Diffraction happen when waves pass through an opening)

(Diffraction happen when waves encounter an obstacle)

Characteristics of Diffracted Wave
1. Wavelength does not change.
2. Frequency does not change.
3. Speed of diffracted does not change.
4. The amplitude of the wave decreases after diffraction.

Factors Affecting the Magnitude of Diffraction

The magnitude of diffraction (or angle of diffraction) depends on

The wavelength
The size of the opening

Experiment 1

Shorter wavelength - The wave spreads to a smaller area

Longer wavelength - The wave spreads to a wider area

Conclusion:
Diffraction is affected by the wavelength. The longer the wavelength, the greater the effect of diffraction

Experiment 2

Bigger opening - The wave spreads to a smaller area

Smaller opening - The wave spreads to a wider area

Conclusion:
Diffraction is affected by the size of the opening. The smaller the size of the opening, the greater the effect of diffraction

6.2.3.2 Refraction of Sound Waves

Posted on April 17, 2020 by user

Experiment 1

Result

The amplitude of the sound wave increases when a balloon filled with Carbon dioxide is placed between the speaker and the microphone.

Explanation

Carbon dioxide is denser than air.
The sound wave is refracted approaching the normal when the wave propagates from the air into the balloon, and away from normal when moving out from the balloon.
As a result, the balloon acts as a biconvex lens and converge the sound wave at a point.

Experiment 2

Result

The amplitude of the sound wave decreases when a balloon filled with Carbon dioxide is placed between the speaker and the microphone.

Explanation

Helium gas is less dense than air.
The sound wave is refracted away from the normal when the wave propagates from the air into the balloon, and closer to the normal when moving out from the balloon.
As a result, the wave is diverged to a wider area and causes the amplitude of the sound wave decreases.

Phenomenon Related to Refraction of Sound Wave