6.1 TRANSVERSE AND LONGITUDINAL WAVE

TRANSVERSE WAVE
Imagine a cork on a pond. Waves make the cork bob up an down but it doesn't move in the direction the wave is going. Light waves vibrate similarly, up and down and also from side to side, while the light energy moves forward. This is the characteristic of Transverse wave.











A Transverse Wave is a wave in which particles of the medium move in a direction perpendicular to the direction which the wave moves. The animation below shows a one-dimensional transverse plane wave propagating from left to right. The particles do not move along with the wave; they simply oscillate up and down about their individual equilibrium positions as the wave passes by. Pick a single particle and watch its motion.

• Transverse waves are mechanical waves. That means they need a solid, liquid, or gas medium to travel through.
• Transverse waves include all electromagnetic (radio wave, microwave, Infra Red, visible light, ultra violet., X-rays and Gamma rays)

LONGITUDINAL WAVE

• A longitudinal wave is a wave in which particles of the medium move in a direction parallel to the direction which the wave moves.
• Example: Sound waves

6.1 WAVEFRONT, AMPLITUDE, PERIOD AND FREQUENCY

WAVEFRONT
A line of surface which joins all points which have the same displacement at the same moment (they are all in phase).




The perpendicular distance between two wavefronts represents one wavelength.






Plane wavefront & Circular wavefront
Amplitudes

• The distance from the maximum displacement to the equillibrium position is called the Amplitudes, a.
• SI unit : meter, m
• Loud sounds have a large amplitude.
• Brightness in light have a large amplitude.

Wavelength
The distance between two adjacent point of the same phase of waves.

• The times taken for the system make a complete oscillation is known as Period, T.
• The number of complete oscillations made by a system in one second is called frequency, f.

The relationship between wavelength, frequency and speed of wave is:















Displacement-time graph










Displacement-distance graph

6.1 DAMPING & RESONANCE

DAMPING

• Damping is the decrease in the amplitude of an oscillating system when its energy is drained out as heat energy.
• The amplitude of an oscillating system will gradually decrease and become zero when the oscillation stops.
• Damping causes a decrease in amplitude and energy but the frequency of the oscillation remain unchanged.

In the diagram on the right the blue spring is experiencing damping but not the black spring. take note of the decrease in amplitude but the frequency is the same. The initial amplitude was 2 unit and it was decreasing. But the frequency remains as 4 complete oscillations in 25 s (0.16 Hz).

There are two types of damping: External Damping and Internal Damping.

1. External Damping

• Is lost of energy to overcome frictional force or air resistance.

2. Internal Damping

• Is due to compression and extension of atoms and molecules of the system.

To enable an oscillating system to go on continuously, an external force must be applied to the system.

• The external force supplies energy to the system.
• Such a motion is called a forced oscillation.

The frequency of a system which oscillates freely without the action of an external force is called the natural frequency.

Application of damping in our daily lives
Vehicle suspension system. A vehicle spring and shock absorber system provides damping to prevent the vehicle from bouncing up and down continuously.







How car suspension work.








RESONANCE

• Occurs when a system/matter oscillate at a frequency equivalent to its natural frequency by an external force.
• Due to resonance, particle/matter oscillate at a higher amplitude.

In the Barton's pendulum on the right when pendulum X was oscillated it supplied energy to the other pendulums making them oscillate. It was observed that pendulum C oscillates with the largest amplitude. This is because pendulum X and pendulum C have the same pendulum length. The natural frequency of an oscillating system depends on its pendulum length. Therefore pendulum X and pendulum C are of the same natural frequency. Hence pendulum C is at resonance.

Video clip for Resonance


Good effects of Resonance in our daily lives

1. A tuner in the radio or television enables you to select the channel we want. Te circuit in the tuner is adjusted until resonance is achieved, at the frequency transmitted by a particular station selected. Hence a strong electrical signal is produced.
2. The loudness of a wind musical instrument like trumpet and flute is the result of resonance of the air.

Bad effects of Resonance in our daily lives

1. A bridge may collapse when the amplitude of its vibration increases because of resonance. The action of the wind can cause the bridge to vibrate with a large amplitude as a result of resonance
2. The loud explosion can cause a glass window to break
3. The high pitch note of an opera singer can cause a thin glass to shatter

6.2 REFLECTION OF WAVES

REFLECTION OF WAVES

• Occurs when a waves strikes an obstacle.
• The wave will change its direction of propagation when it is reflected.

Characteristics of Reflection of waves:

• The value of frequency (f), wavelength ( λ) and speed (v) remain the same after reflection.
• The phenomenon of reflection of waves obeys the Laws of reflection.

Law of Reflection
The incident wave, the reflected wave and the Normal lie in the same plane which is perpendicular to the reflecting surface at the point of incident.
The angle of incident, i is equal to the angle of reflection, r.

Reflection of plane water waves in a ripple tank

• The phenomenon of reflection of water waves can be investigated using a ripple tank.
• The water waves are produced by a vibrating source on the water surface.

A horizontal bar will produced a plane wave.












While a round source will produce circular wave













See video wave reflections from ripple tank.


Diagram shows reflection of waves

WAVE 6.6 Analysing Sound Wave: Sonar

SONAR : TO DETERMINE THE DEPTH OF THE SEABED
1. Sonar: Sonar is reflection from an ultrasonic waves ( ultrasonic echoes)

• to locate underwater objects
• to measure the depth of a sea bed

2. Sonar using ultrasonic waves because :

• has higher frequency
• has more energy, so can move further
• do not produce noise

3. To measure the depth

Attach ultrasonic transmitter to a ship.Use a microphone receiver to detect ultrasonic pulses. Direct the ultrasonic pulses from the transmitter to the seabed. Use microphone receiver to pick up the reflected pulses from the seabed.
Measure the time taken by the pulses to travel to seabed and return. Calculate the depth of the water using the formula d=(v x t) / 2