6.2.2.1 Reflection of Light Wave

  1. The characteristic of reflection of light enables us to see objects. Objects that do not emit light are not seen in the dark. An object only is seen if light is incident on it a reflected back to our eyes

Experiment

  1. The experiment of reflection of light wave shows that reflection of light wave obeys the law of reflection.

 

6.2.2 Reflection

  1. Reflection occurs when an incident wave hits a reflector and reflected back.
  2. The direction of propagation of the wave changed when it is reflected.
  3. The wavelength, frequency and speed of wave remain unchanged.
  4. The amplitude of the wave may or may not change depend on the material of the reflector and the shape of the wavefront.


Reflection of Straight and Circular Wave

  1. Reflection of waves obeys the law of reflection, that is
    1. The angle of incident is equal to the angle of reflection
    2. The incident wave, reflected wave and the normal lie on the same plane.
  2. For reflection of circular wave, the distance of image from the reflector is equal to the distance of source of wave from the reflector.
(Reflection of Straight Plane Wave)

(Reflection of Circular Wave)

 

6.2.1 Ripple Tank


Q & A

What is the function of the Eccentric/Motor?

The function of the eccentric /motor is to produce a reciprocating motion.

Q & A

What is the function of the dipper?
  1. To produce waves of different shape 
  2. Straight parallel waves may be produced by a horizontal wooden bar. 
  3. Circular waves may be produced by a vertical ball-ended rod.

Q & A

What is the function of the sponge beach?

The function of the sponge beach is to prevent reflection of the waves.

Q & A

Explain how the dark and light bands are formed on the screen.
  1. The dark and light bands formed on the screen owing to the refraction of light. 
  2. As shown in figure above, when the light from the light house passes through the area around the peak of a wave, the light will be converged and form a bright band on the screen. 
  3. Conversely, when the light from the light house passes through the area around the trough of a wave, the light will be diverged and form a dark band on the screen. 

Phenomena of Waves

  1. There are 4 phenomena of waves:
    1. Reflection
    2. Refraction
    3. Diffraction
    4. Interference
  2. Diffraction and interference are unique phenomena. Only waves perform these phenomena.

 

6.1.6 Resonance

Natural Frequency

The Natural frequency of an oscillating system is the frequency of the system when there is no external force or forces acting on it.

Damping

  1. Damping is the decrease of amplitude of an oscillating system.
  2. An oscillating system experiences damping when its energy is losing to the surrounding as heat energy.
  3. Usually, the frequency of the system remain unchanged.
(Displacement-time graph of a damped oscillation)

(Amplitude-time graph of a damped oscillation)


Type of Damping

Damping can be divided into:
  1. internal damping, where an oscillating system loses energy due to the extension and compression of the molecules in the system.
  2. external damping, where an oscillating system loses energy to overcome frictional force or air resistance that act on it.

Force Oscillation

  1. In a damped oscillation, external force must be applied to the system to enable the oscillation to go on continuously.
  2. Oscillation with the help of external force or forces is called a force oscillation.

Resonance

In a force oscillation, if the frequency of the external force is equal to the natural frequency of the system, the system will oscillates with maximum amplitude, and this is named as resonance.

Examples of Resonance

  1. Opera singer breaks a wine glass with her voice due to the effect of resonance.
  2. Tacoma Narrow Bridge in USA collapsed in 1940 due to the effect of resonance.
  3. A moving bus produces excessive noise at certain speed when the frequency of the engine equal to the natural frequency of the bus.

Bartons Pendulum

The characteristic of resonance can be demonstrated with a Barton’s pendulum system.


Observation:
1. When pendulum X oscillates, the other pendulums are forced to oscillate.
2. Pendulum D will oscillates with the largest amplitude.

 

 

6.1.5 Displacement-Time Graph

Oscillation

  1. Waves are formed by a series of oscillation.
  2. In order to understand waves, we must understand oscillation.

Technical Terms Related to Oscillation

  1. An equilibrium position is a point where an oscillating object experiences zero resultant forces.
  2. A complete oscillation occurs when the vibrating object:
    1. moves to and fro from its original position and
    2. moves in the same direction as its original motion.



  3. Amplitude is the maximum amplitude of an object from its equilibrium position. The SI unit for amplitude is meter, m.



  4. The greater the amplitude, the greater the mechanical energy possessed by the oscillating system.
  5. Period is defined as the time required for one complete oscillation or vibration .
  6. Frequency, f is the number of oscillation that take place in one second. The SI unit for frequency is Herz (Hz).
  7. Frequency can be related to period by the following equation

    f = frequency
    T = Period


Example:
Given that a pendulum makes 20 oscillations in 25s. Find the frequency of the pendulum.

Answer:
Period,


Frequency



In a displacement-time graph, we can determine
  1. The displacement of the oscillating object at any time.
  2. The amplitude
  3. The period.


Example:

Figure above shows a displacement versus time graph for a vibrating object.
a. Find the amplitude, period and frequency for the vibrating system.
b. What is the displacement of the object at t = 0.3 s,
c. Sketch in the same axis above, a graph of a wave which the frequency and amplitude are half of the wave in the figure above.

Answer:
a.
The amplitude, A = 10cm
The period, T = 0.4s
The frequency,



b. The displacement at 0.3s = -10cm

c.



Comparing Displacement-Time Graph and Displacement- Distance Graph

(Displacement-time graph - Graph of oscillation)

(Displacement-distance graph - Graph of Waves)

  1. Both the displacement-time graph and the displacement distance graph looked similar. However they are 2 different types of graph.
  2. The displacement-time graph illustrate the displacement of an object over time whereas the displacement-distance graph tell the position of the vibrating particles of a wave.
  3. For a displacement- distance graph, the distance between 2 crest/trough represent the period whereas for the displacement-distance graph, it represents the wavelength.

 

6.1.4 Displacement – Distance Graph

  1. A Displacement – Distance graph shows the position of each particle in a wave relative to its distance from a reference point.
  2. The distance between two (2) successive crest or trough is the wavelength.
  3. The maximum displacement of the particles from the equilibrium position (displacement = 0) is the amplitude.
  4. The amplitude of the wave will increase as the energy transfers by the wave increase and vice versa.


 

6.1.3 Transverse Wave and Longitudinal Wave

Waves can be classified into 2 groups

  1. transverse wave
  2. longitudinal wave

Transverse Wave

A transverse wave is a wave where the particles of the medium vibrate in a direction that is perpendicular to the direction of the wave motion.

Example:
Light wave, ripple, radio wave

Longitudinal Wave

A longitudinal wave is a wave where the particles of the medium vibrate in a direction that is parallel to the direction of the wave motion.

Example:
Sound Wave

Transverse Wave – Crest and Trough

  1. When discussing wave, it’s important to know what is meant by the crest and trough of a wave.
  2. The point at which the displacement of the water from its normal level is highest called the crest of the wave
  3. The point at which the displacement of the water from its normal level is lowest called the trough of the wave. 

Longitudinal Wave – Compression and Rarefaction

  1. Unlike transverse wave, longitudinal waves have no crest and trough, instead, they have compression and rarefaction.
  2. In compression regions of longitudinal waves, wave particles of the medium are packed closer.
  3. In rarefaction regions, wave particles of the medium are packed further apart.

Finding Wavelength from a Diagram

Transverse Wave


Wavelength is the distance between two successive crest or trough.

Longitudinal Wave


Wavelength is the distance between two successive compression or rarefaction.

Wave front diagram


Wavelength is the distance between two successive wave front

Example 1:

Figure above shows the propagation of a water wave. What is the amplitude of the wave?

Answer:
Amplitude = 10cm/2 = 5cm

Example 2 :

The figure above shows a transverse wave. The wavelength of the wave is equal to
Answer:



Example 3:

The figure above shows the simulation of longitudinal wave by using a slinky spring. What is the wavelength of the wave?
Answer:



Example 4:

The figure above shows the simulation of transverse wave by using a slinky spring. What is the wavelength of the wave?
Answer:



 

6.1.2 Wavefronts

Phase

  1. A phase is the current position in the cycle of something that changes cyclically.
  2. Two vibrating particles are in the same phase if their displacement and direction of motion are the same.
    1. In phase – Same phase
    2. Out of phase – Different phase
    3. Anti-phase – Phase different = 180°

Wavefront

  1. A wavefront is a line or a surface that connects points that are moving at the same phase and has the same distance from the source of the waves.
  2. When a circular wave is formed, a circular wave front is formed.
  3. Characteristics of wavefront:
    1. wavefronts are always perpendicular to the direction of wave propagation. (As shown in the diagram below)
    2. all the points on a wavefront have same distance from the source of the wave.

Wavelength


The wavelength (λ) is defined as the distance between two successive particles which are at the same phase (exactly the same point in their paths and are moving in the same direction.).


 

6.1.1 Understanding Waves

What is Wave?

  1. A wave is a disturbance or variation that propagates through a medium, often transferring energy.
  2. Waves travel and transfer energy (its amplitude) and information (its frequency) from one point to another, with little or no permanent displacement of the particles of the medium.
Must Know:
Waves transfer energy without transferring physical matter.

 

 

5.5.6 Slide Projector

Function

Bulb
  1. Bulb with high brightness is used.
  2. The bulb must be placed at the centre of curvature of the concave mirror.
Concave mirror
  1. The function of the concave mirror is to reflect and focus light that shines on it to the direction of the condenser.
  2. This is to increase the brightness of the image.
Condenser
  1. The condenser consists of two Plano-convex lenses.
  2. The function of the condenser is to focus all the light that brightens the whole slide.
  3. It also acts as a heat insulator to stop heat from the bulb so it does not spoil the slide.
Slide
  1. The slide acts as the object.
  2. It is located at a distance between f and 2f from the projector lens so that the image produced is real and magnified.
  3. It is purposely placed upside down so that the image forms on the screen looks upright.
Projector Lens
  1. The projector lens projects the image on the screen that is placed a few meters away.
  2. It can be adjusted to focus a sharp image.
Image The image produced is
  1. real (it form on a screen)
  2. magnified
  3. inverted (Since the slide is placed upside down, hence the image looks upright)