Download Senior Science topics Programme

Senior Secondary Science Programme
The Science of Sound and Light (part 3)
Teachers' Notes
Subject
Physics
Level
S4-S6
Duration:
15 minutes
Key Points
1. Describe wave motion in terms of wavelength, wave and phase
2. Reflection, refraction and diffraction of wave
Content:
1.
Resonance
Demonstration:
Two tuning are placed side by side on a table. The two tuning forks are exactly the same, and
they have the same vibration frequency. If we place both tuning forks on a table and hit one of
them, the other one will also vibrate. This is a physical phenomenon known as resonance. Each
object vibrates at a particular frequency according to its own physical properties, size, shape,
mass, etc. For example, if we knock on this table, it always vibrates at a particular frequency.
We call this frequency its “natural frequency”.
Different examples of resonance are illustrated in this segment.
2.
Superposition of wave
When two waves from different sources meet, the resultant displacement of wave is simply the
algebraic sum of the displacements of two individual waves. This is called the principle of
superposition of waves. The same musical notes produced by different musical instruments
sound different to the ear. This is because of the difference in timbre. The shape of the
waveform determines its timbre. The wave generated by a violin can be viewed as the resultant
wave of the superposition of several waves of simple waveforms. When the body of an object
vibrates, it will generate a fundamental tone. The other parts of the object will also vibrate.
These vibrations add up to generate overtones. It is the combination of overtones that
determines the timbre of a sound.
3. Doppler effect
The Doppler Effect is the change in frequency of a wave when there is a relative motion
between the source and the observer. In the case of sound wave, as an ambulance is moving
forward while its sirens are on, passersby at different locations will each hear the siren’s sound
differently. When an ambulance is moving forward while its sirens are on, passersby at different
locations will each hear the siren’s sound differently. Therefore, when an ambulance with its
siren on moves closer to and then drives past an observer, the sound heard will change from a
higher pitched sound to a lower pitched sound. Likewise, in astronomy, the frequency or
wavelength of light emitted by moving celestial bodies also changes. The Doppler Effect is what
causes the seemingly changing colours of the lights. When a star is moving towards or away
from the Earth, its colour will change because of the Doppler Effect. When a star is moving
towards the Earth, the lights it emitted as observed from the Earth will seem to have shorter
wavelengths. The absorption lines in the spectrum shift to the blue side, i.e. to shorter
wavelengths. This is called blue shift. On the contrary, when a star is moving away from the
Earth, the wavelengths of the starlight increase and the absorption lines shift to the red side, i.e.
to longer wavelength. This is called red shift.
4.
Shock wave
When the speed of the source is faster than the speed of the wave in that medium, shock wave
is formed. When a jet is flying at supersonic speed, the sound source is moving faster than the
sound wave it generates. There are no sound waves in front of the jet because the sound waves
can’t catch up with the speed of the jet.. A jet flying at supersonic speed will generate two
shockwaves. The first shockwave is formed when the compressions of the waves generated at
various time instants superimpose to give a conical wavefront. The second one at the rear of
the jet is formed when the rarefactions of the waves generated at various time instants
superimpose. These two shockwaves spread out behind the jet and eventually reach the ground
where it causes the sonic boom. In certain environments such as a nuclear reactor or a particle
accelerator, charged particles can be accelerated to a speed higher than that of light. This will
lead to the generation of electromagnetic radiation similar to a shockwave of light. Then a
characteristic blue glow will be emitted. It is named “Cherenkow radiation” after the scientist
Cherenkow who was the first to detect it experimentally.
Suggested Learning Activities
Preparation before viewing the programme
Teacher may discuss with students to what extent do they know about the resonance of wave.
Activities after viewing the programme
1. Student may, under the guidance of teacher, review the physical concepts presented in the
programme.
2. Teacher can discuss with students to list the examples in daily life which can be used to
illustrate Doppler effect.