Download Senior Science topics Programme

Senior Secondary Science Programme
The Science of Sound and Light (part 2)
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.
Wavelength
Wave is the disturbance of space. For example, the transmission of sound in the air is caused
by the backward and forward movements of air particles in the air, while the propagation of
light is due to the oscillation of a pair of electromagnetic fields that are perpendicular to each
other. Regardless of whether it is the movements of air particles or the oscillation of
electromagnetic fields, when a wave is moving forward, there will be particles or
electromagnetic fields that have the same displacement from the equilibrium position at any
particular time instant. We say that they are vibrating “in-phase” at different positions.
Wavelength refers to the shortest distance between two points in a progressive wave that have
the same displacement from the equilibrium position at any particular time instant. In
transverse wave, wavelength is the distance between two neighboring points which are in
phase. It is usually the distance between two neighboring troughs or crests. In longitudinal
wave, wavelength is the distance between two neighboring compressions or rarefactions. A
wave with a higher frequency has a shorter wavelength, whereas a wave with a lower
frequency has a longer wavelength. Sound or light wave, the velocity of a wave in a particular
medium is constant. So the higher the frequency of a wave, the shorter the wavelength; the
lower the frequency, the longer the wavelength. Various types of sound and light can be
distinguished by their wavelengths. The highest frequency of vibration human eardrums can
sense is 20,000 Hz, which corresponds to a wavelength of about 2 cm. Sounds beyond this
frequency can’t be heard by the human ear. Sounds with a frequency higher than this are called
ultrasonic waves or ultrasounds.
The human eye can sense wavelengths within a range of approximately 400 nm to 800 nm,
namely from red to violet. Humans can’t sense light with a wavelength higher than this range,
such as infrared, microwave and radio wave; or light with a wavelength lower than this range,
such as UV rays, X-rays and Gamma rays.
2.
Reflection of wave
Reflection is a physical phenomenon. It refers to the process when wave passes from one
medium to another and changes its direction of propagation, going back to the original medium.
Sound waves can be effectively transmitted in water, so scientists make use of the echo
produced when sound is reflected to detect objects under the water. We can also make use of
the echo to search for clusters of fish in the ocean. A fish finder emits ultrasonic waves into the
sea. The fish clusters can be found by analyzing the echo received and calculating the time
needed for the sound wave to travel to and back from the target.
Compared with sound, light or electromagnetic wave can be transmitted in the air more
effectively. So by emitting radio wave and receiving the radio wave reflected by the object, the
location of offshore typhoons and other flying objects such as planes can be detected.
3.
Refraction of wave
Refraction is a physical phenomenon. When a wave passes from one medium to another, or
when there is a change in the speed when a wave is travelling in a medium, there is a change in
the direction of propagation. The speed of sound in the air is affected by the temperature. The
higher the temperature, the faster the speed of sound. During the day, the surface of the ground
is heated up by the Sun. The air near the surface is also warmed and its temperature rises,
while the air further up from the ground is cooler and the temperature is lower. Sound travels
faster in hotter air and will refract to the sky. So it is difficult for sound to travel for a long
distance. Conversely, the temperature is cooler during the night. The air near the ground also
becomes cooler, while the air higher up becomes warmer than the air near the ground, so sound
travels faster and further away from the ground than it does near the ground. The sound will
deflect towards the ground surface, making it easier to travel for a long distance. Thus it
becomes possible to hear sounds at night that cannot be easily heard during the day.
A mirage is an optical phenomenon that occurs in Nature. It happens when a virtual image is
formed by refraction as light propagates in the air. Cold air is denser than hot air, and light
travels slower through cold air. When light passes through the boundary between cool and hot
air, it will refract in the direction parallel to the boundary separating the two medium.
Under the scorching Sun, the sand on the ground becomes very hot. The air near the ground
can become warmer than that one meter above it by 5 degrees or more. Light from distant
objects will bend away from the ground towards the sky. When seen by an observer, the light
can easily be mistaken to be light from the ground. The “water” on the ground is actually the
mirror image of the blue sky. It is just like the reflection of the scenery reflected by a lake.
4.
Diffraction of wave
When a progressive wave encounters an obstacle or aperture, the path is deviated from its
original straight line. This phenomenon is known as diffraction.
Because of diffraction, sound can climb over the wall and get around obstacles to reach our
ears. Cellular phones and radio devices can still receive radio waves around corners and in
places blocked by obstacles. The diffraction of radio waves is the reason why we can receive
calls and pick up radio bands from behind a building or around the corner.
Light can also be diffracted. An obstacle is placed between a light source and a screen. A
diffraction pattern consisting of bright and dark bands can be observed.
Suggested Learning Activities
Preparation before viewing the programme
Teacher may discuss with students how they experience the phenomenon of reflection,
refraction and diffraction of wave in daily life.
Activities after viewing the programme
1. Student may, under the guidance of teacher, review the physical concepts presented in the
programme.
2. Teacher may compare the similarity and difference of sound and light in reflection,
refraction and diffraction.