1© Manhattan Press (H.K.) Ltd. Reflection of water waves Application of reflection Application of reflection 9.2 Reflection Phase change on reflection

© Manhattan Press (H.K.) Ltd. 1

• Reflection of water wavesReflection of water waves

• • Application of reflectionApplication of reflection

9.2 Reflection9.2 Reflection

• • Phase change on reflectionPhase change on reflection


Reflection occurs when a wave meets

a barrier or a different medium.

Longitudinal waveLongitudinal wave

Go to

More to Know 1More to Know 1

9.2 Reflection (SB p. 32)


Reflection of water wavesReflection of water waves

Angle of reflection (r) = Angle of incidence (i)



Application of reflectionApplication of reflection

Radar (Radio detection and ranging)

Working principles:Working principles:

- A short pulse of microwave is A short pulse of microwave is emitted in a certain direction.emitted in a certain direction.

- A small portion of the wave is A small portion of the wave is reflected back by the targeted object reflected back by the targeted object and received by the antenna.and received by the antenna.

- The received pulse will be displayed The received pulse will be displayed on CRO and analysed. on CRO and analysed.



RadarRadar

The time between the wave pulse transmission andThe time between the wave pulse transmission and

detection can be measured, so that the distance anddetection can be measured, so that the distance and

position of the object can be detected simply by the position of the object can be detected simply by the

equation below:equation below:

)taken( time Total)(2 distance trip Round )( light of Speed

tdc

2 ctd



RadarRadar

Radar can be used to:Radar can be used to:

1.1. detect aircraft or missiles; detect aircraft or missiles;

2.2. assist navigation of ships assist navigation of ships or aircraft;or aircraft;

3.3. control aircraft waiting to control aircraft waiting to land;land;

4.4. detect rainfall. detect rainfall.



Application of reflectionApplication of reflection

Sonar (Sound navigation and ranging)

Working principles:Working principles:

- Basically same as radar.Basically same as radar.

- The transmitter emits an ultrasonic pulse in a The transmitter emits an ultrasonic pulse in a certain direction.certain direction.

- The reflected wave from the targeted object The reflected wave from the targeted object will then be received and analysed.will then be received and analysed.

2 vtd

- The distance of the object can be calculated The distance of the object can be calculated by: where by: where vv is the speed of the is the speed of the ultrasonic pulse.ultrasonic pulse.



SonarSonar

There is a wide range of application There is a wide range of application of sonar and mainly applied to of sonar and mainly applied to ultrasonic imaging and ocean ultrasonic imaging and ocean technology.technology.

Sonar can be used to:Sonar can be used to:

1.1. scan a foetus in mother’s womb; scan a foetus in mother’s womb;

2.2. monitor the motion of blood in monitor the motion of blood in veins;veins;

3.3. measure the depth of the sea; measure the depth of the sea;

4.4. detect submarines; detect submarines;

5.5. detect shoals of fish; detect shoals of fish;

6.6. detect the flaws in solid materials. detect the flaws in solid materials.



Reflection from the ionosphereReflection from the ionosphere

The lower atmosphere up to 60 km contains almost neutral The lower atmosphere up to 60 km contains almost neutral molecules. However, most of the atmosphere’s free electrons and molecules. However, most of the atmosphere’s free electrons and positive ions (molecules ionized by solar ultraviolet and X-ray) are positive ions (molecules ionized by solar ultraviolet and X-ray) are located at level above 60 km, which is very much ionized.located at level above 60 km, which is very much ionized.

This region of the atmosphere is called the This region of the atmosphere is called the ionosphereionosphere..

There exists a There exists a cut-off frequency cut-off frequency ffcc , approximately 30 MHz (which , approximately 30 MHz (which

depends on the density of free electrons) in the ionosphere. depends on the density of free electrons) in the ionosphere. Electromagnetic waves below this frequency cannot propagate Electromagnetic waves below this frequency cannot propagate through the ionosphere, and therefore are reflected. through the ionosphere, and therefore are reflected.




The reflection from the ionosphere for the short radio wave The reflection from the ionosphere for the short radio wave (frequency ranged from 3 MHz to 30 MHz or wavelength from (frequency ranged from 3 MHz to 30 MHz or wavelength from 15 m to 30 m) enables it to be sent over a long distance in the 15 m to 30 m) enables it to be sent over a long distance in the order of 1 000 km.order of 1 000 km.

On the other hand, waves of frequencies above 30 MHz, for On the other hand, waves of frequencies above 30 MHz, for example, the VHF (very-high frequency) and UHF (ultra-high example, the VHF (very-high frequency) and UHF (ultra-high frequency) waves used in FM (frequency modulation) radio frequency) waves used in FM (frequency modulation) radio and TV (television) broadcasting respectively, can penetrate and TV (television) broadcasting respectively, can penetrate the ionosphere. Therefore, TV broadcast over long distance the ionosphere. Therefore, TV broadcast over long distance requires communication satellites for signal transmission and requires communication satellites for signal transmission and amplification.amplification.









Phase change on reflectionPhase change on reflection

1. According to 1. According to Newton’s 3Newton’s 3rdrd law, the law, the wall exerts an equal wall exerts an equal and opposite force on and opposite force on the string. the string.

2. The force causes an 2. The force causes an inverted pulse to travel inverted pulse to travel along the string in the along the string in the reverse direction. So the reverse direction. So the reflected wave and the reflected wave and the incident wave are out of incident wave are out of phase by phase by radians (1/2 radians (1/2 wavelength).wavelength).



Similar situation in reflection Similar situation in reflection of light waveof light wave


The similar situation occurs in light waves The similar situation occurs in light waves reflection. reflection.

When a light wave from air (less dense When a light wave from air (less dense medium) is incident on a glass (denser medium) is incident on a glass (denser medium) and is reflected, the phase change medium) and is reflected, the phase change is of π radians.is of π radians.


No phase change occurs if …No phase change occurs if …


On the other hand, no phase change occurs On the other hand, no phase change occurs if a light wave from a denser medium is if a light wave from a denser medium is incident on a less dense medium and is incident on a less dense medium and is reflected.reflected.


No phase change occurs if …No phase change occurs if …


1. The end of the 1. The end of the string is attached to a string is attached to a free end (ring) which free end (ring) which can move freely upcan move freely upand down. As the and down. As the wave pulse wave pulse approaches the end, approaches the end, the ring moves up.the ring moves up.

2. As the ring drops, a 2. As the ring drops, a reflected pulse is reflected pulse is produced. Since there is produced. Since there is no external force from no external force from the ring, there is no the ring, there is no phase change in this phase change in this case.case.


End


Stroboscope

A stroboscope is an apparatus that can be used to “freeze” a wave motion. The wave can be “frozen” with the same pattern if the strobe frequency is times the frequency of the wave. In fact, the actual frequency of the wave is the maximum strobe frequency that can “freeze” the wave.

n

1

Return to

TextText


Documents

1© Manhattan Press (H.K.) Ltd. Reflection of water waves Application of reflection Application of reflection 9.2 Reflection Phase change on reflection