2. 1© Manhattan Press (H.K.) Ltd. Reflection of water waves Application of reflection Application of reflection 9.2 Reflection Phase change on reflection Phase change on reflection

3. 2 © Manhattan Press (H.K.) Ltd. Reflection occurs when a wave meets a barrier or a different medium. Longitudinal wave Go to More to Know 1 More to Know 1 9.2 Reflection (SB p. 32)

4. 3 © Manhattan Press (H.K.) Ltd. Reflection of water waves Angle of reflection (r) = Angle of incidence (i) 9.2 Reflection (SB p. 32)

5. 4 © Manhattan Press (H.K.) Ltd. Application of reflection Radar (Radio detection and ranging) Working principles: -A short pulse of microwave is emitted in a certain direction. -A small portion of the wave is reflected back by the targeted object and received by the antenna. -The received pulse will be displayed on CRO and analysed. 9.2 Reflection (SB p. 33)

6. 5 © Manhattan Press (H.K.) Ltd. Radar The time between the wave pulse transmission and detection can be measured, so that the distance and position of the object can be detected simply by the equation below: 9.2 Reflection (SB p. 33)

7. 6 © Manhattan Press (H.K.) Ltd. Radar Radar can be used to: 1. detect aircraft or missiles; 2. assist navigation of ships or aircraft; 3. control aircraft waiting to land; 4. detect rainfall. 9.2 Reflection (SB p. 33)

8. 7 © Manhattan Press (H.K.) Ltd. Application of reflection Sonar (Sound navigation and ranging) Working principles: -Basically same as radar. -The transmitter emits an ultrasonic pulse in a certain direction. -The reflected wave from the targeted object will then be received and analysed. -The distance of the object can be calculated by: where v is the speed of the ultrasonic pulse. 9.2 Reflection (SB p. 33)

9. 8 © Manhattan Press (H.K.) Ltd. Sonar There is a wide range of application of sonar and mainly applied to ultrasonic imaging and ocean technology. Sonar can be used to: 1. scan a foetus in mother’s womb; 2. monitor the motion of blood in veins; 3. measure the depth of the sea; 4. detect submarines; 5. detect shoals of fish; 6. detect the flaws in solid materials. 9.2 Reflection (SB p. 33)

10. 9 © Manhattan Press (H.K.) Ltd. Reflection from the ionosphere The lower atmosphere up to 60 km contains almost neutral molecules. However, most of the atmosphere’s free electrons and positive ions (molecules ionized by solar ultraviolet and X-ray) are located at level above 60 km, which is very much ionized. This region of the atmosphere is called the ionosphere. There exists a cut-off frequency f c, approximately 30 MHz (which depends on the density of free electrons) in the ionosphere. Electromagnetic waves below this frequency cannot propagate through the ionosphere, and therefore are reflected. 9.2 Reflection (SB p. 34)

11. 10 © Manhattan Press (H.K.) Ltd. Reflection from the ionosphere The reflection from the ionosphere for the short radio wave (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 order of 1 000 km. On the other hand, waves of frequencies above 30 MHz, for example, the VHF (very-high frequency) and UHF (ultra-high frequency) waves used in FM (frequency modulation) radio and TV (television) broadcasting respectively, can penetrate the ionosphere. Therefore, TV broadcast over long distance requires communication satellites for signal transmission and amplification. 9.2 Reflection (SB p. 34)

12. 11 © Manhattan Press (H.K.) Ltd. Reflection from the ionosphere 9.2 Reflection (SB p. 34)

13. 12 © Manhattan Press (H.K.) Ltd. Reflection from the ionosphere 9.2 Reflection (SB p. 35)

14. 13 © Manhattan Press (H.K.) Ltd. Phase change on reflection 1. According to Newton’s 3 rd law, the wall exerts an equal and opposite force on the string. 2. The force causes an inverted pulse to travel along the string in the reverse direction. So the reflected wave and the incident wave are out of phase by  radians (1/2 wavelength). 9.2 Reflection (SB p. 35)

15. 14 © Manhattan Press (H.K.) Ltd. Similar situation in reflection of light wave 9.2 Reflection (SB p. 36) The similar situation occurs in light waves reflection. When a light wave from air (less dense medium) is incident on a glass (denser medium) and is reflected, the phase change is of π radians.

16. 15 © Manhattan Press (H.K.) Ltd. No phase change occurs if … 9.2 Reflection (SB p. 36) On the other hand, no phase change occurs if a light wave from a denser medium is incident on a less dense medium and is reflected.

17. 16 © Manhattan Press (H.K.) Ltd. No phase change occurs if … 9.2 Reflection (SB p. 36) 1. The end of the string is attached to a free end (ring) which can move freely up and down. As the wave pulse approaches the end, the ring moves up. 2. As the ring drops, a reflected pulse is produced. Since there is no external force from the ring, there is no phase change in this case.

18. 17 © Manhattan Press (H.K.) Ltd. End

19. 18 © Manhattan Press (H.K.) Ltd. 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. Return to Text 9.2 Reflection (SB p. 32)