PHYSICS: WAVES

PHYSICS: WAVES

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Overview

• A wave is a disturbance that travels across space and time
• Propagation of waves usually involves transference of energy without transferring mass. This is achieved by oscillations or vibrations around fixed locations
• Mechanical waves require a medium for transmission (e.g. sound)
• Electromagnetic waves do not require a medium and can travel in vacuum (e.g. light)
• Longitudinal waves are those with vibrations parallel to the direction of wave propagation. E.g. sound waves
• Transverse waves are those with vibrations perpendicular to the direction of travel. E.g. electromagnetic waves including light
• Waves on a string are an example of transverse waves

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Properties of waves

1. Reflection: It is the change in direction of a wave at the interface between two media. Examples include reflection of light, sound etc
2. Refraction: It is the change in direction of a wave due to a change in its speed. Examples: refraction of light when it passes through a lens
3. Diffraction: Bending of waves as they interact with obstacles in their path. Example: rainbow pattern when light falls on a CD or DVD
4. Interference: Superposition of two waves that come into contact
5. Dispersion: the splitting up of waves by frequency
6. Polarization: the oscillation of a wave in only one direction. Exhibited only by transverse waves (like light), not exhibited by longitudinal waves (like sound)

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Wave properties in everyday life

• The floor of a lake or the ocean appears closer than it actually is. This is because of refraction of light
• The red ring around the Sun is due to diffraction of light
• We can hear but not see across corners, this is because of diffraction of sound (e.g. we can hear but not see a person in the next room)
• The rainbow and the blue colour of sky are both due to dispersion of light
• Sunglasses use polarization filters to block glare

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SOUND WAVES

About Sound

• Sound is a mechanical wave that is transmitted as longitudinal waves through gases, plasma and liquids. However, in solids it can travel as both longitudinal and transverse waves
• Sound cannot travel in vacuum, it needs a medium for propagation
• The speed of sound in air is 330 m/s

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Perception of sound

• The frequency range 20 Hz to 20 MHz is known as the audible range, where human beings can detect sound waves
• The upper frequency limit decreases with age i.e. as we get older, our ability to detect higher pitches (shrills) decreases
• Other species uses different ranges for hearing. E.g. dogs can perceive frequencies higher than 20 KHz
• Increased levels of sound intensity can cause hearing damage. Hearing can be damaged by sustained exposure to 85 dB or by short term exposure to 120 dB sound. A rocket launch usually involves about 165 dB

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Sonar systems

• Sound Navigation and Ranging is a technology that uses sound propagation for navigation and communication
• Primarily used under water because light attenuates very quickly in water whereas sound travels farther
• First developed by R.W. Boyle and A.B. Wood in 1917 in Britain
• Applications include military, fisheries, wave measurement, ocean-floor mapping etc
• Sonar is used by marine mammals (like dolphins and whales) for communication as well
• Bats communicate by means of SONAR at frequencies over 100 MHz (beyond the human range)

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ELECTROMAGNETIC WAVES

Electromagnetic Spectrum

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Electromagnetic radiation and applications

Radiation	Applications
Radio waves	RADAR, TV, cell phones, microwaves
Microwaves	Wi-Fi
Infrared (IR)	Night vision, thermography, imaging
Visible light	Sight
Ultraviolet (UV)	Sun burn, water disinfection
X-rays	Astronomy, medicine
Gamma rays	PET scans, cancer therapy, astronomy, food sterilization

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Radar systems

• Radio Detection and Ranging is a technology that uses radio waves to identify moving and fixed objects
• Developed by Robert Watson-Watt in 1935 in Britain
• Radar works by measuring the waves that are reflected back from an object. Radar can detect objects at ranges where sound or visible light would be too weak
• Applications include aircraft detection, air traffic control, highway speed detection, weather detection etc

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More about electromagnetic waves

• Radio waves are reflected by the ionosphere and hence can be received anywhere on the earth.
• TV transmission penetrates the ionosphere and hence is not received like radio waves. Thus TV transmission is limited to line-of-sight
• At night, the radio reception improves because the ionosphere is not exposed to sunlight and hence is more settled
• Bats communicate by means of SONAR at frequencies over 100 MHz (beyond the human range). Other animals like dolphins and whales use SONAR as well

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JOHAR