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WAVES (EDEXCEL)

  • Waves transfer energy without transferring matter from a point to another.

  • When light waves pass from a phone to your eye, only energy is being transferred.

  • In water waves, it's the wave that travels and not water itself.

  • Direction of propagation: direction in which the wave is moving or travelling.

Mechanical Waves:

  • Need a medium to travel from a point to another.

  • Can't travel in vacuum.

  • E.g. sound waves, sea waves, seismic waves, waves in a rope or spring.

  • All of these require a medium to travel. Sound cannot travel without air, sea waves cannot travel without water.

Electromagnetic waves:

  • Don't need a medium to travel.

  • Can travel in vacuum and through a medium too.

  • E.g. gamma and X-rays, UV rays, visible light, infrared, microwaves, radio waves.

TYPES OF WAVE MOTION

Transverse waves:

  • The direction of vibration (of the source or the medium's particles) is at right angles to the direction of wave motion.

  • High points are called crests, and the low points are troughs.

  • E.g. EM waves, water waves, waves in a rope, seismic S-waves.

Longitudinal waves:

  • The direction of vibration (of the source or the medium's particles) is parallel to the direction of wave motion.

  • E.g. sound.

  • Particles of the source move back and forth along the same line in which the wave is travelling.

Compression and rarefaction:

  • Compression: particles are closest together (high pressure zone).

  • Rarefaction: particles are furthest apart (low pressure zone).

  • Particles do not move along with the wave, they only oscillate up and down their individual positions.

  • Animations in this link will help understand this:

https://www.acs.psu.edu/drussell/demos/waves/wavemotion.html

FEATURES OF A WAVE

Amplitude (A):

  • The height of a crest or the depth of a trough is amplitude.

  • The maximum displacement of a point on a wave from its mean position.

  • The maximum and minimum points of amplitude are same, there's only difference of signs, e.g. +8 and -8.

  • Measured in metres (m).

  • Larger the amplitude, the more the wave's energy.

Time period (T):

  • Time taken to produce one complete wave/oscillation.

Frequency (f):

  • The number of waves passing a point per second (or produced per second) is frequency.

  • Measured in Hertz (Hz).

  • Frequency = 1/T, where T is the time period.

  • Higher the frequency of a wave, smaller its wavelength.

Wavelength (λ):

  • The length of one complete wave is wavelength.

  • It's also the distance between two **consecutive** crests or troughs.

Wave speed (v):

  • The distance travelled by a waveform per unit time.

  • Wave speed = wavelength x frequency.

  • Measured in m/s.

Phase:

  • Two points are in phase if they have the same displacement and motion throughout the wave motion.

  • Points 1 wavelength apart are always in phase.

  • In this displacement-distance graph, points B and D have the same motion and displacement, so they are in phase. A and C are in phase too for the same reason. They have the same speed* in the same direction. Points B and D are not in phase with points A and C, however, because their directions of vibration are opposite.

Wavefront:

  • An imaginary line that joins all crests or troughs of waves travelling parallel to one another.

Reflection at a plane surface:

  • Reflection: the change in direction of a wave as it collides with an obstacle.

  • During reflection, frequency, wave speed and wavelength don't change.

  • The amplitude decreases after reflection due to loss of energy.

  • Larger the amplitude, more the wave's energy.

Drawing reflection:

  • The normal is an imaginary, dotted line that is at a 90 degree angle to the surface.

  • The incident wave is that which is approaching.

  • The reflected wave is the wave that's reflected.

Method:

  • Draw a normal line to the points where wave fronts strike the barriers (make sure it's dotted).

  • Note the angle between the incident wave and the normal (i.e angle of incidence).

  • Draw a reflected wave on the opposite side of the normal at the same angle.

  • The length of the reflected wave should be according to the size of the portion reflected from the barrier.

  • Make sure that the reflected waves are parallel to one another.

Refraction:

  • Refraction is the change in wave speed and wavelength as the wave travels from a medium to another, or from one depth to another.

  • For electromagnetic waves, refraction is when the EM waves slow down or speed up due to the change in medium of the wave in which it travels.

  • For water waves, when a wave travels from deep to shallow water, the wave speed and wavelength decrease.

  • Depth decreases, so wavelength decrease and so does the wave speed and vice versa.

  • Wavelength and wave speed are directly proportional.

  • If the change in depth is perpendicular to the direction of approaching waves, wavelength and wave speed will change and not direction.

  • If the depth of water is changed at an angle, the direction of the waves also changes along with the speed.

  • Unless the source of vibration changes, the frequency remains unchanged.

Diffraction:

  • When waves meet a barrier, they pass through the gap and spread out.

  • The spreading of waves at the edges of obstacles is diffraction.

  • The way the waves diffract depends upon how big the gap is.

  • The wave spreads out more if the gap is made narrower or if the wavelength is increased.

  • It spreads out less with a wider gap or a shorter wavelength.

YS

WAVES (EDEXCEL)

  • Waves transfer energy without transferring matter from a point to another.

  • When light waves pass from a phone to your eye, only energy is being transferred.

  • In water waves, it's the wave that travels and not water itself.

  • Direction of propagation: direction in which the wave is moving or travelling.

Mechanical Waves:

  • Need a medium to travel from a point to another.

  • Can't travel in vacuum.

  • E.g. sound waves, sea waves, seismic waves, waves in a rope or spring.

  • All of these require a medium to travel. Sound cannot travel without air, sea waves cannot travel without water.

Electromagnetic waves:

  • Don't need a medium to travel.

  • Can travel in vacuum and through a medium too.

  • E.g. gamma and X-rays, UV rays, visible light, infrared, microwaves, radio waves.

TYPES OF WAVE MOTION

Transverse waves:

  • The direction of vibration (of the source or the medium's particles) is at right angles to the direction of wave motion.

  • High points are called crests, and the low points are troughs.

  • E.g. EM waves, water waves, waves in a rope, seismic S-waves.

Longitudinal waves:

  • The direction of vibration (of the source or the medium's particles) is parallel to the direction of wave motion.

  • E.g. sound.

  • Particles of the source move back and forth along the same line in which the wave is travelling.

Compression and rarefaction:

  • Compression: particles are closest together (high pressure zone).

  • Rarefaction: particles are furthest apart (low pressure zone).

  • Particles do not move along with the wave, they only oscillate up and down their individual positions.

  • Animations in this link will help understand this:

https://www.acs.psu.edu/drussell/demos/waves/wavemotion.html

FEATURES OF A WAVE

Amplitude (A):

  • The height of a crest or the depth of a trough is amplitude.

  • The maximum displacement of a point on a wave from its mean position.

  • The maximum and minimum points of amplitude are same, there's only difference of signs, e.g. +8 and -8.

  • Measured in metres (m).

  • Larger the amplitude, the more the wave's energy.

Time period (T):

  • Time taken to produce one complete wave/oscillation.

Frequency (f):

  • The number of waves passing a point per second (or produced per second) is frequency.

  • Measured in Hertz (Hz).

  • Frequency = 1/T, where T is the time period.

  • Higher the frequency of a wave, smaller its wavelength.

Wavelength (λ):

  • The length of one complete wave is wavelength.

  • It's also the distance between two **consecutive** crests or troughs.

Wave speed (v):

  • The distance travelled by a waveform per unit time.

  • Wave speed = wavelength x frequency.

  • Measured in m/s.

Phase:

  • Two points are in phase if they have the same displacement and motion throughout the wave motion.

  • Points 1 wavelength apart are always in phase.

  • In this displacement-distance graph, points B and D have the same motion and displacement, so they are in phase. A and C are in phase too for the same reason. They have the same speed* in the same direction. Points B and D are not in phase with points A and C, however, because their directions of vibration are opposite.

Wavefront:

  • An imaginary line that joins all crests or troughs of waves travelling parallel to one another.

Reflection at a plane surface:

  • Reflection: the change in direction of a wave as it collides with an obstacle.

  • During reflection, frequency, wave speed and wavelength don't change.

  • The amplitude decreases after reflection due to loss of energy.

  • Larger the amplitude, more the wave's energy.

Drawing reflection:

  • The normal is an imaginary, dotted line that is at a 90 degree angle to the surface.

  • The incident wave is that which is approaching.

  • The reflected wave is the wave that's reflected.

Method:

  • Draw a normal line to the points where wave fronts strike the barriers (make sure it's dotted).

  • Note the angle between the incident wave and the normal (i.e angle of incidence).

  • Draw a reflected wave on the opposite side of the normal at the same angle.

  • The length of the reflected wave should be according to the size of the portion reflected from the barrier.

  • Make sure that the reflected waves are parallel to one another.

Refraction:

  • Refraction is the change in wave speed and wavelength as the wave travels from a medium to another, or from one depth to another.

  • For electromagnetic waves, refraction is when the EM waves slow down or speed up due to the change in medium of the wave in which it travels.

  • For water waves, when a wave travels from deep to shallow water, the wave speed and wavelength decrease.

  • Depth decreases, so wavelength decrease and so does the wave speed and vice versa.

  • Wavelength and wave speed are directly proportional.

  • If the change in depth is perpendicular to the direction of approaching waves, wavelength and wave speed will change and not direction.

  • If the depth of water is changed at an angle, the direction of the waves also changes along with the speed.

  • Unless the source of vibration changes, the frequency remains unchanged.

Diffraction:

  • When waves meet a barrier, they pass through the gap and spread out.

  • The spreading of waves at the edges of obstacles is diffraction.

  • The way the waves diffract depends upon how big the gap is.

  • The wave spreads out more if the gap is made narrower or if the wavelength is increased.

  • It spreads out less with a wider gap or a shorter wavelength.

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