# 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.