Waves
Waves transfer energy between points, without transferring matter:
o When a wave travels between two points, no matter actually travels with it: The
points on the wave simply vibrate back and forth about fixed positions
The wavelength of a wave is the distance from a point on one wave to the same point on
the next wave. Usually this is measured from the top of one wave to the top of the next
wave
Wavelength is usually measured in meters (a distance)
The amplitude of a wave is its height, measured from the middle of the wave to its top
(or from the middle to its bottom)
Wave fronts are a useful way of picturing waves from above: each wavefront is used to
represent a single wave
The frequency of a wave is the number of waves passing a point (or being created or
received) every second – it is helpful to think of it as being the waves per second
The units of frequency are hertz (Hz)
Wave Front:
It is the imaginary line on a wave that joins all points that are in same phase. It is
usually drawn by joining down all the crests.
Plane dipper can produce plane waves in a ripple tank. Thus plane Wave
fronts are seen.
Spherical dipper on the other hand can provide circular waves in a ripple
tank. These types of waves have circular Wave fronts
Longitudinal & Transverse Waves
Waves can exist as one of two main types:
o Transverse
o Longitudinal
Transverse Waves
For a transverse wave, the points along the wave vibrate at 90 degrees to the
direction in which the wave is moving (the direction of energy transfer)
Longitudinal Waves
For a longitudinal wave, the points along the wave vibrate in the same
direction that the wave is moving in
The Wave Equation
The speed of a wave (v) is related to the frequency (f) and wavelength (λ)
by the equation:
Wave Effects
Reflection
When waves hit an object, such as a barrier, they can be
reflected:
When waves reflect off a barrier, the angle of reflection, r, is equal
to the angle of incidence, i
Refraction
When waves enter a different medium, their speed can change
This effect is called refraction, and it can have two other effects:
o The wavelength of the waves can increase or decrease
o The waves can change direction
o If the waves slow down the waves will bunch together,
causing the wavelength to decrease. The waves will also
start to travel closer to the normal
o If the waves speed up then they will spread out, causing
the wavelength to increase. The waves will also turn
slightly away from the normal
Demonstrating Wave Effects
The ripple tank is used to generate water waves in
laboratory. It is useful in demonstrating wave
properties such as reflection and refraction. It
consists of a shallow tray of water with a
transparent base, a light source directly above the
tray and a white screen beneath the tray to
capture the image of the shadows formed when
water waves spread across the tank as shown
above. Straight waves can be set up by using a
straight dipper, while circular waves can be formed
by using a spherical dipper. Both dipper are
vibrated up and down by a motor. The waves will
be seen in bright and dark patches on the screen
below the tray. These patches show the position of
the crests and troughs of the waves. The dark
patches will correspond to the crests and bright
patches will be the troughs.
Glossary
Crests and Troughs: These are the highest and the
lowest points of a transverse wave, respectively.
For longitudinal waves, the terms compression and
rarefactions are used.
Phase: Any two points are said to be in phase when
they move in the same direction and have the
same speed and in the same displacement from the
rest position
Wavelength( λ): This is the shortest distance
between any two points in a wave that are in phase
such as two successive crests or troughs. The SI
unit is metre (m).
Amplitude: This is the maximum displacement from
the rest or centre position. It is the height of crest
or trough measured from the rest position.
Period (t): Time taken for one point on the wave to
complete one oscillation. This can also be written
as the time taken, to complete one wave.
SI Unit is seconds(s)
Frequency(f): This is the number of complete
waves produced per second.
The SI unit is Hertz.
Wave Speed (V): The distance of the wave moved
in one second in the medium.
V=f ×λ
SI unit: metre per second (ms-1)
Wavespeed = frequency × wavelength
