Field Guide: Waves and Waves Properties
1) Wave Properties
a) Our first sighting of a wave
i) Wave: rhythmic disturbance that carries energy through matter or space
ii) Amplitude: the maximum amount of displacement of a particle on the medium
from its rest position
iii) Frequency: how often the particle of the medium vibrates when a wave passes
through the medium
(Waves - Lesson 1 The Nature of a Wave)
b) Classifying by duration (pulse and continuous progressive)
i) Wave pulse: a single bump or disturbance that travels through a medium
ii) Continuous (progressive) wave: Repeating bumps or disturbances that travels
through a medium
c) Classifying by orientation (Transverse, longitudinal)
i) Transverse wave: a wave that vibrates perpendicular to the direction of wave
ii) Crest: Point of the medium that exhibits the maximum amount of positive or
upward displacement from the rest position.
iii) Trough: The point on the medium that exhibits the maximum amount of
negative or downward displacement from the rest position.
iv) Longitudinal wave: the disturbance is in the same direction as, or parallel to,
the direction of wave motion ie. Sound wave
v) Compression: a point on a medium through which a longitudinal wave is
travelling that has the maximum density.
vi) Rarefaction: a region where the coils are spread apart, thus maximizing the
distance between coils.
d) Wave transports energy not matter
i) As disturbance moves through the medium, energy is transported from one
end of the medium to the other
e) Classifying by appearance
i) Travelling: When a wave travels through a medium, a crest is seen moving
along from particle to particle across a medium.
ii) Standing: a wave that vibrates back and forth from a positive displacement to
a negative displacement; they occur at regular time intervals
f) Classifying by Medium (mechanical and electromagnetic)
i) Mechanical: a wave that is not capable of transmitting its energy through a
vacuum; they require a medium in order to transport their energy from one
location to another. ie. Sound wave
ii) Electromagnetic: a wave that is capable of transmitting its energy through a
vacuum. They are produced by vibration of charged particles.
2) Waves at a boundary
a) Reflection (incident and reflected waves)
i) When any wave meets the boundary between two different media it is
partially reflected and partially transmitted.
ii) This diagram shows that when plane waves are reflected at a boundary. When
working with rays, by convention we always measure the angles between the
rays and the normal (perpendicular to the barrier). This diagram shows when a
single ray of light strikes a smooth mirror it produces a single reflected ray.
This type of ‘perfect’ reflection is different to the reflection that takes place
from an uneven surface.
iii) Diffuse reflection: When a ray strikes an uneven surface, it generates multiple
rays in all direction.
iv) Law of reflection: The incident angle is equal to the reflected angle. The
incident ray, the reflected ray and the normal all lie in the same plane
v) The incident angle is always taken as the angle between the incident ray and
the normal. The normal to a surface is the line at right angles to the surface as
shown above.
b) Refraction (incident and transmitted )
i) If plane waves are incident at an angle on the boundary between two different
medium, the transmitted wave will change direction; this is refraction.
ii) The change in direction is caused by the change in speed.
iii) For instance, in this example, light waves travel faster in air than in glass. So,
when the light waves strike the glass, due to the change in the speed of the
wave, it changes direction.
𝑠𝑖𝑛𝑖
iv) Snell’s law: The ratio 𝑠𝑖𝑛𝑟 = 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡, for a given frequency. The ratio is
equal to the ratio of the speeds in the different media.
𝑠𝑖𝑛𝜃1 𝑉1
=
𝑠𝑖𝑛𝜃2 𝑉2
3) Waves at apertures and obstacles
a) Diffraction
i) Diffraction: the effect of a wave spreading as it passes through an opening or
goes around an obstacle.
ii) When wave encounters obstacles, the waves pass around the barrier into the
regions behind it; subsequently the water behind the barrier is disturbed.
iii) The amount of diffraction increases with increasing wavelength and decreases
with decreasing wavelength.
iv) As wavelength (speed of wave) increases, the amount of diffraction increases.
On the other hand, as the wavelength decreases, the amount of diffraction also
decreases
v) Also, when the distance of an opening increases, there is less diffraction
vi) But, when the distance of the opening decreases, the waves spread around
more.
vii) So, diffraction becomes relatively more important when the wavelength is
large in comparison to the size of the aperture. The wavelength needs to be of
the same order of magnitude as the aperture for diffraction to be noticeable
4) Waves meeting waves
a) Interference (constructive and destructive)
i) If the waves have the same amplitude and the same frequency then the
interference at a particular point can be constructive or destructive
ii) Constructive: Interference that occurs at any location along the medium where
the two interfering waves have a displacement in the same direction. In this
case, both waves have an upward displacement. The medium has an upward
displacement that is greater than the displacement of the two interfering
pulses.
iii) Destructive: interference that occurs at any location along the medium where
the two interfering waves have a displacement in the opposite direction. These
two pulses completely destroy each other when they are completely
overlapped
b) Superposition (patterns and path differences)
i) Superposition: When two waves interfere, the resulting displacement of the
medium at any location is the algebraic sum of the displacements of the
individual waves at that same location.
5) Moving wave source or observer
a) Doppler Effect
i) Doppler Effect: the change of frequency of a wave as a result of the
movement of the source or the movement of the observer
ii) When a source is moving, sound waves are emitted at a particular frequency
from the source. The speed of the sound wave in the air does not change, but
the motion of the source means that the wave fronts are all ‘bunched up’
ahead of the source. This means that the stationary observer receives sound
waves of reduced wavelength.
iii) The overall effect is that the observer will heard sound at a higher frequency
than it was emitted by the source. This applies when the source is moving
towards the observer.
Bibliography
Kirk, T. (2007). Physics for the IB diploma (2nd ed.). Oxford: Oxford University Press.
Physics: Diffraction . (n.d.). Get Homework Help with CliffsNotes Study Guides .
Retrieved September 28, 2012, from http://www.cliffsnotes.com/study_
guide/Diffraction.topicArticleId-10453,articleId-10443.html
Serway, R. A., & Faughn, J. S. (2006).Holt physics. Orlando: Holt, Rinehart and
Winston.
Waves - Table of Contents. (n.d.). The Physics Classroom. Retrieved September 27,
2012, from http://www.physicsclassroom.com/class/waves/