Wave Properties 4.5
Reflection 4.5.1
 A wave reaching the end of its medium, but where the medium is still free to move, will be reflected (b),
and its reflection will be upright.
 A wave hitting an obstacle (fixed end) will be reflected (a), and its reflection will be inverted. It has
undergone a 180o phase change or  change in phase.
 This is because the instant the pulse hits the fixed end, the rope attempts to move the fixed end upwards
 It exerts an upwards force on the fixed end
 By Newton’s third law, the wall will exert an equal but opposite force on the rope
 This means that a disturbance will be created in the rope which, however is downwards and will start
moving to the left
 A wave encountering a denser medium will be partly reflected and partly transmitted; if the wave speed is
less in the denser medium, the wavelength will be shorter.
 The law of reflection: the angle of incidence equals the
angle of reflection.
 Reflection
 Visit this site for more in depth look at reflections.
 http://physicsquest.homestead.com/quest11.html#anchor_
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 These following wavefronts can be used to show
reflection (and refraction and diffraction and interference)
of water waves
The Law for Reflection
 The angle of incidence is equal to the angle of reflection
 Also - The incident ray, the reflected ray and the normal
lie on the same plane
 Use this rule for any ray or wave diagram involving reflection from any surface
 For circular waves hitting a flat reflector, the reflected waves appear to come from a source, which is the
same distance behind the reflector as the real source is in front of it
 Also a line joining these 2 sources is perpendicular to the reflecting surface
 If a plane wave is incident on a circular reflector then the waves are reflected so that they
 Converge on a focus if the surface is concave
 Appear to come from a focus if the surface is convex
 Echos
 In the case of sound, a source of sound can be directed at a plane, solid surface and the reflected sound can
be picked up by a microphone connected to an oscilloscope.
 The microphone is moved until a position of maximum reading on the oscilloscope is achieved.
 When the position is recorded it is found that again the angle of incidence equals the angle of reflection.
Refraction 4.5.2
 Why should a wave change direction when it changes
media?

Think about a car driving from concrete onto sand.
 Straight on
 Concrete to sand at an angle
 Sand to concrete at an angle
 Refraction
 As light travels from a less dense to a more dense
medium, it is bent towards the normal.
 As light travels from a more dense to a less dense
medium it is bent away from the normal
 Snell’s Law
 State that the amount that a wave refracts as it travels between two media of different densities, can be
expressed as a ratio of the speeds of the wave in the two media.

 n1 = sinθ2 = v2
 n2
sinθ1
v1
 n are the refractive indices
 v is velocity of wave
 θ1 = angle of incidence
 θ2 = angle of refraction
 NOT GIVEN FORMULA!!!!
 Where do you get your refractive index of a medium?
 n=c/v
 n is the refractive index
 c is the speed of light in vacuum
 v is the speed of light in medium
 Refractive indices of common media
Example Problem 1
 A ray of light is incident on the surface of the water in a pond with an angle of incidene of 35º. It bends,
producing an angle of refraction of 25.5º. Calculate
 a) the refractive index of the water
 b) the speed of the light in the water
 Answer: 1.33, 2.25 x 108m/s
Example Problem 2
 A ray of light is incident on the surface of the water in a pond with an angle of incidene of 35º. It bends,
producing an angle of refraction of 25.5º. Calculate
 a) the refractive index of the water
 b) the speed of the light in the water
 Answer: 1.33, 2.25 x 108m/s
Diffraction 4.5.3
 Consider every single point on the wavefront of the
wave as itself a source of waves. In other words a
point on the wavefront would emit a spherical
wavelet or secondary wave, of same velocity and
wavelength as the original wave.
 Therefore as a wave goes through a gap or passed
an obstacle the wavelets at the edges spread out.
 This is a demonstration of Huygens’ principle. It
can be used to predict the shapes of these
wavefronts.
 The new wavefront would then be the surface that is
tangent to all the forward wavelets from each point on the old wavefront.
 The amount of diffraction depends on the size of the obstacle compared to the wavelength. If the obstacle is
much smaller than the wavelength, the wave is barely affected (a). If the object is comparable to, or larger
than, the wavelength, diffraction is much more significant (b, c, d).
 Draw different types of diffraction
Examples of Diffraction 4.5.4
 Diffraction occurs when the wavelength of the wave is long compared with the aperture. This explains why
we cannot see around corners. The wave length of light is very short(about 6x10-7m).
 This also explains why water waves and sound waves can be diffracted as their wavelengths are relatively
long
 Diffraction provides the reason why we can hear something even if we cannot see it.
 An ordinary CD, when held at a sharp angle to a light source, will produce a spectrum characteristic of a
reflection grating. The narrow, closely spaced grooves in the disc diffract the reflected light and produce the
interference pattern that separates light into colors.
 A simple transmission grating can be made by looking at the light from a showcase filament with your eyes
nearly closed. Light passing through the narrow openings between your eyelashes will be diffracted and
give rise to an interference pattern with its characteristic bright and dark bands.
Interference; Principle of Superposition 4.5.5
 Principle of super position – the resultant
displacement at any point is the sum of the
separate displacements due to the two waves.
 The superposition principle says that when
two waves pass through the same point, the
displacement is the arithmetic sum of the
individual displacements.
 In the figure below, (a) exhibits destructive
interference and (b) exhibits constructive
interference.
 Constructive interference takes place when the
two waves are ‘in step’ with one another-they are said to be in phase. There is a zero phase difference
between them.
 Destructive interference takes place when the waves are exactly ‘out of step’-they are said to be out of
phase.
 There are several ways of saying this. One could say that the phase difference is equal to ‘half a cycle’ or
‘180 degrees’ or ‘π radians.’
 Constructive interference – mean the amplitudes add together
 Destructive interference – means the amplitudes cancel each other out.
 There are several ways of saying this. One could say that the phase difference is equal to ‘half a cycle’ or
‘180 degrees’ or ‘π radians.’
 For a more in depth look at superposition look at this activity.
 http://physicsquest.homestead.com/quest11ac2.html
 Interference can take place if there are two possible routes for a ray to travel from source to observer. If the
path difference between the two rays is a whole number of wavelengths, then constructive interference will
take place.
 Path difference=nλ
(constructive)
 Path difference=(n + ½ )λ
(destructive)
 For constructive or destructive interference to take place the sources of the waves must be phase linked or
coherent.
 Young’s Double Slit Experiment
 We can do this right now
 http://www.studyphysics.ca/newnotes/20/unit04_light/chp1719_light/lesson58.htm