Post on 27-Jul-2020
Term 1 Week 11
Diffraction
Noadswood Science, 2011
Diffraction
To understand diffraction
Monday, April 9, 2018
Waves
• Waves carry energy without transferring matter
• Waves have three distinct properties – they can be reflected, refracted and diffracted…
Reflection Refraction Diffraction
Diffraction
• What is happening in the picture below (showing waves passing between two islands)…
• Light and sound are both waves – why is it that
you can hear around a corner, but not see?
Waves and boundaries
• A boundary is a place where conditions change.
What a wave does at a boundary depends
on the boundary conditions.
Waves can interact with boundaries in four
different ways...
Properties Of Waves
• Waves can reflect, refract and diffract…
Waves and boundaries
• The wave approaching a boundary is called the incident wave.
• The wave sent from a boundary is the reflected wave.
• A wave that is bent passing through a boundary is called a refracted wave.
This incident plane wave
refracts a circular wave
after passing through a
convex barrier.
Waves and boundaries
• Boundaries that are not straight can be used to change the shape of the wave fronts and therefore change the direction of a wave.
• A sharp boundary creates strong reflections.
• A soft boundary absorbs wave energy and produces little reflection.
Diffraction
• Diffraction is the spreading out of waves – all waves tend to spread out at the edges when they pass through a gap or past an object
• More diffraction occurs if the size of the gap is
similar to the wavelength
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Diffraction through a big gap
Shadow zone
Shadow zone
We often imagine that waves passing through a big
gap will behave like this.
16
Diffraction occurs at edges
In reality the waves curve slightly when they
pass the edges of the gap.
10/04/2018
10/04/2018Diffraction
More diffraction if the size of the gap is similar to the wavelength
More diffraction if wavelength is increased (or frequency decreased)
Diffraction is an effect seen when a wave travels around a corner or through a narrow gap:
Less occurs if
wavelength is
smaller than the
object.
More occurs if
wavelength is
larger than the
object.
The nearer the slit size is to the wavelength, the more the wave will diffract.
•1. The smaller the gap the greater the diffraction.
•2. The longer the wavelength the greater the diffraction
Sound
• The explosion cannot be seen over the hill, but it can be heard - due to diffraction…
Sound• A high frequency (short wavelength) wave does not get
diffracted much, so is more difficult to receive (i.e. TV / FM radio)
Sound• A low frequency (long wavelength) wave is
diffracted more, so is more likely to receive it (i.e. long wave radio)
Diffraction of Sound
DiffractionDiffraction is the bending of waves around
obstacles or the edges of an opening in the
same medium.
The amount of diffraction depends on the
nature of waves and their wavelength.
Sound waves diffract much more than light
waves.
Low-frequency (high-wavelength) sound
waves diffract more than high-frequency
(low-wavelength) light waves.
Sound & Light
• Sound can diffract quite a lot (i.e. you can hear around a corner) because the wavelengths are quite big (around 0.1m in air)
• Long wave radio (long wavelengths) can diffract a great deal, perfect for radio
• Higher frequency sounds have shorter wavelengths so diffract less
• Visible light has extremely short wavelengths, and will only diffract with a very narrow slit
Diffraction
• Sound can diffract through a doorway or around buildings – lower pitched sounds travel better than high-pitched sounds because low-pitched sounds have a long wavelength compared with the width of the gap, so they spread out more
• Ultrasound is sound with a high frequency – it has a very short wavelength compared with most gaps, so there is very little spreading making sharp focusing of ultrasound easier, which is good for medical scanning
• Light has a very short wavelength compared with most everyday gaps such as windows and doors –there is little obvious diffraction, so it produces sharp shadows
• Long wave radio signals are much less affected by buildings and tunnels than short wave radio signals or VHF radio signals – because of diffraction, radio signals can sometimes be received in the shadow of hills
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Sound wave diffraction
•Sound waves have wavelengths from about 15 m
down to 2 cm. Many everyday objects are the same
size as sound waves – this causes a lot of diffraction.
•This is why we can hear someone talking to us who is
not directly in our line of sight. Sound waves diffract
when they pass through doors or move past the edges
of walls.
•Diffraction of sound should not be confused with
reflection of sound (echos).
Waves at a beach
With the wind or Into the wind