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IGCSE Physics

Waves

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Lesson 5 – Diffraction and the wave equation

Aims:

•To recall and use the relationship between the speed, frequency and wavelength of a wave: wave speed = frequency × wavelength, v = f × λ

•To understand that waves can be diffracted through gaps or when they pass an edge, and that the extent of diffraction depends on the wavelength and the physical dimension of the gap.

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The ripple tankA ripple tank is a device used to study the behaviour of waves, because all waves behave in a similar manner.

A ripple tank produces water waves that can be reflected, refracted and diffracted.

paddle vibrates to

produce waves

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Reflection and refraction

paddleWhat do you think will happen if a barrier is placed in front of the water waves?

If it is a plane barrier then the waves are reflected.

barrier

What do you think will happen if a block is submerged in the ripple tank?

The change in depth of the water causes a change in speed of the waves – they are refracted.

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Barrier with large gap

What will happen if the width of the gap in the barrier is larger than the wavelength of the waves?

The waves pass through the gap unchanged apart from slight diffraction of the waves near their ends.

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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.

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Diffraction occurs at edges

In reality the waves curve slightly when they pass the edges of the gap.

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Barrier with wavelength-sized gap

What will happen if the width of the gap in the barrier is similar in width to the wavelength of the waves?

Circular waves are produced. This effect is known as diffraction.

Diffraction• Diffraction is the bending of waves as they pass through a gap or past an edge

When a wave (front) isWhen a wave (front) is Incident on an Incident on an edge or an obstacle/slit/gap Wave edge or an obstacle/slit/gap Wave ‘‘bendsbends’’ into the geometrical into the geometrical shadow/changes direction/spreadsshadow/changes direction/spreads

Diffraction effects are most obvious when the object or aperture causing the diffraction is similar in size to the wavelength of the waves.

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

diffraction of plane waves

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When a water wave hits an obstacle, the waves spread out round the edge and becomes curved. A small gap will cause more diffraction than a large gap.

The wavelength of the wave is not changed in diffraction.

Water wave diffraction

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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).

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Diffraction of SoundHow can the teacher in the corridor hear the school band even though he isn’t in the hall?

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Light wave diffraction

•Visible light waves are tiny less than a millionth of metre in length. There are not many things in our everyday lives that are this small so we do not see any diffraction of light waves.

•We can only see light that comes directly towards us or has reflected off another surface.

•Radio waves are much larger and diffract when they pass hills and buildings so that we can receive signals.

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How can houses in the shadows of hills receive a radio signal?

All electromagnetic radiation travels in

straight lines.Radio waves diffract around the hill to the

house.

Radio wave diffraction

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Summary – Diffraction and the wave equation

•Diffraction is the spreading out of a wave when it reaches a gap.