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Diffraction

19 April 2023

Objectives

Be able to describe what diffraction is and be able to draw diagrams of it.

HSW: AF1: Describe phenomena making use of abstract ideas and models.

Used before in: Will use again in:

PLTS: Independent enquirers: support conclusions, using reasoned arguments and evidence.

Used before in: Will use again in:

KeywordsWavelength, wave-front, shadow region

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Outcomes

All students should be able to: Be able to describe what diffraction is.Most students should be able to: Be able to draw diagrams of diffraction

and describe how different situations alter the amount of diffraction.

Some students should be able to: Give examples of where diffraction

may be noticed in everyday life.

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Starter: How come we can hear sounds round corners but can’t see round corners?

• We will answer this at the end.

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Demonstration time!!

• Your teacher will show you an example of Diffraction on an online Ripple Tank. (offline simulator click here)

• Or perhaps in real life!!

• What happens when the slit is the same size as the wavelength of the wave?

• What about when the slit is a lot bigger?• Can you describe what happens when there is

an obstacle in the middle of the wave and to one side?

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Diffraction diagrams - Copy

• How would you describe how the gap width alters the amount of diffraction?

NB these should be small circular arcs

centred on the edge of the gap

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What is diffraction?

Diffraction is a property of all waves, which can be shown by water waves in a ripple tank.

Here, the waves travel along until they reach a gap. The width of the gap is similar to the wavelength of the waves.

The waves pass through the gap and spread out. This is diffraction.

So, when radio waves travel past an obstacle or through a narrow gap, they bend or spread out due to diffraction.

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How does wavelength affect diffraction?

The amount of diffraction depends on how the wavelength compares with the size of the gap.

What happens if the wavelength is much smaller than the width of the gap?

In this case, only the edges of the wave front are diffracted.

The amount of diffraction depends on wavelength.

The longer the wavelength, the greater the diffraction.

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Copy and complete the following – use the keywords below to help:

Diffraction is a property of all ………. When they pass through a ……… they spread out. The amount of diffraction depends on how the ……….. compares with the size of the gap. The amount of …………. increases as the wavelength becomes ………… and similar the the size of the gap. Diffraction also occurs around ………….. too.

wavelength waves diffraction

obstacles gap smaller

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• The same thing happens in harbours too.

Waves come from the sea

this way

Notice the circular ripples due to the waves diffracting through the gap

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Diffraction in a harbourWaves come from the sea

this way

Notice the circular ripples due to the waves diffracting through the gap

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Tsunami December 2004

• The wave was produced from a long fault on the sea bed. This is why Sri Lanka was badly affected

• Notice the diffraction around Sri Lanka towards southern India

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In hilly areas there may not be a clear path between the transmitter and the radio, so how can a signal be received?

How do radios work in hilly areas?

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Why can I receive radio but not TV?

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Problems with Radio and TV Reception

Long wavelength radio waves diffracted into the “shadow” region

Short wavelength radio waves (for TV) diffracted less

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Plenary: So - How come we can hear sounds round corners but can’t see round corners?

• Draw 2 diagrams to help (one showing light waves and one showing sound waves) and then write an explanation using these keywords: light, small, wavelength, similar, doorframe, sound, large, diffraction.

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Plenary: So - How come we can hear sounds round corners but can’t see round corners?

• Most spreading out of waves (diffraction) occurs when the wavelength is a similar size to the size of the gap.

• So the sound spreads out and can be heard around the corner,• But the light does not spread out and so the band cannot be seen.

My Answer:

•Light has a small wavelength.

•The wavelength is a lot smaller than the size of the gap (the door frame)

•Sound has a large wavelength similar to the gap size.

•How did you do? WWW/EBI