Term 2 Week 1

Diffraction

Diffraction

To understand diffraction

Friday, May 3, 2019

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.

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Diffraction occurs at edges

In reality the waves curve slightly when they

pass the edges of the gap.

03/05/2019

03/05/2019Diffraction

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

• Lower pitched sounds travel better than high-pitched sounds

• Reason :- 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

Interference

• Where two waves meet their effects are added together – this is called interference

• Constructive interference is caused when the waves arrive in step, reinforcing each other to give a wave of greater amplitude

Interference

• Where two waves meet their effects are added together – this is called interference

• Destructive interference is caused when the waves arrive out of step, cancelling one another out

Interference Of Light

• If a laser is shone on two slits very close together, the diffracted beam can be seen on a screen to have bright and dark bands on it: -

– The bright bands show constructive interference of light

– The dark bands show destructive interference of light