Diffraction To understand diffraction Wednesday, April 29, 2015
Waves Waves carry energy without transferring matter Waves have three distinct properties – they can be reflected, refracted and diffracted… ReflectionRefractionDiffraction
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?
Properties Of Waves Waves can reflect, refract and diffract…
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
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 wavelength is increased (or frequency decreased)
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)
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
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
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