Diffraction

Noadswood Science, 2011

Diffraction

To understand diffraction

Tuesday, April 18, 2023

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?

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