Waves

It’s amazing what you can see on Google Earth.  Take a look at the sea around the Raglan to Kawhia coastline, for example.

You can immediately see why it attracts surfers – on the day the current satellite image was taken, there was a near-perfect set of waves rolling in off the Tasman. There’s a lot of wave effects you can make out on this image.

Out at sea, the waves are about 300 m apart, but closer to the shoreline they become closer together. That’s simply because deep water waves slow down as the depth of the sea reduces – since their frequency (the number of waves per second) remains the same, their wavelength (distance between the crests) has to reduce. You can see this effect beautifully at the entrance to Aotea harbour.

This effect also means that waves tend to come into shore reasonably perpendicular to the shore (have you ever wondered why waves never travel parallel to the beach no matter what way the wind is blowing?) – you can see this too on the image, on the coastline just north of the bush-clad Mt Karioi.

I’ve also had a look for ‘diffraction’ on the image – that’s where the wave ‘bends around’ a headland. It’s not the same process as the bending in shallow water, but the effect is somewhat similar – however, I can’t see any obvious examples.

Similar effects are seen with light waves – light slows down when it travels through certain materials (e.g. glass) – this is how a lens is able to bend light. In an optical fibre, these effects are exploited to ensure that all the light travels along the fibre, and none of it leaks out the side. And some of the latest technological developments are with negative refractive index materials – that’s kind of like water waves moving a negative depth of water – and the effects that can produce are quite bizzare indeed.

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