Summer Holiday Science – The Physics of Waves
Many of us will be spending our summer holidays on the beach, making sandcastles and dipping our toes in the ocean. But have you pondered questions such as where do breakers come from and why is the sea blue in the first place? These questions can be answered with one word – waves.
Why Is the Sea Blue?
Pour some water – from the kitchen sink or the sea – into a glass and it appears clear, with no detectable blue colour. So why is it what the ocean appears to range between colours of blue and green, depending on the location. Many people think that the sea is just reflecting the colour of the sky, but that’s only half the answer. Remember those experiments in school where you used a glass prism to split light into the colours of the spectrum? Each colour of the spectrum travels at a different wavelength. Red and orange have a long wavelength whereas at the other end of the spectrum, blue and green is short wavelength light. Bodies of water, including lakes and seas, absorb long wavelength light better than short wavelength light. When the white light enters the water, it’s mostly the blue and green light which is reflected, making the water appear blue. The same thing happens with the sky.
Biology also has a role to play in the colour of the oceans. The principle of absorbing the red and orange frequency of light can be affected by impurities in the water. Sand, silt, rubbish and plants can all interfere with the way in which light is absorbed, making the water look brown, or even yellow. The most important role however is played by phytoplankton, tiny single-celled algae. These little organisms absorb electromagnetic radiation from both the blue and red end of the light spectrum, so seas with a high concentration of phytoplankton will appear green rather than blue.
What Causes the Waves?
Waves are all down to the moon and the tides, right? Well, yes and no. The ocean’s motion is a complex field of science, with factors as diverse as the global climate and weather systems in the upper atmosphere determining how water flows around the world. The main factor as to whether there are waves on the oceans, and how large those waves are, is the wind. The largest waves are found on parts of the coast where there are large oceans with nothing blocking the waves from crashing on shore – which is why Cornwall in the UK and the Californian beaches are so well known for their great surfing waves. The speed of the wind also affects the waves, as does the length of time we experience high winds from. If we’re lucky enough to experience a period of calm and settled weather over the summer periods, the result will be gentle waves which are perfect for paddling, not so great for surfing.
So Where Does the Moon Fit In?
Tides shouldn’t be confused with waves as although they both affect the movement of the sea, waves are caused by wind and weather patterns whereas the moon controls tides. The simplistic explanation is that the moon’s gravity pulls the water in our seas towards it, causing a high tide. Tides are also affected by the rotation of the earth. Rotational force also creates a “bulge” of water on the opposite side of the earth to the bulge caused by the moon. If the moon remained static in the sky, the rotation of the earth would mean tides switching between high and low every 6 hours. But the moon is moving too, and that means that rather than it being 12 hours exactly from one high tide to the next, it’s actually 12 hours and 25 minutes. This is why lengthy books of tide times and tables are published – to help sailors keep track of changing tide times. At times of year when the moon, sun and earth are all in alignment this can cause the phenomenon of Spring Tides, which despite the name can occur at any time of year. Spring tides are much higher and lower than average, and last for a few days until the earth and other celestial bodies move out of alignment once again.