The presence of a medium (such as air or water) does effect the electric and magnetic fields, because media are made up of atoms, which are composed of positive and negative electric charges. First, a medium is not needed, as electric and magnetic field can exist in a vacuum. Okay, so for light we now have the wave speed and the "displacement." Let's address a couple other elements of light as a wave. Don't worry that this doesn't make much sense right now – it should be a bit clearer when you get to Physics 9C and study electricity & magnetism. So the "displacement" of such a wave is actually the electric and magnetic field vectors (both types of fields are waving simultaneously, with each inducing the other) in the space through which the light wave is traveling. It is traditional to denote this speed with a lower-case 'c': The wave equation included physical constants from both electricity and magnetism, and extracting the wave speed from this equation resulted in a number Maxwell was already familiar with – the speed of light. This is a recipe for propagation of these fields, and the equation he derived for this propagation was exactly the wave equation! So he predicted, from results taken from experiments in electricity and magnetism, that an electromagnetic wave could be produced. He showed that changing electric fields could induce magnetic fields, while changing magnetic fields could in turn induce electric fields. It all came together with an amazing (for the time) effort in mathematics by a man named James Clerk Maxwell. Electric currents were found to affect compass needles, and magnets moving near wires were found to create electric currents. It started becoming clear that the two forces, while different, had some links. Unlike every other wave we have seen, it doesn't require any medium at all! So what do we use as the "displacement" for our wave function?īack in the 19th century, physicists studied extensively the subjects of electricity (lightning, shocking your finger on a doorknob, balloons sticking to your hair, etc.) and magnetism (compasses, sticking things to your refrigerator, etc.). But we also know that we can see light from the sun, moon, and stars, which means that light waves can travel through the vacuum of space. We know that light is a wave based on how it behaves – it exhibits the same properties of other waves we have examined – it interferes with itself, it follows an inverse-square law for intensity (brightness), and so on. This difficulty gets greatly magnified for the case of light. The jump from mechanical waves to sound was a difficult one, mainly because the "displacement" of the wave changed from matter that oscillates back-and-forth, to (in the case of sound in a gas) oscillations in pressure or density.
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