Electromagnetic+Waves

The most obvious electromagnetic wave we encounter is perhaps visible light, as we have an organ specifically designed to detect it. However there is much more the electromagnetic spectrum than just visible light. In fact we can also detect another frequency of the electromagnetic spectrum quite well: infrared radiation. We detect this as heat, for instance when we are in the sun, or in front of a fire. But we are blind to the rest of the spectrum, which stretches from radio waves (with the lowest frequency and longest wavelength) through microwaves, infrared, visible, ultraviolet and x-rays to gamma rays and cosmic rays.

The image on the left (from wikipedia) shows...

Light and vision
Light is an electromagnetic wave. Some of its behaviours are best explained by analogy with waves. Some if its behaviours are best explained by analogy with particles.

Wave nature of light.
The speed of light depends on the medium (e.g. air, water, glass) that it travels through. The more optically dense the material, the slower light travels. By analogy with a water wave (an easily visualised transverse wave), when a water wave moves into shallower water it slows down. Therefore if a straight water wave approaches shallower water at an angle, the wave changes direction as the part of the wave that has reaches the shallow water slows, while the part that has not reached the shallow water continues on at a greater speed:



An easy way to think of this is to imagine that the wave-crests are likes of soldiers marching along a field, and the shallower water is an area where the field become more muddy, slowing the soldiers down. In this way you can imagine the line slowing down, resulting in a kink in the line.

In the same way as light moves into a different medium it changes direction. As light enters a more optically dense medium it is bent towards the normal; similarly as it moves into a less optically dense medium it is bent away from the normal. The action of a lens can be explained following these rules, as can the displacement of an image in water (e.g. a coin in the bottom of a mug can be hidden by pouring water in).

By considering the **refraction** of waves in more detail we can understand the **dispersion** of sunlight by a prism, producing a **spectrum.** The frequency (f) and wavelength (l ) of waves are linked by the equation:

v = fl, where v = speed of the wave.

As we have already discussed, the speed of a wave changes depending on the medium that it is travelling through. (Remember, it is the speed of light in a vacuum that determines the cosmic speed limit - light travelling through air travels more slowly, and light travelling through water more slowly still). If the speed changes then so must either f or l.

As the frequency and amplitude of a wave is determined by the source, it is the wavelength that is reduced as the speed is reduced. So for light, the frequency (which determines the colour we see) and amplitude (i.e. the brightness of the source) is determined by the source, and the wavelength changes depending on the medium, with the wavelength being shorter as the speed reduces.

A useful summary from minutephysics (a youtube channel): media type="youtube" key="FAivtXJOsiI" height="315" width="560" align="left" //An aside on descriptions of light as a wave://

//For light a couple of these concepts require further explanation as we only think of light being a wave to help explain how it behaves in certain situation.//

//Light can be thought of as a particle (a photon) where the energy (or frequency) of the photon is determined by the change in energy levels of the electrons in the atoms giving off the light. As a result, one should not think of the amplitude of a light wave in the same way as the amplitude of a water wave. The amplitude of a light wave is effectively the rate at which photons are being emitted.//

//Similarly, we have to be careful talking about the speed of light decreasing as it moves through different mediums, as the speed of light is a constant, c (approx 300,000,000 m/s). However, the time it takes the photons to travel through a optically dense medium is increased by the time it takes for the atoms in the medium to re-emit the light. Importantly, as the photons hit the atoms of the medium the time it takes for them to be re-emitted depends on how closely the frequency of the absorbed light matches the resonant frequency of the electrons of the atom. Therefore light of different frequencies will be slowed down by different amounts by the interactions as they travel through the medium.//


 * Light moving through prisms**

As sunlight moves into a prism, the different frequencies of light are slowed down by different amounts (test idea: do water waves of different wavelengths get refracted at different angles when moving into a shallower area - probably not as the effect for light is caused by the absorption and re-emission of photons), so they are refracted by different amounts (but all towards the normal). Then as they exit the prism they are all refracted by different amounts away from the normal. But as a prism does not have parallel faces, these refractions are in the same direction in respect to the original beam of light, so the different frequencies of light exit the prism in different directions, creating a rainbow.


 * Microwaves**

1GHz (30cm wavelength) to 300GHz High frequency - more information Transmit via "tight beam" making it difficult to intercept

//Discovery// -CMB

//Sources//

//Uses// Phones use two bands, one from around 800 to 900Mhz (very short-wave radio), the other from 1.8 to 1.95Hz (very long wave microwaves).