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CSEC Physics: Electromagnetic Waves

Electromagnetic waves are the opposite of mechanical waves. The waves we discussed previously (i.e. sound waves) are mechanical waves, as they require a medium to pass through like air or water. Electromagnetic waves are produced by oscillating electric and magnetic disturbances, or when electrically charged particles move through a vacuum or matter. These electric and magnetic waves are transverse waves that move perpendicular to one another, and, just like mechanical waves, have characteristics like amplitude, wavelength and frequency.

Electromagnetic waves all share a specific set of properties:

  1. Electromagnetic waves can travel through vacuums (empty space). Most other types of waves must travel through a medium. For example, sound waves need either a gas, solid, or liquid to pass through in order to be heard.

  2. Electromagnetic waves all travel at the same speed through a vacuum. That speed, the speed of light is always a constant. (Speed of light (c) : 2.99792458 x 10^8 m/s)

  3. Wavelengths are measured between the distances of either crests or troughs. It is usually characterized by the Greek symbol (λ)

There are different types of electromagnetic waves based on their wavelengths. The range of electromagnetic waves is known as the electromagnetic spectrum.

The electromagnetic spectrum (as seen above) encompasses everything from visible light (what humans can see) to infrared light and gamma radiation. As you can see, the wavelength increases (and frequency decreases) as you move to the right of the spectrum (towards radio waves).

Radio Waves are about 10^3 m in wavelength and have the lowest energy levels of any of the waves on the spectrum. Radio broadcasts, TV broadcasts and cell phones all transmit this type of wave. They are often used in radar systems to map the surface of the earth or even to illustrate underwater terrain.

Microwaves are around 10^-2 m in wavelength. You are probably familiar with the household appliances known as microwaves which use microwaves to generate intermolecular friction by vibrating water molecules resulting in heat. Microwaves are also used to broadcast information through space, and are sometimes used in certain radars to predict weather conditions.

Infrared Rays are approximately 10^-5 m in wavelength and are invisible to the human eye. However, this type of wave can be felt as heat or thermal energy. All objects in the universe technically give off infrared radiation, and thus, thermal cameras work by detecting minute differences in the frequencies of infrared to determine the temperature of something being viewed.

Infrared waves are also used in devices like remote controllers to communicate commands to another device.

Visible Light, between 390 and 780 nm in wavelength, is the only type of electromagnetic wave that the human eye can see unaided. Each colour that we see has a particular wavelength that falls within this tiny slice of the electromagnetic spectrum.

Ultraviolet Rays, though they have a smaller wavelength than visible light and are invisible to humans, some animals can see them, such as bumblebees. The Sun emits light at all the different wavelengths in the electromagnetic spectrum, but it is ultraviolet waves that are responsible for causing sunburns. Scientists use special devices (such as spectrographs) to detect ultraviolet light, allowing them to determine the locations and vague appearances of distant stars and planetary systems. The hottest and most active bodies in the cosmos give off large amounts of ultraviolet light (as well as electromagnetic waves at higher wavelengths!).

X-Rays have wavelengths less than 3 nm and are often classified based on energy rather than wavelength because their wavelengths are so small. X-rays were first observed and documented in 1895 by Wilhelm Conrad Roentgen, a German scientist who found them by accident when experimenting with vacuum tubes. Only a week later, he was able to take an X-ray photograph of his wife's hand that clearly showed her bones. Why do X-rays allow us to take 'pictures' of our bones and teeth through our skin? Well, it all comes down to the density of your bones and teeth and the sensitivity of X-ray film to X-rays. When an X-ray is taken, an X-ray film is placed on the other side of what is being observed before the rays are shot through the body. Your bones and teeth are more dense than any of the fleshy material of your body, and so, they absorb the rays more. What you see on the X-ray film is essentially a shadow projected onto and captured on the film.

.Gamma Rays, wavelength less that 0.03 nm, have the smallest wavelengths of all the electromagnetic waves. They are generated by radioactive atoms and nuclear explosions. Gamma-rays are the most energetic form of light and are produced by the hottest regions of the universe. They are also produced by such violent events as supernova explosions or the destruction of atoms, and by less dramatic events, such as the decay of radioactive material in space. Things like supernova explosions (the way massive stars die), neutron stars and pulsars, and black holes are all sources of celestial gamma-rays. Satellites like the Compton Observatory allow us to observe gamma rays from outer space, as most of it is absorbed by our atmosphere. Gamma Rays also kill living cells, and are utilized in certain treatments (radiotherapy) to kill cancer cells.

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