Electromagnetic spectrum

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The electromagnetic spectrum consists of electromagnetic radiation of widely differing wavelengths. Radio is at one end, it has wavelengths of well over a mile. Gamma is at the other end, we do not have instruments capable of directly measuring the wavelength of some of the harder gamma radation, it's that small, well into subatomic measurements.

The effect of this radiation depends on the amount of energy per quantum it carries, high energies give high frequencies and short wavelengths, and vice versa. There is one rule is always obeys, regardless of the circumstances. It always travels at the speed of light, relative to the observer, regardless of the observer's velocity.

Radio is at the weak end, low energy and long wavelength. It's used for transmission of data, via modulation. Television, mobile phones, wireless networking and amateur radio all use this.

Microwaves come next, they can cause entire molecules to resonate. This resonance cause heating and so microwaves can cook things by forcing water molecules to resonate.

After that is infra-red. This makes chemical bonds resonate. When a chemical bond resonates, the vibations become the "internal energy" of the molecule. The molecule becomes hot. In the bulk substance, it becomes hot when it's molecules' bonds are all resonating. You touch it, you feel it being warm or you lose the tip of your finger, depending on how violent the resonation is. After infra-red comes visible light. When this is scattered or reflected, we can see the scattered or reflected light, so we can infer the existence of the object. I can see the light scattered from my room's light by my keyboard, so my brain infers that the keyboard must exist.

Next comes ultraviolet. This is light too blue to be seen. It was discovered to be useful for astronomy by a Mariner probe at Mercury, which detected UV that "had no right to be there". The dying probe was turned over to the UV team full time. It turned out to be a star, but UV astronomy was born. Being very energetic, UV can break chemical bonds. Chlorine will not react with an alkane with no ultra-violet. Give it UV, it reacts quickly. This is because the UV breaks the bond holding chlorine atoms into molecules of Cl2. Lone atoms are extremely reactive and will react with the otherwise almost-inert alkanes. It also makes a mess of DNA, causing cell death at best and uncontrolled cell reproduction at worst. Cancer.

After UV come x-rays. Hard x-rays are shorter wavelength, soft are not. X-rays are used for seeing through some things and not others as well as high energy physics and astronomy. Black holes and neutron stars emit x-rays, they are invaluable in their study.

After hard x-rays come the gamma rays. These are the highest energy photons, having no limit to their infinitesimal wavelength. They are useful to astronomers in the study of high energy objects or regions and find a use with physicists thanks to their penetrative ability and their production from radioisotopes.

Note that there is no clear distinction between any form of electromagnetic radiation and those either side of it. They just gain more character of one and lose character of the other gradually. Red light can resonate certain chemical bonds for example.