Light is not all in one form of substance. It has a dual nature, meaning that it consists of part particle and part wave. These particles, or massless bundles of concentrated electromagnetic energy, are called photons. In 1675, we discovered that the speed of light is at a finite speed and can be measured. A man named Olaus Roemer first discovered the speed of light by measuring the periods of Jupiter’s moons. Io, which is the most inner moon, is able to be seen through a telescope and measured to revolve around Jupiter in 42.5 hours. As the Earth moved closer to Jupiter, the periods were faster, but as the Earth moved farther away, it was slower. The calculated speed for the speed of light is 300,000 km/s. Albert Michelson invented another way to find the speed of light, by using a mirror arrangement. A mirror was adjusted so a beam of light was reflected to a stationary mirror located on a mountain 35 km away, and then reflected back to the octagonal mirror and shown into the eye of a surveyor. He spun the octagonal mirror at a high rate. He divided the 70km round trip distance and came up with the experimental value of the speed of light, 299,920 km/s. A light year is the distance that light travels in one year. Transparent material is material that light can pass through. Glass and water are two examples of this. When a light hits these materials, they vibrate. A wave a light traveling through a pane of glass sets up vibrations in the atoms that create a bunch of absorptions and reemissions that pass the light energy through the material and out the other side. There is a time delay because of this and the average speed of light in glass is lower.
Light is energy that is emitted by accelerating electric charges; many times in atoms. An electromagnetic wave is where energy travels in a wave that is partly electric and partly magnetic. An electromagnetic spectrum is the range of the electromagnetic waves.
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