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Light is part of the electromagnetic spectrum, the spectrum is the collection of all waves, which include visible light, Microwaves, radio waves (AM, FM, SW), X-Rays, and Gamma Rays.
In the late 1600s, important questions were raised, asking if light is made up of particles, or is it waves?
Sir Isaac Newton, held the theory that light was made up of tiny particles. Around the same period, Christiaan Huygens, believed that light was made up of waves vibrating up and down perpendicular to the direction of the light travels. Huygens' theory was the successful theory of light wave motion in three dimensions. Huygens, suggested that light wave peaks form surfaces like the layers of an onion. In a vacuum, or other uniform mediums, the light waves are spherical, and these wave surfaces advance or spread out as they travel at the speed of light. This theory explains why light shining through a pin hole or slit will spread out rather than going in a straight line.
Newton's theory came first, but the theory of Huygens, better described early experiments.
At the time, some of the experiments conducted on light theory, both the wave theory and particle theory, had some unexplained phenomenon, Newton could not explain the phenomenon of light interference, this forced Newton's particle theory in favor of the wave theory. This difficulty was due to the unexplained phenomenon of light Polarization - scientists were familiar with the fact that wave motion was parallel to the direction of wave travel, NOT perpendicular to the to the direction of wave travel, as light does.
In 1803, Thomas Young studied the interference of light waves by shining light through a screen with two slits equally separated, the light emerging from the two slits, spread out according to Huygens' principle. Eventually the two wave fronts will overlap with each other, if a screen was placed at the point of the overlapping waves, you would see the production of light and dark areas.
Later in 1815, Augustin Fresnel supported Young's experiments with mathematical calculations.
In 1900 Max Planck proposed the existence of a light quantum, a finite packet of energy which became known as the Photon.
From the early experiments and theories, both the Wave theory, and the Particle theory were correct - light is both a wave and a particle, most of the time light behaves as a wave.
Light can exhibit both a wave theory, and a particle theory at the same time.
The speed of light in a vacuum is a universal constant, about 300,000 km/s or 186,000 miles per second. The exact speed of light is: 299,792.458 km/s
It takes approximately 8.3 min for light from the sun the reach the earth (150,000,000 / 300,000 / 60 = 8.3)
Taking the distance of the sun from Earth into account, which is 150,000,000 km, and the fact that light travels at 300,000 km/s, it shows in someway how fast light actually travels.
The following diagram, showing the electromagnetic spectrum, gives you an Idea of where different frequencies fit, and also the size of the spectrum, in-comparison to what we actually see - Visible Light!