Light

Different have attempted to measure the speed of light throughout history. attempted to measure the speed of light in the seventeenth century. An early experiment to measure the speed of light was conducted by , a Danish physicist, in 1676. Using a , Rømer observed the motions of and one of its , . Noting discrepancies in the apparent period of Io's orbit, he calculated that light takes about 22 minutes to traverse the diameter of Earth's orbit.[15] However, its size was not known at that time. If Rømer had known the diameter of the Earth's orbit, he would have calculated a speed of 227,000,000 m/s.

Another more accurate measurement of the speed of light was performed in Europe by in 1849. Fizeau directed a beam of light at a mirror several kilometers away. A rotating was placed in the path of the light beam as it traveled from the source, to the mirror and then returned to its origin. Fizeau found that at a certain rate of rotation, the beam would pass through one gap in the wheel on the way out and the next gap on the way back. Knowing the distance to the mirror, the number of teeth on the wheel, and the rate of rotation, Fizeau was able to calculate the speed of light as 313,000,000 m/s.

carried out an experiment which used rotating mirrors to obtain a value of 298,000,000 m/s in 1862. conducted experiments on the speed of light from 1877 until his death in 1931. He refined Foucault's methods in 1926 using improved rotating mirrors to measure the time it took light to make a round trip from to in California. The precise measurements yielded a speed of 299,796,000 m/s.[16]

The effective velocity of light in various transparent substances containing ordinary , is less than in vacuum. For example, the speed of light in water is about 3/4 of that in vacuum.

Two independent teams of physicists were said to bring light to a "complete standstill" by passing it through a of the element , one team at and the in Cambridge, Massachusetts, and the other at the , also in Cambridge.[17] However, the popular description of light being "stopped" in these experiments refers only to light being stored in the excited states of atoms, then re-emitted at an arbitrary later time, as stimulated by a second laser pulse. During the time it had "stopped" it had ceased to be light.

The study of light and the interaction of light and is termed . The observation and study of such as and the offer many clues as to the nature of light.

Refraction

Refraction is the bending of light rays when passing through a surface between one transparent material and another. It is described by :

where θ1 is the angle between the ray and the surface in the first medium, θ2 is the angle between the ray and the surface normal in the second medium, and n1 and n2 are the , n = 1 in a and n > 1 in a .

When a beam of light crosses the boundary between a vacuum and another medium, or between two different media, the wavelength of the light changes, but the frequency remains constant. If the beam of light is not (or rather normal) to the boundary, the change in wavelength results in a change in the direction of the beam. This change of direction is known as .

The refractive quality of is frequently used to manipulate light in order to change the apparent size of images. , , , and are all examples of this manipulation.

There are many sources of light. A body at a given temperature emits a characteristic spectrum of radiation. A simple thermal source is sunlight, the radiation emitted by the of the at around 6,000 kelvins (5,730 degrees Celsius; 10,340 degrees Fahrenheit) peaks in the visible region of the electromagnetic spectrum when plotted in wavelength units Another example is , which emit only around 10% of their energy as visible light and the remainder as infrared. A common thermal light source in history is the glowing solid particles in , but these also emit most of their radiation in the infrared, and only a fraction in the visible spectrum.

The peak of the black-body spectrum is in the deep infrared, at about 10 wavelength, for relatively cool objects like human beings. As the temperature increases, the peak shifts to shorter wavelengths, producing first a red glow, then a white one, and finally a blue-white color as the peak moves out of the visible part of the spectrum and into the ultraviolet. These colors can be seen when metal is heated to "red hot" or "white hot". Blue-white is not often seen, except in stars (the commonly seen pure-blue color in a flame or a 's torch is in fact due to molecular emission, notably by CH radicals (emitting a wavelength band around 425 nm, and is not seen in stars or pure thermal radiation).