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1 Visible light

Visible light is that portion of the electromagnetic spectrum between the frequencies of 7.5×10¹⁴  hertz (abbreviated 'Hz') and 3.8×10¹⁴ Hz. Since the speed (v), frequency (f or ν), and wavelength (λ) of a wave obey the relation:

and the speed of lightCherenkov effect in a "swimming pool" nuclear reactor. The effect is due to electrons moving faster than the speed at which light moves in water. The speed of light (denoted as c reputedly from the Latin celeritas "speed", and also known as Einstein's con in a vacuum c is fixed, visible light can also be characterised by its wavelength of between 400  nanometreA nanometre ( American spelling: nanometer is 10−9 metres—or one billionth of a metre. Its symbol is nm . It is an SI measure of length, commonly used in measuring the wavelengths of visible light, ultraviolet radiation and gamma rays; amongst others (abbreviated 'nm') and 800 nm (in a vacuumThe article on the vacuum cleaner is located elsewhere. In physics, a vacuum is the absence of matter in a volume of space. A partial vacuum is expressed in units of pressure. The SI unit of pressure is the pascal (abbreviated to Pa in usage). It can also).

Visible light excites the rod cellRod cells are photoreceptor cells in the retina that function in less intense light. Rods are named for their cylindrical shape. They are concentrated at the outer edges of the retina (see peripheral vision). There are about 120 million rod cells in the hs and cone cellCone cells or cones are cells in the retina which only function in relatively bright light. There are about 6 million in the human eye, concentrated at the fovea and gradually becoming sparser towards the outside of the retina. Cones are less sensitive ths in the retinaeye cross-sectional view. Courtesy NIH National Eye Institute. Many animals have eyes different from the human eye. The retina is a thin layer of cells at the back of the eyeball of vertebrates and some cephalopods; it is the part of the eye which transdu of the human eye, creating electrical nerve impulses that travel up the optic nerve to the brain, producing vision.

2 Speed of light

Main article: Speed of light

Although some people speak of the "velocity of light", the word velocity should be reserved for vector quantities, that is, those with both magnitude and direction. The speed of light is a scalar quantity, it has only magnitude and no direction, and therefore speed is the correct term.

The speed of light has been measured many times, by many physicists. The best early measurement is Ole Rømer's (a Danish physicist), in 1676. By observing the motions of Jupiter and one of its moons, Io, with a telescope, and noting discrepancies in the apparent period of Io's orbit, Rømer calculated a speed of 227,000  kilometres per second (approximately 141,050  miles per second).

The first successful measurement of the speed of light using an earthbound apparatus was carried out by Hippolyte Fizeau in 1849. Fizeau directed a beam of light at a mirror several thousand metres away, and placed a rotating cog wheel in the path of the beam from the source to the mirror and back again. At a certain rate of rotation, the beam could 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 measured the speed of light as 313,000 kilometres per second.

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