Light?

[Nitro]

Can anyone explain the definition of light?

[Mr. Bombastic]

Light is electromagnetic radiation as far as I know. We can sense some wavelengths with our eyes, just as we can sense vibrations in the air around us with our ears.

Edit: from Dictionary.com...

light1 ( P ) Pronunciation Key (lt)
n.
Physics.

1.
Electromagnetic radiation that has a wavelength in the range from about 4,000 (violet) to about 7,700 (red) angstroms and may be perceived by the normal unaided human eye.

2.
Electromagnetic radiation of any wavelength.

[gwbaltzell]

Not the first time I would disagree with a definition found at dictionary.com. Though their next entry " The sensation of perceiving light; brightness: a sudden light that made me blink. " is valid. Its the "Electromagnetic radiation of any wavelength. " I have trouble with. Thought technically correct the word by itself usually refers to visible light.
Electromagnetic radiation falls into a number of ranges. The lowest frequency (longest wavelength) is usually called radio waves. These being further broken down ranges such as VLF - Very Low Frequency, such as the signal that sent with time info. for clocks at 60 kHz., VHF, UHF, (very high, ultra high) were TV is. Or classed by wavelength, which is the speed of light divided by the frequency. LW - long wave, MW - medium wave, SW - shortwave, and microwave. Above these are various ranges of infra-red or heat, then the narrow range of visible light, followed by the ultra-violets, and x-ray. Next up we have the ranges of ionizing radiation associated with breakdown of atomic particles: alpha, beta, and gamma rays.

Wavelegnth is normally express in meters. Wavelengths of visible light (color) in nanometers (nm) which could, of course, be converted to other units of length such as the angstrom. The meter itself is now defined in terms of the speed of light (in a vacuum).

Hz is Hertz. A unit of measure of frequency that used to be called cycles per second.

BTW when refering to the speed of light, the broader term is in use. Not just visible light.

[Nitro]

Correct me if I'm wrong, but I would think by saying "Visible Light" automatically means that visible light does not include all light. Therefore the definition of light would have to include all wavelengths.

[gwbaltzell]

What I was trying to say, and not doing a very good job, is that the word light is most commonly use to refer to the visible portion of electromagnetic radiation spectrum. In some instances it's broader meaning is used. Only a few examples come to mind: Speed of light, and light up a target with radar. It is seldom used when speaking of radio waves, x-rays, or ionizing radiation. When you cook popcorn in a microwave oven you don't normally say your going to shine a bright light on it. However, many of us do say "nuke" which implies ionizing radiation which microwaves are not.

I hope this sheds some light on what I was trying to say.

Light is an eletromagnetic wave - an electric wave running orthogonal with a magnetic wave, both moving orthogonal in the direction of travel. The electric wave induces the magnetic wave which in turn induces the electric wave, and so on...

Light, on the quantum scale, are very small packets of energy called photons in which their energy is equivalent to their frequency. If the frequency decreases, so does the energy. Light usually travels as a wave - if the light is stopped for any reason, it can appear as a particle. Light therefore exhibits the properties of wave/particle duality.

Light has no effective rest mass. Instead, light has momentum which is usually calculated from its energy/frequency. It is the momentum of light that can push a solar sail along, for example.

Light travels a c, about 300,000 kilometers per second. Light is constant in that it will always travel at this speed in a vacuum, but may travel slower moving through a meduim such as glass or water.

If you move towards a light source, the light propagating form the source will appear blueshifted, hence will have increase in energy/frequency - and if moving away, will redshift and decrease in energy/frequency. This is usually referred to as the Doppler effect.

[gwbaltzell]

The speed of light in a vacuum is exactly 299,792,458 m·s
One of the seven international standard units is the length unit meter. Which is now defined as 1/299,792,458 of the distance light in a vacuum travels in 1 second. The other six are:<ul type="square">[*]kilogram - mass[*]second - time[*]ampere - electric current[*]kelvin - thermodynamic temperature[*]mole - amount of a substance[*]candela - luminous intensity[/list]All other units are currently derived from these seven.

Note that the luminous intensity definition is at one specific color. The exact definition is:

The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 x 10^12 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.

This frequency is the wavelenght (in a vacuum) of 555 nm
To convert from a radiometric unit (Watts) to a photometric unit (candela), or the other way around, you must know the wavelength(s) of the light source and apply a factor based on the scotopic response of the human eye. That factor is 1 at 555nm and less than one at any other color, really wavelength. The term color is actually applied to perception and not true wavelength.