What is light?
Visible light is that part of the electro-magnetic spectrum that lies between
the wavelengths of ultraviolet and infrared. That's probably more that you
need to know for the purposes of home aquaria.
White light is all colors
When we see a rainbow, we are seeing white light split up into it's
component colors, hence the expression "all the colors of the rainbow".
Sunlight is different in different places in the world
Sunlight contains, more or less, equal portions of all colors of sunlight.
Northern sunlight, that is, sunlight in areas north of the fortieth parallel,
has more blue than equatorial sunlight because of absorption of all other
colors, or wavelengths of light, by the atmosphere.
Blue pictures underwater
This is the same effect that causes underwater photos taken below three
feet to be so blue. Just as the atmosphere absorbs non-blue light so does
water, except water absorbs non-blue light at a much greater rate. Almost
all non-blue light below three feet of water is absorbed.
How is light measured?
Light quality is expressed and measured in many ways. Light color can be
measured in degrees Kelvin (K) and the color rendering index of a light
source can be measured and expressed as CRI.
Color temperature - degrees K
White light can have different "warmth". A bit more red/yellow and white
light appears "warmer". A bit more blue and light appears "cool". This can
be quantitatively assessed by the assigning of a color temperature, given
in degrees Kelvin. Think of color temperature as the color of a block or
iron as it is heated to various high temperatures. A warm, reddish light is
around 3500 degrees Kelvin, and above 6000 degrees Kelvin the light takes
on a bluish tone. Sunlight is somewhere around 5000 degrees Kelvin.
The first part of the paragraph is misleading. Although people may think of
blue as a "cooler" color than red, it is actually hotter. For the physicists
out there, iron is acting as a black body here.
Color rendering index (CRI)
The color rendering index identifies the degree of color shift objects
undergo when illuminated by a particular light source. In simpler terms,
the CRI expresses the degree to which a light source renders the true
color impression. The CRI is an index and ranges from 0 to 100. A light
source having a CRI of 100 means objects illuminated by it look like they're
supposed to; that is their natural color is not distorted. A light source
having a very low CRI would tend to make objects appear to be a different
shade or even color that they really are. An example of light with a high
CRI is, obviously, sunlight. Some fluorescent tubes such as Daylight,
Chroma 65 or Vita-Lite have a very high CRI. Some light sources such as
Gro-Lux or sodium vapor lamps have very low CRI's.
