posted on Mar, 18 2005 @ 10:56 PM
"Visible light" is in fact a very narrow range of the EM spectrum which can also be understood as an energy scale. High frequency (low wavelength)
radiation has high energy as defined by the equation E = h(nu). h is Planck's constant, equal to 6.626e-34 J * s and nu is the frequency of the
wave. This equation can also be re-written in terms of wavelength: E = hc/(lambda) where c is the speed of light, and lambda is the wavelength.
The EM spectrum extends beyond UV on the high energy side to include X-rays, gamma-rays, etc. On the low energy side, it goes down into microwaves,
and lower. Any old modern physics text will go through this in a lot more detail.
In quantum mechanics, UV and visible radiation is often (always?) generated by changes to the electronic structure of atoms and molecules. IR
radiation is generally associated with the variation of the vibrational frequency of molecules. Microwaves are generally associated with changes to
the frequency of molecular rotations. UV and visible light can be absorbed by molecules and internally converted to lower energy light (ie. IR &
We see visible light because our eyes are only sensitive to a narrow range of frequencies. You cannot see light unless it is directed towards your
eye. For example, the air between a light bulb and an object is filled with the EM wave, but you only see the object that reflects or scatters the
light. Light is reflected/scattered off objects and into our eyes. The rods & cones absorb the photons and generate electrical signals that our
brain interprets as a 'color'. Other parts of our eyes (the cornea, for example) absorbs some IR frequencies and prevents the photons from reaching
the retina. The eye is unbelievably sensitive to light. Some believe that the eye is efficient enough to detect individual photons at some
Sorry, that's a lot of rambling. Hope you find it helpful.