It seems to everyone
that what we see is just what's there.
In about 1685, Isaac Newton discovered realms of vision beyond human vision.
Since Newton, science
has discovered much, much more that lies beyond human perception.
(Many other animals have known about some of this for millions of years.)
Most of what our eyes
see is light that is reflected from things.
Except light from things that glow--like candles, electric light bulbs, stars, the sun, red hot objects...
Those glowing things--the sun, the lamps, etc--supply the light that gets reflected.
One interesting fact
that science discovered is that everything "glows."
The higher the temperature, the more to the left in that spectrum is the "light" given off.
(Energy that is emitted by "glowing" is defined as "heat" -- which means that, to science, "light" is generally a form of heat.)
For the heat emitted
by something to be visible to the human eye,
its temperature must be above about 1000 degrees F.
A 1000-degree glowing object will look deep red to the human eye.
electric iron is not far in temperature from being visible
and it leaves a trail on the cloth being pressed.
But even a warm human
body emits "light" (heat) not very far
from that narrow visible band in the spectrum.
The floor that has just been walked on by a barefoot person has a trail of warm footprints.
tissue that is feverish emits more infrared than does healthy tissue.
Hot tea is poured into a tea cup, and the infrared camera sees the level of the
tea in the teapot and in the tea cup.
You can see the hot steam!
wall that is a little warmer because it has a fire behind it
emits much more light (infrared) than a cooler wall.
Infrared vision lets
us see all of this.
And so those who understand such things have invented
several kinds of infrared vision.
Forward-Looking InfraRed camera
Yet another advantage
that infrared vision gives us is this:
Infrared vision sees through most haze and smoke.
That's because of a phenomenon physicists call "Rayleigh scattering"...
Scattering makes the sky blue and the sunsets red (especially when the air is smoky).
It's the wavelength
of the light that counts.
If the wavelength is longer than the size of the haze or smoke particles,
then the light is not much scattered.
smoke particles are larger than the wavelength of deep red light.
And so the visible light gets scattered and we can't "see through it."
Most smoke particles are smaller than the infrared seen by the infrared cameras.
And so the infrared light is not scattered, and we can "see through it."
Infrared cameras usually
use lenses of materials that are opaque to visible light
but transparent to infrared.
Semiconductors are usually the materials of choice: silicon, germanium, gallium arsenide, e.g.
The same characteristic that makes them semiconductors, makes them absorb visible light.
(Visible light is absorbed because it generates the "semiconducting" electron-hole pairs; infrared lacks the energy needed.)