Invisibility is an interesting concept. Invisible means not
visible. Things can be invisible to humans because they do not
give off, reflect or refract visible light. This of course means
that these things might be visible to animals who see UV, IR
or other forms of light. Normal objects become invisible to humans
in the dark or when visually blocked out by other objects. An
apple is not visible to the viewer if on the other side of the
wall. Some camouflage attempts to make objects pseudo-invisible,
or otherwise consciously unnoticeable from the surroundings.
Even if the viewer's eyes see the camouflaged bird in the brush,
he doesn't consciously notice it. The camouflaged bird doesn't
stand out in the crowd. This is a psychological invisibility
rather than the real optical invisibility where the eyes receive
no visible light from the object.
Something is referred to as perfectly invisible when
it is completely transparent, meaning light moves through it.
Someone who is perfectly invisible would be blind, as eyes absorb
light when seeing
