Reflection
Much of the light that a satellite can sense is similar to
the light we see with out eyes - reflected off of a surface or
other boundary. For satellites looking down at Earth, for
example, light comes from a source first, usually the Sun.
This light hits Earth and is reflected by both the atmosphere,
the surface, and about everything between. A lot of this
reflected light is scattered which can interfere with getting
a clean image from the satellite's sensors. This is where
having a selective sensitivity to specific wavelength can come
in handy. Different materials and shapes scatter or reflect
light in different ways, but they also interact with different
wavelengths differently as well. Most reflections tend to be
at least somewhat diffuse, that is, they tend to have the
appearance of scatter off of a surface but still adhering to
the Law of Reflection, that is, an incoming ray of light
reflects off a surface at the same angle it approached the
surface(Knight).
So when selecting a sensor, we can use one that is sensitive
to wavelengths that scatter less, or are reflected more often
by a specific structure or material so that it is detected
more prominently by the satellite. Trying to separate clouds
from a snowy ground in a satellite image relies on this!
Earth's clouds are highly reflective in the visible spectrum,
as is snow. However, clouds are mostly water vapor and water
vapor tends to absorb light in the infrared wavelengths,
whereas snow reflects much of it. So when a region is being
shined on by the Sun, we can pick out the two by looking at
infrared wavelengths instead of visual ones.