We look for infrared emissions for three reasons. First is the process called "cosmological redshifting."
The expansion of the universe causes all galaxies to move away from one another, stretching the light from those galaxies as it travels across the universe. As a light wave stretches, it moves toward the red end of the spectrum — thus the name, redshifting.
If an object is extremely far away, the light stretches so much that it moves beyond the end of the visible light spectrum into the invisible infrared. So to see the farthest and earliest galaxies in the universe, we have to be able to look at the light that reaches us in the form of infrared radiation.
Second is infrared's ability to penetrate the dark clouds of dust present in the universe.
Third is the simple fact that some things predominantly emit infrared radiation. Not all objects glow with their own light, but even the dimmest objects give off some infrared. Older planets, dust around stars, the early stages of star formation, and clouds of dust drifting in space are all visible in infrared light.
[VISIT The Infrared Universe: See the cosmos shift between visible and infrared light]
Webb will see entirely in infrared light, bringing us pictures of a universe we've never been able to see before in such detail. But infrared astronomy brings its own set of complications.
Everything made up of atoms emits infrared light. When the atoms in an object collide or vibrate, their electrons are bumped up to a higher energy level. As those electrons return to their normal energy level, they give off radiation. Much of this radiation escapes as infrared. Warm objects, which have a great number of excited electrons, emit lots of infrared. Just like a flashlight beam being lost in the glare of the Sun, the more infrared there is all around, the harder it is to distinguish distant infrared sources.as cold as possible, so the infrared produced by its own excited atoms doesn't overwhelm the distant emissions it's designed to detect.