Why is the James Webb telescope infrared?

Answer 1

The primary goals of Webb are to study galaxy, star and planet formation in the Universe. To see the very first stars and galaxies form in the early Universe, we have to look deep into space to look back in time (because it takes light time to travel from there to here, the farther out we look, the further we look back in time). The Universe is expanding, and therefore the farther we look, the faster objects are moving away from us, redshifting the light. Redshift means that light that is emitted as ultraviolet or visible light is shifted more and more to redder wavelengths, into the near- and mid-infrared part of the light spectrum for very high redshifts.

Therefore, to study the earliest star formation in the Universe, we have to observe infrared light and use a telescope and instruments optimized for this light. Star and planet formation in the local Universe takes place in the centers of dense, dusty clouds, obscured from our eyes at normal visible wavelengths. Near-infrared light, with its longer wavelength, is less hindered by the small dust particles, allowing near-infrared light to escape from the dust clouds. By observing the emitted near-infrared light we can penetrate the dust and see the processes leading to star and planet formation. Objects of about Earth's temperature emit most of their radiation at mid-infrared wavelengths. These temperatures are also found in dusty regions forming stars and planets, so with mid-infrared radiation we can see the glow of the star and planet formation taking place. An infrared-optimized telescope allows us to penetrate dust clouds to see the birthplaces of stars and planets.

(Also, at one of the other commenters, Webb is not Hubble's replacement, but rather a successor.)

Answer 2

The James Webb Space Telescope is designed to observe in the infrared range because infrared light can penetrate through cosmic dust clouds, allowing it to observe objects that are otherwise hidden in visible light. This enables it to study the early universe, distant galaxies, and the formation of stars and planets.