infrared radiation
/ in-fra-RED /
When you stand near a warm radiator or feel heat radiating from sun-baked pavement after dark, you are sensing infrared light. Infrared sits just beyond the red end of the visible spectrum, with wavelengths a little longer than red light, roughly from 0.7 micrometers out to a few hundred micrometers. Our eyes cannot see it, but our skin feels it, and it is the band in which warm-but-not-glowing objects shine most brightly.
Infrared matters enormously in astronomy for two reasons. First, anything at the temperatures common in the universe — cool stars, planets, clouds of dust warmed by nearby stars, the disks where new planets form — radiates most of its light in the infrared, simply because cooler objects glow at longer wavelengths (Wien's law). Second, infrared light passes through dust far more easily than visible light does, so it lets us peer inside the dark cocoons where stars are being born and through the dusty heart of our own galaxy. Astronomers split the band into near-, mid-, and far-infrared, each probing different temperatures.
Because warm things glow in the infrared, an infrared telescope must be kept extremely cold, or its own heat would blind it — which is why observatories like the James Webb Space Telescope carry giant sunshields and operate far from Earth's warmth. Water vapor in our atmosphere absorbs most infrared, so much infrared astronomy is done from high, dry mountaintops or from space. The same physics powers everyday tools: thermal cameras that 'see' body heat are simply infrared cameras.
A dense, dark cloud of gas and dust looks completely black in a normal photograph. Photograph the same patch of sky in infrared and the cloud turns transparent, revealing newborn stars glowing inside that visible light could never show us.
Infrared sees through dust and reveals cool objects — newborn stars, planets, and warm gas.
Infrared is not the same as heat itself; it is the light that warm objects emit. And it is not harmful at normal levels — your own body radiates infrared continuously, which is exactly what night-vision and thermal cameras detect.