• Tags: infrared, visible, thermal
• Difficulty: Foundation

Infrared (700 nm - 1 mm, 3 x 10^11 - 4 x 10^14 Hz) is produced by all warm bodies (thermal radiation). Wien's law: $lambda_{max}$ = 2898/T (in micro-meters when T in Kelvin). Human body (~310 K) emits peak IR at ~9.4 um. IR applications: thermal imaging (night vision, medical diagnostics), remote controls, fiber-optic communication (1550 nm window), greenhouse effect (CO2 and H2O absorb IR, trapping heat). Visible light (400-700 nm) is the narrow window detected by human eyes. Produced by electronic transitions in atoms. VIBGYOR from short to long wavelength: Violet (380-450 nm), Indigo (450-475), Blue (475-495), Green (495-570), Yellow (570-590), Orange (590-620), Red (620-750). White light is a mixture of all visible wavelengths. The Sun's peak emission is in the visible range — not coincidental, as eyes evolved to detect the most abundant radiation.