• $summary_{type}$: concept
• $word_{count}$: 170

Visible light (400-700 nm): the narrow band detected by human eyes. Produced by electronic transitions in atoms. VIBGYOR from violet (400 nm, highest frequency) to red (700 nm, lowest). White light contains all visible wavelengths. The Sun's peak emission is in this band. Ultraviolet (10-400 nm): produced by very hot objects and gas discharges. UV-B causes sunburn and vitamin D synthesis. UV-C is germicidal. The ozone layer (stratospheric O3) absorbs harmful UV-B and UV-C — ozone depletion is a critical environmental issue. X-rays (0.01-10 nm): produced by decelerating fast electrons hitting metal targets (Bremsstrahlung) or inner-shell transitions. Used in medical imaging, crystallography (Bragg law: 2dsin(theta) = nlambda), security screening. Gamma rays (lambda < 0.01 nm): from nuclear transitions and particle annihilation. Highest energy, most penetrating. Used in cancer treatment, sterilization, and nuclear spectroscopy. All regions travel at c in vacuum.