Gustav Robert Kirchhoff, German physicist, c 1870s.
© Science Museum / Science & Society Picture Library
Engraving. Gustav Robert Kirchhoff (1824-1887) distinguished himself in many fields, notably in the study of electricity, heat, optics and especially for his contributions to spectrum-analysis. He formulated Kirchhoff's law of radiation, the key to the whole thermodynamics of radiation, and his electromagnetic theory of diffraction is today still the most commonly used in optics.