Planck's law describes how objects emit radiation based on their temperature. It states that the intensity of radiation emitted by a black body at a specific temperature varies with wavelength. The law shows that hotter objects emit more radiation at shorter wavelengths, which is why they appear brighter and bluer. This law is fundamental in the field of quantum mechanics and was formulated by Max Planck in 1900. It laid the groundwork for understanding thermal radiation and led to the development of quantum theory, which revolutionized physics by explaining the behavior of energy at microscopic levels.