Planck's law
Topic: Solar Radiation, Heat Balance and Temperature · NCERT: Class 11, Ch 8 "Solar Radiation, Heat Balance and Temperature"
Meaning
Planck's law is a rule about how objects give off energy as radiation. The hotter a body is, the more energy it sends out, and the shorter the waves of that energy. This is why the very hot Sun sends out mostly short waves, while the much cooler Earth sends out long waves. It explains why sunlight passes through the air easily but the Earth's own heat gets trapped.
Example
The Sun's surface is about 5,800 K, and its radiation peaks at about 0.5 µm, which is visible light. The Earth is about 288 K, and its radiation peaks at about 10 µm, which is infrared (heat waves we cannot see).
Don't confuse with
- Stefan-Boltzmann law: it deals only with the amount of energy. That energy rises with the fourth power of the absolute temperature (temperature counted from absolute zero, in kelvin).
- Wien's law: it deals only with wavelength. The peak wavelength (the wave length at which a body gives off the most energy) gets shorter as temperature rises.