Blackbody Radiation

A blackbody is an idealized object that absorbs and emits radiation perfectly, with no reflection getting in the way. Heat it up, and it emits a continuous spread of electromagnetic wavelengths, not just one color. Plot power emitted against wavelength and you get a smooth hump: the blackbody curve. As temperature increases, two things happen at once. The whole curve rises, because total power radiated grows fast with temperature, proportional to T^4. And the peak of the curve slides toward shorter wavelengths, which is why a piece of iron glows dull red at lower temperatures, then orange, then white as you keep pumping in heat. White hot means so much of the curve has shifted into the visible range that all the colors are blazing at once.

That peak location follows Wien's law: peak wavelength times temperature equals a constant. Double the temperature and the peak wavelength cuts in half. This is why a 6000 K object, plotted next to a 3000 K object on the same axes, shows a curve that is both shifted left and dramatically taller, not just moved sideways. A cool object, including you, still radiates continuously, just with its peak sitting out in the infrared, invisible to your eyes but real and measurable.

This connects forward to the photon picture of light: blackbody radiation was the puzzle that first forced physicists to treat light as quantized energy packets rather than a continuous wave, setting up the photoelectric effect and the quantum ideas that follow in this unit.