It was in Planck’s study of blackbody radiation that quantum mechanics actually made its first appearance, though not in fully developed form. We will focus on how blackbody radiation relates to the sun as a source of light.
Let’s start with ordinary objects first. All objects (you, your chair, anything) emit electromagnetic radiation, or light, because all objects contain little particles that jiggle around and accelerating charges produce electromagnetic radiation. However, most objects also reflect light. A blackbody is then defined as an object from which electromagnetic radiation emanates purely due to the thermal motion of its charges (the jiggling of the particles, a.k.a. the temperature), and therefore blackbodies for the most part only absorb and don’t reflect light (hence the name “blackbody”)1.
The sun is an object that radiates like a blackbody; it makes sense, it would seem to produce much more electromagnetic radiation that it would be reflecting. In the quantum picture, which correctly described blackbody radiation only when the radiation was viewed as quanta (or photons) of energy, the sun should emit a maximum of photons at a certain wavelength and emit less and less photons of wavelengths on either side, as pictured in Solar Radiation Outside the Earth’s Atmosphere.
An object with a cavity as depicted above can act as a blackbody, because it absorbs light and doesn’t reemit this radiation until its energy is in thermal equilibrium with the particles inside the body itself. In other words, the photons bounce around and lose energy until they have an energy based almost purely on the temperature of the object.
1. Harris, Randy. Modern Physics. 2nd edition. San Francisco: Pearson Education, Inc., 2008.