Radiation occurs when heat transfers from a high temperature region to a lower temperature region without the movement of particles. Therefore radiation can travel though a vacuum, i.e. it does not need a physical medium such as a liquid or a solid.
Most radiation comes in the form of infrared heat from the sun. It sustains life here on the earth and travels through a vacuum to reach the earth. That vacuum is about 150 million km (93 million miles) of mostly empty space.
Absorption of radiation
Dull, black, matte surfaces are good at absorbing radiation but not so good at reflecting it.
White, shiny, smooth surfaces on the other hand are very good at reflecting radiation while these are poor at absorbing it.
Surfaces that can lose heat quickly emitting it to its surroundings are called good emitters. Surfaces that are good emitters are the same as the surfaces that are good absorbers.
|Good absorbers||Good reflectors|
|Good emitters||Bad emitters|
Conduction and convection both require a material medium. Conduction is the transfer of heat by the vibration of the molecules. Convection occurs by the motion of the fluid itself, carrying heat with it. Thus, neither of these can occur in a vacuum.