In the radiation mode of heat transfer, heat energy gets transferred through all media and also through the vacuum. The heat energy gets transferred as electromagnetic waves.
Conduction is the process of transmission of energy from one particle of the medium to the other but here each particle of the medium stays at its own position.
Convection is the process of heat transfer by the bulk movement of molecules within fluids such as gases and liquids. The initial heat transfer between the object and the fluid takes place through conduction, but the bulk heat transfer happens due to the motion of the fluid.
Radiation can be described as energy or particles from a source that travels through space or other mediums. Light, heat, microwaves, and wireless communications are all forms of radiation.
The conduction, convection, and radiation are illustrated in the figure below.
In a vacuum, heat gets transferred by the radiation mode of heat transfer.
The radiation mode of heat transfer does not need any medium to propagate. Heat gets transferred in the form of electromagnetic waves.
For example in space sunlight is transferred by radiation.
