Principle

There are three types of heat transfer by means of which thermal
energy is transferred from a hot body to a colder body. Matter is
necessary for thermal conduction and thermal convection with the
aid of which this transfer takes place. Thermal radiation does not
require matter. This is also the case with regard to the radiation
of the sun which travels through space.

How much energy an object absorbs depends on the color of its
surface. A black tube and a test tube are filled with water in an
experiment and illuminated with an incandescent bulb. The heating
of the water is measured.