Q.What are the different modes of transfer of heat?

When thermal radiation is incident on the surface of any object, the incident energy is, in general, split three ways: some is absorbed, some is reflected, and some is transmitted straight through. Three dimensionless coefficients -- the coefficient of absorption a, the coefficient of reflection r, and the coefficient of transmission tr -- give the fraction of incident energy disposed of in each of these three ways, and they always satisfy a + r + tr = 1.

These coefficients let real substances be classified by how they respond to heat radiation: a diathermanous substance (e.g. glass, dry air, hydrogen) transmits a significant fraction of incident radiation without being a particularly good absorber or reflector; an athermanous substance (e.g. water, wood, metal) is largely opaque, transmitting almost none of the incident radiation; a perfect reflector reflects essentially all incident energy (r = 1); and a perfect blackbody absorbs essentially all incident energy (a = 1). Crucially, these coefficients are not fixed constants of a material -- they depend on the wavelength of the incident radiation too, so a substance can behave very differently (e.g. transparent versus opaque) at different wavelengths.