For a more exact explanation, you need to look at thermodynamics, heat and energy transfer texts.
Heat transfer uses 3 ways, convection, conduction and radiation. Convection warms air which moves because warm air is lighter and floats upward. Conduction warms by touch (the basis of convection). Heat lamps are radiation. Use only heat lamps and your pipes will burst (they only heat where they are pointed)!
When the lamp gets hot, that is wasted energy. For a more efficient process use QH bulbs.