Q. When is heat radiated from one body to another?
A. When the two bodies are separated by a non-conducting medium: thus the sun radiates heat towards the earth, because the air comes between (which is a very bad conductor).
Q. On what does radiation depend?
A. On the roughness of the radiating surface: thus if metal be scratched, its radiating power is increased, because the heat has more points to escape from.
Q. Does a fire radiate heat?
A. Yes; and because burning fuel emits rays of heat, therefore we feel warm when we stand before a fire.
Q. Why does our face feel uncomfortably hot, when we approach a fire?
A. Because the fire radiates heat upon the face; which (not being covered) feels the effect immediately.
Q. Why does the fire catch the face more than the rest of the body?
A. The rest of the body is covered with clothing, which (being a bad conductor of heat) prevents the same sudden and rapid transmission of heat to the skin.