Rollo’s father then told him to go back from the fire as far as he could go, and still feel the heat upon his hand, and then he interposed the book between the fire and his hand just as before. Rollo still found that he could feel the heat from the fire instantly, after his father took the book away.
“Now, that is radiation,” said Mr. Holiday.
“Radiation?” replied Rollo.
“Yes,” said his father. “Heat radiates from a fire, in all directions, through the air. Heat radiates, too, from all bodies that are hot, or warmer than the air around them. Go and hold your hand near the stove pipe, and see if you can feel the heat radiating from the stove.”
Rollo did so. He said he could feel the heat very plainly, though he had to hold his hand nearer than was necessary when he was feeling the heat of the fire.
“Yes,” said his father; “that is because the pipe is not so hot as the fire. If the pipe was as hot as the fire, it would be red hot, and then you could feel the radiation as far as you do that of the fire. However, there is some radiation, though the pipe is not very hot. There is always radiation when the thing is warmer than the air around it. There is radiation from your face.”
Here Rollo put his hand up towards his face, to try whether he could feel the radiation.
“You can’t feel it very well with your hand,” said his father, “because your hand is warm, and so that radiates too. But if you hold your hand up very near to your face, perhaps you will feel both to be warmer, for your hand will radiate to your face, and your face will radiate to your hand.”
Rollo then put his hand up very near to his cheek, and, after holding it there a minute, he said that he perceived that it felt warm.
“At any rate,” said his father, “you understand what radiation is. The heat that comes from the sun is radiated.”