In conduction and in convection we need some STUFF. In this mechanism called RADIATION things go better with nothing in between! Which is a strange business.
A. We energize an incandescent lamp. Nearly INSTANTLY we FEEL the heat energy on the arm a foot or so away — but the lamp bulb still is unheated. The radiation passes through the glass envelope - falls on the flesh - is absorbed - and this is commuted to thermal energy IN THE FLESH. The mechanism is very complicated.
B. The classical RADIOMETER: Here is a device that nearly every¬ body has seen. Strangely enough - even physicists do not thoroughly understand it! Radiation falls upon the vanes. The black faces retreat. A good inquiry to investigate is this: How would this enchanting device work if placed between two big cakes of ice? Try it.
C. THE CASE OF THE THREE CANS: One is shiny - one is black - one is covered with a thin layer of asbestos. Thus we would say that this one is insulated. We fill them equally with hot water. They cool at different rates - obviously. And we can hardly believe it: the asbestos covered can — the insulated can - cools off the fastest! HINT: The surface of this can is very very rough thus exposing very much area for radiation losses!
D. THE CASE OF TWO THERMOMETERS: The bulb of one is black - the bulb of the other is white. We place them in the Sun. What do we see AT ONCE? Answer: The black one rises faster - sooner — but in due course they both come to read the same. These ideas are very important.
E. THE CASE OF THE TWO THERMOMETERS: The bulb of one is wrapped with cotton batting LIGHTLY - LOOSELY — the bulb of the other is wrapped with the same mass - the same weight - the same amount - of batting - but tightly. How do they be- have in the Sun? Answer: The tightly wrapped one climbs higher sooner - faster - quicker — but in due course they come to the same reading. The air lodged in the loose wrapping is a good ther¬ mal insulator.
F. A Dewar flask - which is a thermos bottle — utilizes this mechanism. Quiet air is a good insulator — but better still - NO air at all is a good insulator. So the thermos bottle has two glass walls - an inner one and an outer one - and the space between evacuated - mostly. Also: The outer wall and the inner wall are
both shiny - for good reflection. G. An interesting problem:* THE CASE OF THE BLACK COFFEE AND CREAM: a. We pour a cup of hot coffee black. b. We are on the verge of adding cream when the phone rings. c. We wish to answer the phone and return to find the coffee as hot as possible. d. QUESTION: Do we add the cream before going to the phone or after? e. ANSWER: We add the cream BEFORE! See why? And do not say that the cream holds the heat in or some such worth¬ less phrase.
H. We show a chart of ELECTROMAGNETIC RADIATIONS - the E-M SPECTRUM. Of the entire range we know the visible light is but a tiny part. Human vision, although a wonderful mechanism, utilizes only a small - very small - part of the whole thing. And what lies beyond we can not now say. There may be wavelengths yet undiscovered.
Demonstrations in Physics was an educational science series produced in Australia by ABC Television in 1969. The series was hosted by American scientist Julius Sumner Miller, who demonstrated experiments involving various disciplines in the world of physics. The series was also released in the United States under the title Science Demonstrations.
This program was a series of 45 shows (approximately 15 minutes each) on various topics in physics, organized into 3 units: Mechanics; Heat and Temperature / Toys; and Waves and Sound / Electricity and Magnetism.