Radiation

Introduction

When we hold one end of a metal rod in our hand insert the other end in a gas flame, heat reaches our hand conduction along the rod. If our hand is held above the flame heat reaches it by way of the upward-moving convection currents of hot gas. When we hold our hand at one side of hot object our hand still becomes warm, even though conduction through the air is negligible and the hand is not in the path of convection currents. Energy now reaches the hand by radiation.

The term radiation refers to the continual emission of energy from the surface of all bodies. The energy is called radiant energy and is carried by electromagnetic waves. Here also – just as in the case of light waves – we assume that there exists an all pervading hypothetical medium – either – through which the wave is propagated. Being electromagnetic waves, thermal radiation travels with the velocity of light, viz, 3 x 10¹º cm/sec. in free space. When they fall on a body that is not transparent to them they are in part reflected and in part absorbed. A material medium absorbing thermal radiation gets heated.

It has been found that the wave-length of thermal radiations lie on the larger side of the wave-length of red light. They, therefore, fall under the class of infra-red radiation. In view of their large wave-lengths the Angstrom, the familiar unit of measurement of optical wave, is not a convenient unit in this region.

Properties of radiation

The fact that at an eclipse the heat and the light of the sun are cut off simultaneously indicates that radiant heat and light travel through space with the same velocity. This suggests that radiant heat and light are essentially the same in nature, except that they differ in their wave-lengths. Radiant heat is invisible light. The following is the summary of the properties of thermal radiation in a homogeneous medium.

(1)    They travel in straight lines.

(2)    They obey the laws of reflection and refraction of light.

(3)    They exhibit the phenomena of interference, diffraction ad polarization.

(4)    They travel through empty space with the velocity of light in all directions.

(5)    The amount of heat received per second per unit area of a given surface, i.e. the intensity of radiation, by absorption of thermal radiation emitted by a source of heat at a constant temperature is inversely proportional to the square of the distance between the source and the absorbing surface. This is known as inverse square law.

(6)    They produce heat in a body by which they are absorbed.

(7)    They do not heat the medium through which they pass.

(8)    They produce photo-electric current in cesium photo-electric cell.

Diathermanous and athermanous substances

Those substances which allow heat radiations to pass through them are called diathermanous whereas those which absorb heat radiations are called athermanous or a diathermanous. These terms are analogous to the terms transparent and opaque in light. Among solids, quartz, rock salt and sylvine are diathermanous. In general, those bodies which are transparent to light are diathermanous. However, there are some exceptions. Dry air is transparent to heat whereas moist air is opaque to it. A solution of iodine in carbon disulphide is transparent to heat but is opaque to visible light. Further, it was shown by Tyndall that glasses like hydrogen, oxygen, nitrogen are diathermanous whereas ammonia, absolute alcohol, carbon dioxide and water vapor are athermanous.

Temperature radiation

If a body is maintained at a constant temperature and continuously emits radiation without undergoing any change in its structure, the radiation is called pure temperature radiation. At low temperature a body emits radiations primarily of longer wave-lengths, but with the increase of temperature the proportion of shorter wave-length radiations increases. This radiation becomes apparent as visible light when the body is red hot. Further increase in the temperature increases the proportion of short wave-length radiation, the body ultimately becoming white hot and emitting radiations all visible wave-lengths. The distribution of energy among the various wave-lengths depends upon the nature of the surface as well as the temperature of the body.

Prevost’s theory of exchanges

Before 1792 scientists were in belief that hot bodies emit hot radiations and cold bodies unit emit cold radiations. This means that if we stand near a fire we get hot radiations from the fire and feel warmth. It is observed that if a body at a high temperature is placed inside a constant temperature enclosure, it radiates heat and the temperature of the body falls till it is the same as the enclosure. This implies that the body stops radiating heat when it is at the same temperature of the surroundings. Again on lowering the temperature of the enclosure, it will be observed that once again the body starts radiating heat.

Pierre Prevost of Geneva in 1792 discarded the old assumptions and put forward his theory of exchange which states that all bodies radiate heat continuously at all the temperatures above absolute zero respective of their surroundings. This means that a process of exchange of heat is in action between the body and its surroundings, in which each one radiates heat and at the same time absorbs heat.

In the case of a hot body, placed in an enclosure at a lower temperature, the heat radiated by the body per second is greater than the amount of heat received by the body per second from the surroundings, as a result the temperature of the body falls till the body attains the temperature of the enclosure. In the case of a cold body placed in an enclosure at a higher temperature, the amount of heat received by the body per second is greater than the amount of heat radiated by the same body per second; as a result the temperature of the body rises till it is the same as that of the enclosure. In the case of a body at the temperature of the enclosure, the amount of heat radiated per second by the body is equal to the amount of heat received by the body per second from the surroundings, as such there is neither gain nor loss of heat and the body remains at the same temperature of the enclosure. Thus a state of dynamic equilibrium is said to have reached.