Radiation Definition

Radiation is the emission or transmission of energy in the form of waves or particles through space or through a material medium.

Radiation is defined as the transfer of heat from one place to another place without heating the intervening medium. This process is accomplished through the use of electromagnetic waves, which are created by the superposition of electric and magnetic fields perpendicular to each other. These waves are capable of carrying energy.

The word “radiation” has generally been adopted to refer to the phenomenon of waves that typically emanate from a source.

Important Types of Radiation

In Physics, we study different forms of Radiation. The common ones include:

Electromagnetic Radiation: These include radiations such as microwaves, infrared, ultraviolet, radio waves, x-rays, and gamma radiation (γ), visible light.

Particle Radiation:

• Alpha Radiation (α)
• Beta Radiation (β)
• Neutron Radiation

Acoustic Radiation: Some of the popular types are Sound, Ultrasound, and Seismic Waves.

Gravitational Radiation: This is a type of radiation that typically manifests as gravitational waves or disturbances in the curvature of spacetime.

Additionally, radiation is mainly classified as either ionizing or non-ionizing radiation, depending on the energy of the radiated particles.

Properties of Radiation

(a) All objects emit radiation due to their temperature being above absolute zero, and they absorb some of the radiation that is emitted by other objects.

Maxwell, on the basis of his electromagnetic theory, demonstrated that all radiations are electromagnetic waves, and that their sources are vibrations of charged particles in atoms and molecules.

(c) A body emits more radiation at a higher temperature, and less radiation at a lower temperature.

(d) As the temperature increases, the wavelength corresponding to maximum emission of radiations shifts from longer wavelengths to shorter wavelengths. This causes the colour of a body to change, and radiations from a body at NTP have predominantly infrared waves.

Thermal radiations travel with the speed of light and in a straight line.

(f) Radiations are electromagnetic waves and can also travel through a vacuum.

Similar to light, thermal radiations can be reflected, refracted, diffracted, and polarized.

Radiation from a point source obeys the inverse square law (intensity is inversely proportional to the square of the distance, i.e. 1/r²).

Prevost Theory of Exchange

According to this theory, all bodies radiate thermal radiation at all temperatures. The amount of thermal radiation radiated per unit time depends on the nature of the emitting surface, its area and its temperature. The rate is faster at higher temperatures. Besides, a body also absorbs part of the thermal radiation emitted by the surrounding bodies when this radiation falls on it.

If a body radiates more than what it absorbs, its temperature falls. Conversely, if a body radiates less than what it absorbs, its temperature rises. Finally, if the temperature of a body is equal to the temperature of its surroundings, it radiates at the same rate as it absorbs.

Black Body Radiation

Fery’s Black Body

Perfectly Black Body

A perfectly black body is one that absorbs all heat radiation of any wavelength that is incident on it. It neither reflects nor transmits any of the incident radiation, and thus appears black, regardless of the color of the incident radiation.

In actual practice, no natural object has the exact characteristics of a perfect black body. However, lamp-black and platinum black are a good representation, as they absorb nearly 99% of the incident radiation. The most basic and widely-used black body was invented by Fery, which consists of an enclosure with a small opening that is painted black on the inside. This opening acts as a perfect black body.

The cone opposite to the opening prevents any radiation from being reflected back directly, thus ensuring that any radiation that falls on the opening goes inside and has very little chance of escaping the enclosure before getting absorbed through multiple reflections.

Absorption, Reflection, and Emission of Radiations

$$Q = Q_r + Q_t + Q_a$$

$$1 = \frac{Q_r}{Q} + \frac{Q_t}{Q} + \frac{Q_a}{Q}$$

$$1 = r + t + a$$

Where r is the reflecting power and a is the absorptive power.

and t = transmission power.

(i) r = 0, t = 0, a = 1, perfect black body

(ii) r = 1, t = 0, a = 0, perfect reflector

(iii) r = 0, t = 1, a = 0, perfect transmitter

Absorptive Power

The fraction of incident radiation absorbed by a body is referred to as its absorptive power.

$\begin{array}{l}a=\frac{\mathrm{Energy}\mathrm{absorbed}}{\mathrm{Energy}\mathrm{incident}}\end{array}$

As all the radiation incident on a black body are absorbed, the absorptivity of a black body is equal to 1.

Emissive Power

Emissive power is the energy radiated per unit time per unit area along the normal to the area.

\begin{array}{l}E = \frac{Q}{\Delta A \cdot \Delta t}\end{array}

Unlike absorptive power, emissive power is not a dimensionless quantity.

Spectral Emissive Power E_l

The total emissive power, E, is related to the spectral emissive power, $E_l$, as follows:

$\begin{array}{l}E={\int }_0^{\mathrm{\infty }}{E}_{\lambda }{d}_{\lambda }and\frac{dE}{d\lambda }=E_{\lambda }\end{array}$

Emissivity

$\begin{array}{l}e=\frac{EmissivepowerofabodyattemperatureT}{EmissivepowerofablackbodyatsametemperatureT}=\frac{E}{E_o}\end{array}$

Kirchoff’s Law

The ratio of the emissive power to the absorptive power for the radiation of a given wavelength is the same for all substances at the same temperature and is equal to the emissive power of a perfectly black body for the same wavelength and temperature.

$$(\frac{E(body)}{E(black body)}=a(body))$$

Therefore, we can conclude that good emitters are also good absorbers.

Wien’s Displacement Law and the Nature of Thermal Radiations

From the energy distribution curve of black body radiation, the following conclusions can be drawn:

(a) The greater the temperature of a body, the larger the area under the curve, indicating that more energy is emitted by the body at a higher temperature.

(b) The energy emitted by the body at different temperatures is not consistent. For both long and short wavelengths, the energy emitted is comparatively low.

(c) For a given temperature, the wavelength (lm) with the highest energy emitted (El) is the one at which the emission is maximized.

(d) As the temperature of the black body increases, the maxima of the curves move to shorter wavelengths.

It was established experimentally that, as the temperature of a black body increases, the wavelength (λm) corresponding to the maximum intensity of emission decreases inversely. i.e.

$$\lambda \propto \frac{1}{T}$$

$$\lambda = \frac{b}{T}$$

This is known as Wien’s Displacement Law.

Here, Wien’s constant is b = 0.282 cm-K.

Ex: Calculate the temperature of the solar surface assuming the surface of the sun is perfectly absorbing (a = 1) using the intensity maximum near the wavelength range of 470 nm of solar radiation.

Sol. Since a = 1, the sun can be assumed to be emitting as a black body.

From Wien’s law for a black body

$$\lambda mT = b$$

\begin{array}{l}T=\frac{b}{\lambda_m} = \frac{0.282\ (cm\ K)}{(470\times10^{-7}\ cm)}\end{array} = 600.43 K ≃ 6125 K

Applications of Radiation

1. Radiation and radioactive substances are frequently employed in the medical domain for diagnosis, treatment, and research. For instance, X-rays are a well-known example of this. Additionally, radiation therapy is also utilized for cancer treatment.

2. Modern communication systems rely heavily on electromagnetic radiation.

Radioactive atoms are used in the process of carbon dating to determine the age of materials.

4. Neutron activation analysis is a process used to determine the composition of materials through radiation.

On the other hand, radiation can also be harmful to living beings. It can cause burns, and in more serious cases, cancer or genetic damage. Furthermore, radioactive waves have been linked to causing cancer in humans.