[Physics Class Notes] on Continuous Spectra of Electromagnetic Radiation Pdf for Exam

Electromagnetic radiation is related to both electric and magnetic fields. You have studied the concept of electromagnetic radiation in earlier classes. Let us talk about electromagnetic radiation.

Reasons Behind the Continuous Spectra of Electromagnetic Radiation

Electromagnetic radiation emits a continuous electromagnetic spectrum. Warm substances generate such kinds of emissions. The heat generated from these substances accounts for the irregular motions of the molecules, atoms, and individual electrons. The rapid motion of all these entities increases with an increase in temperature. Owing to its lighter weight, the electrons exhibit an irregular charged oscillatory motion with a continuous spectrum of frequencies due to the irregular thermal-induced motions.

Every oscillation with a defined frequency is regarded as a small antenna that can receive or emit electromagnetic radiation.  For example, when iron is heated gradually to higher temperatures, it first glows with a red hue that transforms into yellow and then into white. This phenomenon can be considered as a representation of all the colors included in the visible spectrum. 

You can feel the infrared waves by feeling the heat even before the iron starts to glow red. On the other end of the spectrum, you will be able to detect ultraviolet radiations emitted from white hot iron by placing a photographic film near it.

Different Materials Emit Different Types of the Continuous Electromagnetic Spectrum

It is wrong to conclude that all materials heated to the same temperature will emit the same spectral distribution and amount of electromagnetic waves. However, iron exhibits such spectral distribution, a piece of glass placed next to it will not do so, although it will be hotter than the iron (emits more infrared rays). 

Such an observation is justified by the law of reciprocity, which states that the strength of radiations from a body depends on its ability to absorb heat. It needs its tiny antennas to receive frequencies of that range. Since glass cannot absorb red color, it cannot glow with a red hue. However, it is a better emitter/absorber than iron in the infrared spectrum, and therefore, it emits more heat. Such selective absorptivity and emissivity are essential in our understanding of different natural phenomena like the greenhouse effect. 

Application of Differential Emissivity/Absorptivity of Different Materials in our Daily Lives

Metals exhibit lesser emissivity in the infrared range. Therefore, a tungsten filament present inside the lightbulb can emit a large proportion of visible light at a high temperature of 2500K but does not produce much heat. Such phenomena are required since we need the light but not the heat. 

Similarly, the light emitted from the candle is due to the very hot soot particles (carbon) in the flame. Soot can strongly absorb and emit visible light. However, the flame on the gas ovens in the kitchen appears pale, although it is hotter than the flame on a candle. This phenomenon is due to the absence of soot in the flame of gas ovens. 

Stars emit light comprising a wide radiation spectrum due to the high temperature of gases on their surface. For example, the Sun’s surface is about 5800 K, and it emits a wide radiation spectrum. Every square meter of the solar surface emits radiation of about 60 million watts, comparable to any average commercial power generating station that supports around 30,000 households.

An X-Ray tube also functions using the properties of the electromagnetic spectrum.

Factors Attributing to the Spectral Composition- The Concept of Blackbody

Everybody has its characteristic spectral composition that it emits when heated at different temperatures. Such spectral composition depends on the materials present in the body. This is not the case for a theoretically ideal absorber radiator. An ideal radiator or an absorber will emit or absorb radiations with the same frequency. Such an ideal radiator or an absorber is called the blackbody, and its radiation spectrum is called the blackbody radiation. The only parameter that determines the radiation spectrum of a blackbody is the temperature.

The concept of blackbody has been a hot topic of research. Scientists consider these objects to be the known parameter for any comparative studies since their properties can be exactly identified. Such information can then be used to compare with real-world objects and decipher the reasons why these objects deviate significantly from the ideal cases. An example of a black body can be considered as a cavity in a piece of coal that can be seen through a small opening.