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Lesson 3.1
Lesson 3.1 |
Lesson 3.1 Spectrum Overview This lesson deals with various forms of electromagnetic radiation. On completion of this lesson you should be able to explain the relationship between wave speed, frequency and wavelength. MINI LAB
CHOICE OF ACTIVITIES
Radiation: Electromagnetic waves are moving disturbances in the electric and magnetic fields around us. These waves transport energy and usually radiate outwards in all directions from the source that creates the waves. Light: Electromagnetic radiation that can be detected by the eye. Frequency: When something occurs in a regular, repeating pattern, the number of times that this occurs in a particular period of time is known as its frequency. The SI unit of frequency is the Hertz. 1 Hz is equal to 1 per second. Spectrum: A spectrum is an arrangement of types of waves in order of frequency.
Invisible Radiation Many forms of electromagnetic radiation are invisible. These include, in order of increasing frequency: Radio waves, microwaves, infrared radiation, ultraviolet light, x-rays and gamma rays. The range of visible frequencies is situated between the frequencies of infrared and ultraviolet radiation. We are familiar with the heating effect of infrared radiation and the sunburn caused by ultraviolet radiation. Electric grills cook food using infrared radiation. Remote control units for entertainment centers typically send coded sequences of infrared frequencies to a receiver on the main unit. These sequences are interpreted to activate controls in the unit. In many cases, the emitting diode on the remote unit and the detecting diode on the main unit are hidden or partially hidden by a dark material that is transparent to infrared radiation but relatively opaque to visible light. Most types of glass, on the other hand, are relatively opaque to infrared and ultraviolet radiation. Frequencies Electromagnetic waves cause electric and magnetic fields to oscillate without permanently changing their state. Different forms of electromagnetic radiation have different frequencies. The number of oscillations per second is the frequency. This is a measure of the number of oscillations per second or cycles per second. 1 Hertz (Hz) = 1 oscillation, or cycle, per second. The Visible Spectrum A spectrum is an arrangement of types of waves in order of frequency. Electromagnetic waves in the visible region of the spectrum have frequencies ranging from 4 x 1014 to 7.5 x 1014 Hz. Although the number of different frequencies – and colors - in the visible spectrum is theoretically infinite, the actual frequencies are finite and are determined by the atomic structures in the emitting source. The number of frequencies is however very large and it is difficult to identify transition points between different types of radiation. For example, the value of the frequency that separates red light from orange light is not easy to define. We tend to refer to bands of frequencies that cover radiation of a particular type.
The Electromagnetic Spectrum
The diagram below shows different types of electromagnetic waves with their typical wavebands. The diagram shows frequency increasing from left to right. This diagram also shows various wavebands for different types of radiation. Gamma radiation, for example, has frequencies in excess of one million trillion Hz. Gamma ray frequencies are typically in the range: 1020 to 1024 Hz. FM Radio frequencies are much smaller. They range from roughly 87 million to 107 million Hertz (megahertz) The types of subatomic transformations that can cause emission of electromagnetic radiation set the upper and lower limits of the frequencies in the electromagnetic spectrum. For example, movements of electrons associated with particular orbitals in atoms are responsible for near-infrared, visible, ultraviolet and x-ray radiation. Gamma rays are associated with nuclear reactions and radio waves are associated with molecular vibrations and nuclear or electron spin flips. Frequencies and Sources of Radiation The table below shows various types of radiation, typical frequency ranges and the types of transitions that are associated with each type of radiation.
Health and Radiation Forms of radiation with higher frequencies, such as x-rays and gamma rays, are generally dangerous because they are able to penetrate human tissue easily. They also tend to initiate chemical reactions that are destructive to life, promote mutation of cells and cause cancer. Radiation forms with lower frequencies are relatively harmless at low intensities. At higher intensities, these can cause heating, discomfort and damage to materials. Microwaves cause certain types of molecules to vibrate – notably water and sugars. Concentrated microwaves can be used to cook food as a result of the heat generated by vibrating water and sugar molecules. Telecommunications Radio waves with higher frequencies are able to carry higher volumes of information than those with lower frequencies. Microwaves transmission systems are therefore commonly used for linking telephone networks and for data transmission. Unlike radio waves with lower frequencies, microwave communications are hindered by obstacles such as building and hills between the transmitter and receiver. Microwave receivers need to be within sight of the transmitter. For this reason, microwave dishes (receiving and transmitting antennae) are often seen on hilltops and on specially constructed towers or masts. Certain transparent materials of extreme purity are able to transmit light with specific frequencies over extremely long distances without significant absorption of the light. Optical fibers made from such materials trap light inside the fibers due to total internal reflection. These are used to carry signals over large distances without losses due to heating and magnetic influences that contribute to losses in the quality that are common in electronic, wire-based systems
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