Applications of electromagnetic radiation

Student ID 9214122

Vacation Essay 2015

School of Physics and Astronomy

The University of Manchester

1. Introduction

As Evans states, "If the entire electromagnetic spectrum were represented by the keys of a piano...the visible part would correspond to less than one piano key!" [1] Even if we were to use only the visible part of the spectrum, there would still be a number of applications for electromagnetic (EM) radiation. With the whole spectrum at our disposal, we are provided with a wealth of potential uses. Only a tiny fraction of these applications will be discussed in this essay.

2. Gas lenses

Normal glass lenses, such as t h o s e i n v a r i f o c a l eyewear, are used to focus light towards a

Fig 2. Diagram of normal converging lens, used to focus light to a point.

Fig 1. The EM spectrum, showing the relative size of the visible spectrum compared to the rest of the EM wavelengths. [2]

desired point, for example to correct vision defects such as shortsightedness. To achieve the same result, gas lenses can be used. [3] They work on the principle that the amount of refraction of light depends upon the density ρ of the gas. For a gas at constant pressure p which obeys the ideal gas law its density is inversely proportional to its temperature T, as shown in Equation 1;

ρRT = pMr

where Mr is the relative molecular weight expressed in kgmol−1 and R is the molar gas constant, with a value of 8.314 Jmol−1K−1. Consequently, to focus light, the temperature of an initially cool gas is raised by an external heating element to decrease the density and increase the refraction of the light.

These gas lenses can be used in particle accelerators, where beams of EM radiation need to be focused to collide with targets of particles or other similar beams. Another application of gas lenses is to carry modulated light, as is found in telephone messages.

3. Fibre Optics

Linked to the use of gas lenses to carry telephone messages are fibre optics. These work when light is totally internally reflected within a fibre optic cable. A condition for this to work is that the light is confined to a small angle from the axis of the cable, to ensure that the light is not refracted out of it. For light inside a glass of refractive index n surrounded by air the possible angles θ at which light can deviate from the axis are shown by Inequality 2;

θ < 900 - sin-1(1/n)

if θ is measured in degrees. Practically, therefore, there cannot be a sudden curve in the cable and it has to be more or less straight.

! 2

(1)

Fig 3. Diagram of light passing through a glass, as is used in fibre optics. The blue beam passes parallel to the axis of the cable. The red beam takes a zigzag path as it entered the cable at an angle θ to the axis. It travels through the cable by reflections at the boundary between the glass and the outer medium. All the angles shown in the diagram are the same.

θ

θ θ

θ θ

θ

(2)

Since the light is nearly on the axis of the cable, the signal propagates at close to the speed of light in the medium. Indeed, a pulse travelling on the axis (or parallel to the axis) of a straight cable will propagate at the speed of light in that medium. This result makes communication with fibre optics faster than that with wires which are affected by electrical resistance. [4] Virgin Media famously uses Usain Bolt to advertise their ‘superfast’ fibre optic services.

Another advantage of f ibre opt ics over conventional methods o f i n f o r m a t i o n transmission is that there are little losses, compared to the large heat loss that occurs in normal wires.

In most uses, the light passes in a glass which is surrounded by another glass of l o w e r r e f r a c t i v e index; a requirement for to ta l in ternal

reflection.

Not only can visible ‘light’ be used in fibre optics, but the principle can be extended to infrared or ultraviolet radiation. It is, however, visible light that is used for communication.

Fibre optics are not only restricted to use in communication. They are also used in medicine to study the interiors of bodies. The size of the cable can be adapted to fit inside natural small holes or those created by doctors.

A problem associated with fibre optics is dispersion; where a signal spreads to become almost unrecognisable by the time it reaches the other end of the cable. To overcome this, pulses have to be separated by appropriate time intervals to ensure successive signals are not interpreted as being different parts of the same signal.

At the beginning of the use of fibre optics the absorption of light by the glass was a problem, meaning that almost no light would emerge on the other side. This prevented fibre optics from being properly applied. Since then, better glass has been developed which transmits the majority of light.

! 3

Fig 4. Picture of Usain Bolt showing his trademark ‘bolt’ for a Virgin Media advert. [5]

4. Radar

An application of EM radiation that might not have as much of an everyday use as fibre optics, but is still widely used in navigation, is radar.

Radar was first developed during the second world war by the allied troops to track down enemy fighter planes. Certainly, it was crucial to their success in a number of aerial battles.

For this use, radar works by the fact that radio waves are reflected by conductors [4] such as the metal of planes. The time difference between the emission of the initial signal and detection of the reflected signal is measured and since the signals travel at the speed of light, the distance to the object is calculated.

Nowadays, radar can still be used as radio waves are reflected differently by sea and land. This helps commercial airline pilots and air traffic control to navigate the planes when conditions are unfavourable, such as at night or when visibility is reduced as a result of fog, for example.

5. Antennae

The radio waves needed for radar communication might be produced by antennae; devices which transmit and receive radio wave signals [3]. Antennae always have a length which is an integer number of half wavelengths of the waves used. This is so that a standing wave pattern is formed between the ends of the antennae. A method to reduce cost and save space is to build a vertical conducting mast planted in the ground. Here, the length only has to be half of that required to satisfy the standing wave condition (an integer number of quarter wavelengths of the waves used). The other half is provided by the image of the mast in the ground. Around the mast, a conducting sheet can be placed to improve efficiency.

Usually an antenna emits uniformly, which is why antennae arrays are used when a stronger signal is needed in a particular direction. This is achieved by using interference patterns produced by multiple sources of radio waves. The antennae can be spaced

! 4

Fig 5. Diagram of the scan a radar might produce. The green dots could be enemy planes, for example.

Fig 6. Diagram of a simple antenna.

differently or have their relative phases changed to produce the desired pattern. An area where antennae arrays are used in this way is satellite communication to and from space with the ground.

6. Cerenkov radiation

Cerenkov radiation is a phenomenon observed when a particle with mass travels faster than the speed of light in that medium. This may seem to contradict Einstein's postulate of Special Relativity that nothing can travel faster than the speed of light, but the solution here is that light does travel slower than 3x108 ms−1 (c) when it is in a medium of refractive index n. Its speed limit v is now given by Equation 3;

v = c/n .

There is no such barrier to particles with mass travelling in different media as their speed only depends on the energy supplied to them. When a particle with mass is travelling faster than the speed of light in that medium it emits photons of EM radiation. In the same way, an object moving in air at a speed faster than that of sound in air emits a sonic boom. Examples of this are a fighter plane or the sound of a whip cracking (the tip of the whip travels faster than the speed of sound even when the person's hand movement is nowhere near this speed). The photons emitted by the speeding particle can be used to measure its energy [3] (therefore mass) and thus is useful in the field of particle physics to help identify particles.

7. Strain study

An application of EM radiation in engineering is one discovered by James Clerk Maxwell [6] to study the strain an object might be put under when it is used for its desired purpose. First of all, a transparent structure of the object has to be built. Then, polarised light is passed through the object as it is bent and pressured in various ways. The light is split into its component colours in the transparent medium (according to its refractive index) and the distribution of colours can be studied to find the weak parts of the object. To finish the process, engineers improve the parts that were found to be weak before building the final

! 5

Fig 7. Diagram showing a particle with mass travelling through a medium. The straight arrow shows the particle’s trajectory and the yellow arrows are photons b e i n g e m i t t e d b y t h e particle.

(3)

product. This technique is now common practice during the design process in engineering.

8. Conclusion

Even before the discovery that radio waves, micro-waves, infrared radiation, visible light, ultraviolet radiation, X-rays and gamma rays are all fundamentally the same and only differ in energy, there were innumerable uses of EM radiation. These uses have been better understood since the pioneering work of Michael Faraday, Maxwell and Einstein in electromagnetism. This essay only manages to scratch the surface of such uses. The applications of EM radiation are not just confined to physics, they are found in all fields of science and technology, and extend into everyday life. The incredible number of applications for EM radiation continues to grow and will likely continue growing far into the future.

References

[1] Rh. Evans, “The Cosmic Microwave Background”, Springer (2014), Chapter 3.

[2] T h e e l e c t r o m a g n e t i c s p e c t r u m , I m a g e S o u r c e : h t t p s : / /commons.wikimedia.org/wiki/File:EM_spectrum.svg [Accessed on 29th July 2015] [3] P. Lorrain, “Electromagnetic fields and waves”, W. H. Freeman & Co (1970), Chapters 13 and 14.

[4] W. J. Duffin, “Electricity and Magnetism”, Mc-Graw Hill (1990), Chapter 14.

[5] Virgin Media Superfast, Image Source: https://recombu.com/digital/article/uk-superfast-broadband_M10887.html [Accessed on 29th July 2015] [6] B. Mahon, “The Man Who Changed Everything”, Wiley (2004), Chapter 3

! 6

Fig 8. Example of image produced for strain study. [7]

[7] S t r a i n S t u d y, I m a g e S o u r c e : h t t p : / / f i l e . s c i r p . o rg / H t m l /10-7401420_31911.htm [Accessed on 29th July 2015]

Appendix

The number of words in the main text of this document (excluding headings, captions, equations, references, appendix) is 1542.

This document was last edited on 3/08/2015 at 16:49.

! 7