1. Some general points on the properties of electromagnetic radiation

This page will answer many questions about the electromagnetic spectrum e.g.  

Be able to understand that all electromagnetic waves are transverse and that they travel at the same speed in a vacuum.  

Be able to understand that the electromagnetic spectrum is continuous from radio waves to gamma rays, but the radiations within it can be grouped in order of decreasing wavelength and increasing frequency.  

Be able to describe the properties and uses of the continuous electromagnetic spectrum including in order and there are various sections to work through.

we are talking about ...

radio waves, microwaves, infrared radiation (ir), visible light spectrum, ultraviolet light (uv), X-rays, gamma radiation, ALL of which all travel at the speed of light !!!

Waves for communication and to provide evidence that the universe is expanding

Electromagnetic (EM) radiations are an example of transverse waves - the oscillations are at 90^o to the direction of travel..

Unlike longitudinal sound waves, which need a material for the vibrations to pass through, electromagnetic radiation can pass through a vacuum. But, note that many materials are partially or wholly transparent to various electromagnetic radiations.

All EM radiations are oscillating (vibrating) electric and magnetic fields.

The energy is transferred in little packets of energy called photons.

All the types of EM radiation travel at the same speed in vacuum ('empty space'), with little differentiation in air, which has a very low density. The speed of light in vacuum is 3.0 x 10⁸ m/s.

However, in passing into transparent dense materials like glass or water the speed is significantly reduced and this reduction depends on the wavelength (see Refraction and the visible light spectrum - prism investigations notes).

EM radiations have a huge variety of wavelengths and corresponding frequencies, whose ranges are quoted in the 2nd table below including the trends from radio waves to gamma radiation.

The seven 'types' of radiation are primarily grouped on the basis of their individual properties and effects - which can be quite different because of the difference in energy carried by the EM radiation.

The higher the frequency of the EM radiation, the shorter the wavelength and the greater the energy transferred.

We, and other animals and plants, can detect some of the EM radiations e.g. our skin detects the infrared heat radiation from the Sun, our eyes detect the colours of the visible spectrum and many insects can detect uv radiation.

 

All EM radiations are emitted from a source and spread out in all possible directions.

The EM waves will continue to travel through any medium until they are absorbed - which may be just a surface itself, or partially absorbed or most passes through e.g. visible light through colourless glass.

 

The sources of EM radiation are even more varied than the seven types, but they all involve energy changes of all atoms, all molecules and even the nucleus of some atoms.

All sources are described in the individual sections.

 

When any type of EM radiation is absorbed by a material it is no longer energy in EM wave form.

The production and absorption of radiation involves energy store exchanges

The EM radiation is converted into another form of energy.

Much of it on absorption eventually ends up as heat - increasing the thermal energy store of the absorbing material or surroundings.

The dissipation of energy is mainly due to the radiation spreading out in all directions from the source.

This also means that signals e.g. radio waves, become much weaker the further from the transmission source.

One exception to this, are laser beams, which can be highly focussed into a narrow coherent beam.

 

Three higher energy EM radiations (uv, X-rays and gamma) can initially cause ionisation - the process of knocking off outer electrons of atoms to create positive ions (See section 10 for more details). That is why these three are referred to as ionising radiations.

A lot of the new technology in industry, medicine and university research that developed through the 20th century and on into the 21st century involves the use of all types of EM radiation.

This is not without issues that must be resolved.

The dangers from EM radiation often depend on how much energy they carry.

For increase in energy and danger it is usually ...

gamma  >  X-ray  >  uv  >  visible  >  infrared  >  microwave  >  radio

in order of decreasing energy of photon ===>

The risks and benefits of any new technology must be carefully evaluated and the use of such technologies must safe and carefully regulated.

that is not to say certain technologies should be banned, but risks must be assessed and appropriate safeguards put in place.

In each of the 7 sections I've described the properties and uses of EM radiations.

When waves meet a boundary they can be absorbed, reflected, refracted or transmitted. What happens depends on the properties of the wave and the nature of the boundary. The result of the boundary - EM radiation interaction can be used to investigate things you cannot see by other methods.

Gamma ray, infrared and X-ray EM radiations are all used in medical imaging techniques to help doctors diagnose medical conditions.

Ultrasound is also used, but this is NOT an EM radiation.

The techniques used in medical imaging are potentially dangerous, so to use them, you have to compromise and hopefully get a good quality image without harming the patient.

I've also described possible hazards and how they are minimised, but also examples of where danger to life is balanced against trying to save life - a good example is cancer treatment using gamma radiation and the diagnostic uses of X-rays.

Four familiar 'parts' of the electromagnetic spectrum and the wave equation

The higher the frequency, the shorter the wavelength.

You should know these trends and the order with respect to seven electromagnetic radiations

Electromagnetic radiation	Radio waves TV and radio	Microwaves cooking	Infrared radiation heat radiation	Visible light eye - vision
Energy	========= increasing energy of radiation (photon) ======>
Frequency	=== increasing frequency of radiation (Hz) ======>
Wavelength	====== decreasing wavelength of radiation (m) ======>
'picture trend'

These four EM radiations are all part of daily life!

 The radio waves can be split into:

long-wave radio (LW), medium-wave radio (MW), short-wave radio (SW), very high frequency radio (VHF) and ultra high frequency radio (UHF)

The full range of electromagnetic radiation is tabulated below.

The greater range of the whole electromagnetic spectrum

Seven varieties of electromagnetic radiation are grouped by their wavelength and frequency:

Type of electromagnetic radiation ===>	Radio waves	Microwaves	Infrared radiation	Visible light	Ultraviolet light	X-rays	Gamma rays
~wavelength range/m	>10-1	10-4 to 10-1	7 x 10-7 to 10-4	4 x 10-7 to 7 x 10-7	10-8  to 4 x 10-7	10-9 to 10-8	< 1 x 10-9
~typical wavelength/m	103	10-2	10-5	5 x 10-7	10-8	10-9	<10-12
~wavelength range/nm	> 108	105 - 108	700 - 105	400 - 700	10 - 400	1-10	< 1
~frequency range/Hz	< 3 x 109	3 x 109 to 3 x 1012	3 x 1012 to 4.3 x 1014	4.3 x 1014 to 7.5 x 1014	7.5 x 1014 to 3 x 1016	3 x 1016 to 3 x 1017	> 3 x 1017
Photon energy trend	=========== increasing energy of radiation photons  ========>
Frequency trend	=========== increasing frequency of radiation  (Hz) =========>
Wavelength trend	========== decreasing wavelength of radiation (m/nm) =========>
'picture trend' !!	================================>

Ultraviolet light, X-rays and gamma rays would not normally be part of daily life!

From left to right: same speed, increasing frequency, decreasing wavelength and increasing energy of photon.

Our senses can only detect specific regions of the spectrum e.g. eyes detect visible light and its different colours and our skin detects infrared radiation as 'heat'.

 

EM radiation always transfers energy from an emitter source to an absorber.

Think about ..

a radio transmitter and receiver

an electric heater uses infrared radiation to transfer energy from the hot element thermal energy store to the thermal energy store of the room and you!

 

This has particular relevance for health and safety issues because generally speaking the electromagnetic radiation is most dangerous from the three on the right.

Remember that when EM waves meet a boundary they may be absorbed, reflected, refracted or transmitted and sometimes several these of effects at the same time!

What happens at a boundary depends on the materials at the boundary and the wavelength of the electromagnetic radiation (i.e. the type of EM radiation).