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Radiation

 

The word radiation must be used with care. It can mean two things. Either it can be a shortened form of the expression Electromagnetic Radiation, or it can mean the radiation we now know consists of particles. Examples of particles which can form 'radiation' are electrons, protons, neutrons, alpha particles and the bare nuclei of other elements.

 

Radiation Belt

 

Image showing the mirror point on EarthA ring-shaped region around a planet in which electrically charged particles (usually electrons and protons) are trapped by the planet's magnetic field. The radiation belts surrounding Earth are known as the Van Allen belts.

 

The very first Earth-orbiting satellite the Americans launched in 1958, Explorer 1, carried a Geiger counter which detected two huge doughnut-shaped rings of charged particles surrounding the Earth. James Van Allen headed the team of scientists who investigated these bands, hence the name Van Allen belts.

 

Image showing the Earths magnetic field

 

The inner radiation belt results from the interaction between cosmic rays and the Earth's atmosphere. The outer zone is created by interactions between the ionosphere and the solar wind.

 

The inner Van Allen belt extends over altitudes from about 2000 to 5000 km and contains mainly protons. The outer Van Allen belt is about 6000 km thick centred at about 16000 km from the Earth.

 

Radiative zone

 

An interior layer of the Sun between the core and the convection zone, where energy travels outward by radiation. See the section on the solar energy cycle for more details.

 

Radio star

 

The 'radio stars' used to measure the twinkling effect caused by the solar wind are not stars at all, but are actually quasars.

 

Radio waves

 

The lowest energy (longest wavelength) photons in the electromagnetic spectrum.

 

Brainiac

What's the wavelength of the BBC Radio 1 broadcasts?

 

Radio 1 reminds you constantly that they broadcast between 97 and 99 MHz. But what does that mean in terms of wavelength? Recall the relationship between a wave's speed or velocity ( v ), frequency ( f ) and wavelength (Lambda):

 

speed = frequency x wavelength

This can be written as:

wavelength = speed / frequency

 

We know that the velocity of all electromagnetic waves is approximately 3 x 108 ms-1. Let's take 98 MHz (98 million hertz, or 98 x 106 hertz) as typical of Radio 1’s broadcast frequencies. This gives us:

wavelength = 3 x 108 / 98 x 106 = 3.06 m

So Radio 1 broadcasts at a wavelength of approximately 3 metres.

 

Brainiac

What's the easy way to remember v= f xLambda in all its various forms? Look at the diagram below.

 

 

Speed triangle

 

Cover up (put your mouse over) the quantity you want to find and the diagram shows you how to find it when you know the other two. So for example, if you know the speed ( v ) and the wavelength (Lambda) and you're asked to calculate the frequency, cover the f and you're left with v above the dividing line and Lambda below it, so f = v /Lambda.

 

Rainbow

 

Image of a rainbow over a parkA rainbow is one of the most fascinating physical events in nature. All of us learn songs or rhymes at school for remembering the colours of the rainbow, such as, 'I can sing a rainbow' or:

 

 

 

 

 

 

 

 

Closeup image of a rainbow“Richard Of York Gave Battle In Vain”

Richard - Red

Of - Orange

York - Yellow

Gave - Green
Battle - Blue

In - Indigo

Vain - Violet

 

Maybe you could make up your own memory jogger? Over 300 years ago, Sir Isaac Newton discovered that white light was made up of a mixture of different coloured light and he suggested there were seven basic colours. Each ray of coloured light is a wave of electromagnetic radiation and each wave has a particular wavelength. When you pass white light through a prism (a triangular glass block) it refracts (appears to bend) and disperses (splits up) into its component colours.

 

Image of light passing through a prism

 

Water droplets can act in the same way as small prisms. When they do and they create a spectrum of the sunlight, we see a rainbow

 

Redshift

 

The name given to the lengthening of the wavelength of radiation. It is often a particular case of the Doppler Shift, although it can have other causes.

 

RoemerRoemer, Ole Christian (1644-1710)

 

Danish astronomer and physicist who was responsible for making many new scientific instruments, including a new type of thermometer - see the Factary entry on temperature. Using observations of the moons of Jupiter, he was also the first person to get a reasonably accurate value for the speed of light.

 

Image of William Conrad RontgenRontgen, William Conrad (1845-1923)

 

A German scientist who discovered X-rays in 1895. In 1901 he was awarded the first ever Nobel prize for physics for this discovery.

 

Reliability

 

Reliability is a very important concept in science. It means that you can depend upon something. If a scientist carries out an experiment and obtains a set of results which shows a pattern, the experiment and results are reliable if they can be repeated, time after time. Reliable results are only obtained through fair tests (this is where only one variable is tested at a time, keeping all others constant).

 

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