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The World Communicates Focus 3Recent technological developments have allowed for greater use of the electromagnetic spectrum

Class Quiz on Focuses1 and 2Objectives 1 20Monday 1/3/09Electromagnetic Waves What do you already know?Electromagnetic WavesElectromagnetic SpectrumThe full range of wavelengths of all electromagnetic waves.We have made arbitrary divisions in the spectrum to divide these waves into families or bands . Examples include; radio waves, light waves, ultraviolet waves, X-rays. In reality the spectrum is a continuum.

Electromagnetic WavesDo not require a medium for propagation so can travel through the vacuum of space.

Travel at the speed of light in a vacuum. c = 3 x 108 ms-1 , 300 million ms-1

EM waves are transverse waves they consist of an oscillating magnetic and electric field that are perpendicular to each other.

http://www.phys.hawaii.edu/~teb/java/ntnujava/emWave/emWave.html

Electromagnetic WavesSelf propagating the oscillating electric field induces a magnetic field and the oscillating magnetic field produces an electric field... and so on...

Can be produced by oscillating electric charges. The waves produced have the same frequency as the oscillating particles.

The waves can be detected as they produce electrical responses in the medium that they pass through.

http://phet.colorado.edu/simulations/sims.php?sim=Radio_Waves_and_Electromagnetic_Fields

Comparison Compare sound waves and electromagnetic waves.

Compare: Show how things are similar or differentScaffold for ComparisonsWave typeSoundElectromagneticSimilarities Differences Sources and Methods of Detection for the Wave Bands of the EM Spectrum Complete the worksheet The Electromagnetic Spectrum and Detecting the Bands.Identify the wave bands used in communication.

Attenuation of LightAttenuation of EM waves refers to the reduction in amplitude or intensity of EM waves as it passes through a medium.

We experience this in our everyday lives in a number of ways.

Our mobile phone signal strengths decrease the further we are from the phone tower.

As you travel away from Sydney the signal from Sydney based radio stations gradually decrease until you can no longer receive the station.

The further the distance we are from a source of light the lower the intensity of the light appears to be

Some real science...Your Task:

Design a valid and reliable experiment to determine the mathematical relationship between the intensity of light and the distance from the source.

Resultsdistance from source (m)Intensity (lx)Results: Graph of I vs dIn order to be able to work out the mathematical relationship between the variables we need to obtain a straight line so that we can use y = mx + b. These results suggest that the relationship between Intensity and distance may be a y = 1/x type relationship.

Next step is to graph Intensity vs 1/d and see what shape our graph is.Resultsdistance from source (m)Intensity (lx)1/d (m-1)Graph of I vs 1/dGraph of I vs 1/d2distance from source (m)Intensity (lx)1/d (m-1)1/d2 (m-2)Graph of I vs 1/d2Determining the Gradient of a lineSelect two points on the line of best fit (do not use data points!).

Select points that are well apart and easy to read on the graph.

Draw a gradient triangle and use rise over run.

Inverse Square LawFrom the prac we determined that Intensity (I) and distance (d) from the source were related by an inverse square relationship.

Where I = Intensity (lux)d = distance from the source (m)

This is known as the Inverse Square Law.We will see many other examples of Inverse square relationships throughout this course.

What implications does this law have for communication using electromagnetic waves?

Under the spotlight!Example: A spotlight is used to illuminate an actress on the stage. When the spotlight is 1m from the actress the intensity of light falling on her is 10 000 lux. If the spotlight was moved back another metre how would the intensity of light falling on the actress change?