Making Waves

Get a ballpoint pen and rub it on your clothes.   Hopefully you will have created a static electric charge on the end of your pen.   You can check this by seeing if it will pick up bits of fluff or tiny pieces of paper.   Now wave your charged ballpoint back and forwards in the air once every second.   You are making electromagnetic waves. You are broadcasting radio waves!   Don't try tuning in, even if you have a long wave radio, because the waves you are making are extremely long and extremely faint.

You can work out the length of your radio waves by using the wave formula:

wavelength = wave speed / frequency

The wave speed of all electromagnetic waves in a vacuum is 300,000,000 metres per second.   So with waves of a frequency of 1 per second (1 Hertz) the wavelength is 300,000,000 metres!   And as you waved your pen backwards and forwards at a pathetic speed, that enormously long wave had a miniscule amount of energy. 

You can think of making electromagnetic waves  whilst sitting in the bath.   If you move a hand forward and  backwards in the water,  waves will travel to the other end of the bath.   If you move your hand backwards and forwards faster, you will make more waves travel towards the end of the bath.  You have made shorter wave lengths by putting in more energy.   Now move you hand backwards and forwards at the same rate but this time further forwards and further backwards. Your hand will have to move faster to do this.   You are putting more energy in again. You will have made waves of the same wavelength but with a greater height or amplitude.   You may also have flooded the bathroom.   This experiment does not work in the shower.

Astronomical objects transmit their electromagnetic waves in a variety of ways.  Firstly using their heat.   The hotter they are, the faster their atoms vibrate and the shorter the wavelengths of EM waves they make.   That's why when you hold your hand a few millimetres from your cheek, you can feel the heat.Your warm body is giving off long wave infrared.   As you heat objects up they make shorter and shorter EM waves.   Eventually they enter the visible spectrum and glow red hot, then orange, yellow, white and blue.   That's why the hottest stars, like Rigel, are blue.  So why aren't there any green stars?  The reason is that there is always a spread of wavelengths.   Green is smack in the middle of the visible spectrum.   When the spread of wavelengths is centred on the middle of the spectrum the star appears to us a white.   So objects radiate a broadband of EM waves.   The spread of wavelengths is called blackbody radiation.   The graphic below shows the spread of EM radiation given off by three stars.  

Graphic showing the blackbody radiation of three stars

Remote Sensing Tutorial at Goddard Space Flight Centre