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Energy, Wavelength and Electron Transitions

What is light and how is it created?  The creation or origin of light is directly related to the structure of atoms – specifically where the electrons are around the nucleus.  This in turn determines the characteristics of atoms and how they react.

Light, more correctly defined as “visible light”,  is one form of electromagnetic radiation -  energy that travels through space  in waves at approximately 186,000 miles per second.    Other forms of electromagnetic radiation include x-rays, infrared radiation (heat energy), gamma rays, microwaves and radio waves.  All travel at the “speed of light” or 186,000 miles per second and all travel in the form of waves.  What are the characteristics of waves?

If you sit on a surfboard just offshore at the beach, you will find yourself bobbing up and down in the water.  This is  due to energy being transferred from out in the ocean in towards the shore.  This energy is being carried by the water in the form of waves.   If you have ever been pummeled by a breaking wave at the beach, you know that a lot of energy has been transferred!  As you bob up and down, you are riding different parts of the wave: when you go up, you are on a peak or crest and when you go down, you are in a trough.  The distance between two peaks or between two troughs is called the wavelength of the wave.  The number of peaks that pass by a given point (you!) every second is called the frequency of the wave.   How fast a given peak moves through the water is called the speed of the wave.   The speed, frequency and wavelength of a wave are related by the following equation:

speed = (frequency)   X  (wavelength)

c =  n l

If we are talking about light, c= 186,000 miles per second or about 3.0 x 108 meters per second (m/s).  The frequency of a wave has the units  waves/second or hertz.  The units of wavelength are in meters/wave.  Using this information, you can determine the length of any wave of electromagnetic radiation given the frequency of that radiation.   Let’s try one:

What is the wavelength of a wave transmitted by the FM radio station 100.5, the FOX?  First, we know that the speed of the waves is 3.0 x 108 m/s.  On the FM band, 100.5 is the radio frequency and means 100.5 Megahertz or 100.5 x 106 hertz (waves/second).    Rearranging the above equation and solving for   , gives us:

 l = 3.0x108 m/s =3.0 meters/wave 100.5 x 106 wave/s

The frequency of a wave is related to how much energy is used to generate it.  Take a rope or a long spring and extend it between 2 people.  Move the spring to create a wave pattern.  Count the number of waves between the ends of the spring.  Then shake the spring much harder and you will see more waves but a smaller distance between them.  By putting more energy into a wave, we increase the frequency and reduce the wavelength.  In other words, the waves with higher frequency (and shorter wavelengths) have more energy.  In visible light, red light has a certain characteristic wavelength that is longer than blue light.  Their frequencies are therefore reversed: red light has a lower frequency while blue light has a higher frequency.  Blue light has higher energy than red light  since its wavelength is shorter (and its frequency higher).  In fact, each color of light (or any other type of electromagnetic radiation) has its own characteristic energy which is based on its wavelength and frequency.  Our eyes are designed by nature to detect different wavelengths of light which our brain translates into color.  Our eyes are not designed to detect radiation outside the visible light wavelengths.  For instance, we cannot see x-rays or radio waves.  Photographic film can detect x-rays and a radio receiver can detect radio waves.  X-rays have very short wavelengths (high frequencies) and therefore have high energies – they can penetrate the body.  Radio waves are the lowest energy radiation and therefore have the longest wavelengths (lowest frequencies).

So why do we see colored light when we energize elements in the gas phase?  Why do we see a different color for each element?  What is different about each element :  The number of electrons and their arrangement in space.  We shall now begin to see how the emission of light helped to determine something of where electrons reside in the atom.

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