It’s a kind of energy called “electromagnetic (EM) radiation”. There are other kinds of EM radiation too (radio waves, microwaves, x-rays, etc.).Light travells very fast and straight about 186,000 miles per second [300,000 kilometers per second], so light from the sun takes about 8 minutes to go 93 million miles [149 million kilometers] to earth.

                 Light has some characteristics in common with all forms of electromagnetic energy. These include wavelength, frequency, and speed of propagation.  These characteristics are actually related to each other, so that any one can be calculated if the other two are known.


Since light is a repeating waveform in motion, it is possible to measure the physical distance between matching points of adjacent cycles of the waveform.  


λ =  Distance between two waves .It is the Greek letter Lambda”.

The wavelength can actually be measured between any two corresponding points on the waveform. It is convenient to use the most positive point or the most negative point

                     Speed of Propagation

The speed at which light travels through any medium is determined by the density of that medium. The presence of matter, even transparent matter, will slow the light down.

                    Frequency and Period

In any electromagnetic wave, it takes time for the energy in the wave to change from electrical format to magnetic and then back again. The amount of time required to do this twice, covering one complete cycle, or wavelength of the signal is known as the period of the wave.

       The frequency of the wave is the  reciprocal of the period. ie, the frequency is the number of cycles of the waveform that occur in one second of time. For many years this was simply measured in units of cycles per second. Recently, however, the specific name hertz  .In general equations, the letter f is used to indicate frequency in hertz.

                                               c  =  f . λ


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