You may remember that sound is also a wave with characteristic wavelengths and frequencies. (Sound is traditionally characterized by its frequency, such as 2 KHz – two thousand vibrations per second). Sound waves need a medium to move through such as air, water or even solids. It was believed that light too must have a medium to travel through. Since light reaches us from the stars, it was proposed in 1690 that this medium, called the “luminiferous ether” or just “ether”, must exist throughout space, but it would have rather bizarre properties. Ether had to be dense enough to transmit light waves efficiently, but transparent enough to see through for great distances. Scientists postulated what ether’s physical attributes should be and devised sophisticated experiments to detect it; they found nothing. In 1887, Albert Michelson and Edward Morley conducted an experiment that failed to detect the existence of the ether. It is perhaps the most famous “failed” experiment in physics. SO without ether to move through could light really be a wave?

Light can travel in a vacuum because it is a series of electric and magnetic fields, each generating the other – it is not a wave of pressure variations, as is sound. Electric and magnetic fields do not need a medium to travel through. In the late 1800’s Michael Faraday demonstrated that changing electric fields generate magnetic fields and changing magnetic fields generate electric fields, a property called electromagnetic induction.

So light is just a series of electric fields generating magnetic fields, generating electric fields, and so on. Both fields are perpendicular to the direction of movement of the wave, and both fields move at the speed of light.

 

​Michael Faraday(image from AIP Emilio Segre Visual Archives, E. Scott Barr Collection)

 

Wavelength

(Image: close up picture of ether – it doesn’t exist.)