Electromagnetic Spectrum

Light

We are mostly blind. Our eyes can only see a tiny portion of the radiation available to us from the universe. It turns out that this is not entirely a bad thing. If you could see all of the light around you, the world might seem a confusing place. We call the full breadth of radiation in the universe the “Electromagnetic Spectrum” or EMS, or sometimes just “light”. All light is composed of oscillating electric and magnetic fields. The knowledge we have today about the nature of light has come from over three centuries of experimentation and discovery by many great scientists including Isaac Newton, Michael Faraday, John Ritter, James Clerk Maxwell, William Herschel, Paul Hertz, and Albert Einstein. In this module and the next you will learn that light can behave as both a wave and as a particle, each with a characteristic wavelength, frequency and energy. We start with the evidence for the nature of light.

Light is Particles in Motion

Isaac Newton – we never seem to escape him – proposed in 1675 that light was made of small bits of matter, which were traveled in all directions from a source. Newton believe that light was traveling particles or corpuscles rather than waves because light traveled in straight lines, whereas waves, as in a pond of water, bent around obstacles. Reflection of light could be easily explained if light were made of a stream of particles, for they would bounce off any surface they hit.

Light is Wave Motion

But there was also compelling evidence that light traveled as a wave. Waves certainly existed for everyone saw them flow through water, so why should light waves be different? Additionally, the English scientist Thomas Young demonstrated in about 1800 that if light passed through a slit in a piece of opaque material, the light would spread out in all directions as it moved away from the slit. That was exactly what a beam of light does – it spreads  in all directions from its source. Young then put two slits near each other and shined a beam of light through them. He discovered that the spread of light from each slit overlapped, producing a series of bright images of the slit with dark spaces between them. This again, is due to the waves interacting with each other. Where they come together in phase, or constructively as physicists say, they make a bright image, and where the two are out of phase, they cancel each other, destructively, and no light appears. This is absolute proof that light travels as a wave. Take that, Newton.