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Light and Colour

Light is something we experience everyday, and has been studied intensely for centuries. And yet, there are still many mysteries related to light. Physicists believe that light can teach us much about the origin and development of the universe.

Ok, so there is a lot we do not know yet about light. But here is a list of things we do know (or think we do):

  • Light is electromagnetic radiation
  • This means it is a combination of an electrical field and a magnetic field. What we call light is visible light, and is only a tiny part of the EMR spectrum, somewhere in the middle as wavelengths go.

  • Light, as all EMR radiation, has no mass, but it does have energy
  • We sense warmth in sunlight. An irradiated object may absorb, reflect or retransmit light energy, or let it pass through.

  • Light travels in straight lines called rays
  • Actually, it would more correct to say that what we call a straight line is the path light travels. When light is reflected and focused, the perception of straight paths of light creates the images of objects that are not really there. Light will bend through media, such as water and the atmosphere, and will even be bent a little by gravity!

  • Light has wave characteristics
  • Light diffracts through small apertures in much the same way as water ripples do. But light does not go around corners like water waves. However, low-frequency EMR, radio, can bend around objects.

  • Light is also a particle
  • Light is not a continuous disturbance of a medium, like waves are in water. Light is a stream of individual particles called photons.

  • Light does not require a medium to travel through
  • Light is electromagnetic radiation (EMR), like radio and infrared. Light can travel through vacuum. When light passes through a translucent medium, it is slowed down by it, and refracts (bends).

  • The speed of light is the universe's speed limit
  • This is an important premise of Einstein's theory of Relativity, and as a result for our understanding of time and space. But is it true? And why is it so? Physicists are still unsure. This is one of things they are trying to find out at the LHC, at CERN, Switzerland.

Newton's Opticks

In the 1660s and 70s Isaac Newton conducted investigations into the nature of light. He shuttered up his workroom, and allowed only a thin shaft of light to come in. He then experimented with mirrors, lenses and prisms. The results of his work were his Laws of Optics and the invention of a totally new type of instrument: the reflecting telescope.

At the time of Newton, there had been several major advancements in optics. Robert Hooke had used the new microscope to examine and draw tiny insects and microbes for the first time. Telescopes had been used for 60 years, since Galileo had developed the astronomical telescope in 1609. However, neither of these great scientists had understood the nature of light itself, and how colour was created from white light.

Colour

Rainbow spectrum through a prism
Rainbow spectrum of visible light caused by refraction of differing wavelengths of light colours

In fact, Hooke had an idea that colour was 'added' to 'blank light' as it passed through coloured glass or reflected from a coloured surface. Newton found that he could generate all the colours of the rainbow from a single, uncoloured glass prism.

Primary colours
White light is composed of red, green, and blue primary colours.

When white light passes through a prism, it bends. However, since the degree of refraction depends on the frequency of the light, the white lightray divides into its component colours, and we see a rainbow, or visible light spectrum, consisting of 7 base colours.

Rainbow

When white light passes through a prism, the different wavelengths cause the colours to refract to different degrees. The result is a rainbow of the seven fundamental colours.

R O Y G B I V = Richard of York Gave Battle in Vain

Spectrum
Spectrum of visible light: the different wavelengths of colours cause them to refract to different degrees

Michelson-Morley Experiment

The Michelson-Morley experiment was a very significant 'negative result'. The purpose of the experiment was to measure the change in speed of light in different directions through the presumed universal ether. Instead of finding this, the experiment demonstrated instead that the ether does not exist.

The experiment also led to the invention of the instrument, the interferometer. the interferometer splits a lightbeam into two perpendicular beams, which are then relflected and brought back together, so their interference pattern may be measured. This allows the wavelengths of light to be determined by comparitive means, rather than the more difficult absolute measurement through other techniques.

Content © Andrew Bone. All rights reserved. Created : February 12, 2014 Last updated :February 14, 2016

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