Wavelength & Speed of Light

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This diagram shows the waveform of rays of red light (electromagnetic radiation) travelling through equally sized blocks of different transparent materials.

  • The top block contains a vacuum so the speed of light, the wavelength and the distance travelled are all shown to be 100%.
  • The diagram then illustrates the relative speed, wavelength and distance travelled as light passes through each of the other materials.
  • The speed of light in a vacuum is 299,792 kilometres per second. Different media reduce that speed by different amounts depending on their optical density.
  • Notice that whilst the speed, wavelength and the distance travelled is different for each material, the frequency of the wave remains the same.
  • Remember that frequency involves counting the frequency with which oscillations of a wave pass a given point in a given amount of time.
  • In this diagram, there are seven wave-cycles in each case before each wave reaches the yellow line. The yellow line indicates the distance the light travels in each case over the same period of time. So the frequency is seven in every case.

Description

Wavelength & Speed of Light

TRY SOME QUICK QUESTIONS AND ANSWERS TO GET STARTED
A light wave in a vacuum travels at 300,000 kilometres (km) per second! Or to be exact, 299,792 km/sec.
Yes! The wavelength and speed at which light travels change as they travel through different media but frequency remains the same.

About the diagram

About the diagram
  • This diagram shows the waveform of rays of red light (electromagnetic radiation) travelling through equally sized blocks of different transparent materials.
  • The top block contains a vacuum so the speed of light, the wavelength and the distance travelled are all shown to be 100%.
  • The diagram then illustrates the relative speed, wavelength and distance travelled as light passes through each of the other materials.
  • The speed of light in a vacuum is 299,792 kilometres per second. Different media reduce that speed by different amounts depending on their optical density.
  • Notice that whilst the speed, wavelength and distance travelled is different for each material, the frequency of the wave remains the same.
  • Remember that frequency involves counting the frequency with which oscillations of a wave pass a given point in a given amount of time.
  • In this diagram, there are seven wave-cycles in each case before each wave reaches the yellow line. The yellow line indicates the distance the light travels in each case over the same period of time. So the frequency is seven in every case.
Understanding the diagram:
  • The speed, wavelength and distance travelled by the wave decreases as it passes through the list of materials because the optical density (and so the index of refraction) of each material is progressively increasing.
  • Although the speed, wavelength and distance travelled by the wave decreases as it passes through each material, the number of waves generated over the same period of time remains the same.
  • Because the frequency remains constant the colour of the ray remains the same.

Some key terms

Wavelength is a measurement from any point on the path of a wave to the same point on its next oscillation. The measurement is made parallel to the centre-line of the wave.

  • The wavelength of an electromagnetic wave is measured in metres.
  • Each type of electromagnetic radiation, such as radio waves, visible light and gamma waves,  forms a band of wavelengths on the electromagnetic spectrum.
  • The visible part of the electromagnetic spectrum is composed of the range of wavelengths that correspond with all the different colours we see in the world.
  • Human beings don’t see wavelengths of visible light, but they do see the spectral colours that correspond with each wavelength and the other colours produced when different wavelengths are combined.
  • The wavelength of visible light is measured in nanometres. There are 1,000,000,000 nanometres to a metre.

Any material through which an electromagnetic wave propagates (travels) is called a medium (plural media).

Summary

About sections (temp)

The electromagnetic spectrum includes electromagnetic waves with all possible wavelengths of electromagnetic radiation, ranging from low energy radio waves through visible light to high energy gamma rays.

The perception of colour by an observer results from properties of light that are visible to the human eye. The visual experience of colour is associated with terms like red, blue and yellow.

The frequency of electromagnetic radiation (light) refers to the number of wave-cycles of an electromagnetic wave that pass a given point in a given amount of time.

The speed (or velocity) of a light wave is a measurement of how far it travels in a certain time.

  • The speed of light is measured in metres per second (m/s).
  • Light travels through a vacuum at 300,000 kilometres per second.
  • The exact speed at which light travels through a vacuum is 299,792,458 metres per second.
  • Light travels through other media at lower speeds.
  • A vacuum is a region of space that contains no matter.
  • Matter is anything that has mass and occupies space by having volume.
  • When discussing electromagnetic radiation the term medium (plural media) is used to refer to anything through which light propagates including empty space and any material that occupies space such as a solid, liquid or gas.
  • In other contexts empty space is not considered to be a medium because it does not contain matter.

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