Wavelength & Speed of Light

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.
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Description

Wavelength & Speed of Light

TRY SOME QUICK QUESTIONS AND ANSWERS TO GET STARTED
What is the speed of light in a vacuum rounded up to the next thousand?
300,000 kilometres per second.
Does the speed at which light travels change as it propagates through different transparent media?
Yes! The speed of light is affected by the medium through which it propagates.
Does the wavelength of light change as it travels through different media?
Yes! The wavelength and speed at which light travels change as it travels through different media but frequency remains the same.
What speed does a light wave travel at in a vacuum?
A light wave in a vacuum travels at 300,000 kilometres (km) per second! Or to be exact, 299,792 km/sec.

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.
    • Optically rare medium (air) = a faster medium  = smaller index of refraction.
    • Optically dense medium (glass) = a slower medium = larger index of refraction.
  • 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

Medium

In physics and optics, a medium refers to any material through which light or other electromagnetic waves can travel. It’s essentially a substance that acts as a carrier for these waves.

  • Light is a form of electromagnetic radiation, which travels in the form of waves. These waves consist of oscillating electric and magnetic fields.
  • The properties of the medium, such as its density and composition, influence how light propagates through it.
  • Different mediums can affect the speed, direction, and behaviour of light waves. For instance, light travels slower in water compared to a vacuum.
  • Examples of Mediums:
    • Transparent: Materials like air, glass, and water allow most light to pass through, with minimal absorption or scattering. These are good examples of mediums for light propagation.
    • Translucent: Some materials, like frosted glass or thin paper, partially transmit light. They allow some light to pass through while diffusing or scattering the rest.
    • Opaque: Materials like wood or metal block light completely. They don’t allow any light to travel through them.
Wavelength

Wavelength measures a complete wave cycle, which is the distance from any point on a wave to the corresponding point on the next wave.

  • While wavelength can be measured from any point on a wave, it is often simplest to measure from the peak of one wave to the peak of the next or from the bottom of one trough to the bottom of the next, ensuring the measurement covers the whole of the cycle.
  • The wavelength of an electromagnetic wave is usually given in metres.
  • The wavelength of visible light is typically measured in nanometres, with 1,000,000,000 nanometres making up a metre.
  • Radio waves, visible light, and gamma waves for example, each have different ranges of wavelengths within the electromagnetic spectrum.
Speed of light

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.
Frequency

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.

  • Frequency is measured in Hertz (Hz) and signifies the number of wave-cycles per second. Sub-units of Hertz enable measurements involving a higher count of wave-cycles within a single second.
  • The frequency of electromagnetic radiation spans a broad range, from radio waves with low frequencies to gamma rays with high frequencies.
  • The wavelength and frequency of light are closely linked. Specifically, as the wavelength becomes shorter, the frequency increases correspondingly.
  • It is important not to confuse the frequency of a wave with the speed at which the wave travels or the distance it covers.
  • The energy carried by a light wave intensifies as its oscillations increase in number and its wavelength shortens.
Electromagnetic spectrum

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.

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