Frequency of Electromagnetic Waves

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This diagram looks at the frequency of electromagnetic waves.

  • It shows that 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 frequency of an electromagnetic wave can be thought about in the same way as the frequency of trains that pass through a railway station. If a train passes through a station every 10 minutes then the frequency is six trains per hour.

Remember that:

  • The frequency of a wave should not be confused with the speed at which the wave travels or the distance it travels.
  • The term frequency refers to the measurement of the frequency of wave oscillations that pass a given point in a given amount of time.
  • The unit of measurement of frequency is the hertz. One hertz equals one wave-cycle per second.
  • Because the frequency of some electromagnetic waves is so small, Hertz is sub-divided into kilohertz, megahertz, gigahertz and terahertz.
  • The wavelength and frequency of light are closely related. In any given medium, the higher the frequency, the shorter the wavelength.
  • The amount of energy transported by a light wave increases with the frequency of oscillations (wave-cycle) and as the length of each oscillation decreases.
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Description

Frequency of Electromagnetic Waves

TRY SOME QUICK QUESTIONS AND ANSWERS TO GET STARTED
What is meant by the frequency of a wave?
The frequency of an electromagnetic wave is a measurement of the number of wave oscillations passing a given point in a given period of time.
What does the frequency of a wave measure?
The frequency of a wave is a measurement of the number of waves passing a given point in a given time!
How is frequency affected as light travel from air into glass?
The frequency of incident light is unchanged as it travels from air into glass so its colour remains the same.
Shorter wavelength = higher frequency or lower frequency?
Shorter wavelengths = Higher frequency.
How is frequency affected as light travel from air into raindrops?
The frequency of incident light is unchanged as it travels from air into water and undergoes refraction.

About the diagram

About the diagram
  • This diagram looks at the frequency of electromagnetic waves.
  • It shows that 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 frequency of an electromagnetic wave can be thought about in the same way as the frequency of trains that pass through a railway station. If a train passes through a station every 10 minutes then the frequency is six trains per hour.
Remember that:
  • The frequency of a wave should not be confused with the speed at which the wave travels or the distance it travels.
  • The term frequency refers to the measurement of the frequency of wave oscillations that pass a given point in a given amount of time.
  • The unit of measurement of frequency is the hertz. One hertz equals one wave-cycle per second.
  • Because the frequency of some electromagnetic waves is so small, Hertz is sub-divided into kilohertz, megahertz, gigahertz and terahertz.
  • The wavelength and frequency of light are closely related. In any given medium, the higher the frequency, the shorter the wavelength.
  • The amount of energy transported by a light wave increases with the frequency of oscillations (wave-cycle) and as the length of each oscillation decreases.
Understanding the diagram:
  • The diagram shows that the frequency of waves is counted in wave-cycles. One wave-cycle is shown as a dotted yellow line that follows the wave.
  • The dotted yellow line below the wave measures three complete cycles. The measurement is from the peak of the first wave to the peak of the third.
  • The point at which measurement of passing wave-cycles is taken is shown as a vertical line with a clock face below it.
  • The measurement involves calculating how long it takes for the three wave-cycles to pass the clock.
  • The diagram shows that the time it takes in this example is one second.
  • The frequency, in this case, is 3 wave-cycles per second which equal 3 Hertz (Hz).

Some key terms

Wave-cycle

A wave-cycle refers to the path of a wave measured from any point through the course of a single oscillation to the same point on the next oscillation.

  • Imagine a wave-cycle as a series of points marked on the path of the wave between one crest and the next.
  • All electromagnetic waves share features such as crests, troughs, oscillations, wavelength, frequency, amplitude, direction of travel.
  • Whilst a wave-cycle is the path from one point on a wave during a single oscillation to the same point on completion of that oscillation, wavelength is a measurement of the same phenomenon along the axis of the wave.
Electromagnetic wave

An electromagnetic wave carries electromagnetic radiation.

  • An electromagnetic wave is formed as electromagnetic radiation propagates from a light source, travels through space and encounters different materials.
  • Electromagnetic waves can be imagined as synchronised oscillations of electric and magnetic fields that propagate at the speed of light in a vacuum.
  • Electromagnetic waves are similar to other types of waves in so far as they can be measured in terms of wavelength, frequency and amplitude.
  • We can feel electromagnetic waves release their energy when sunlight warms our skin.
  • Remember that electromagnetic radiation can be described either as an oscillating wave or as a stream of particles, called photons, which also travel in a wave-like pattern.
  • The notion of waves is often used to describe phenomena such as refraction or reflection whilst the particle analogy is used when dealing with phenomena such as diffraction and interference.

 

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Wavelength

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.
Electromagnetic radiation

Electromagnetic radiation is a type of energy more commonly simply called light. Detached from its source, it is transported by electromagnetic waves (or their quanta, photons) and propagates through space at the speed of light.

  • Electromagnetic radiation (EM radiation or EMR) includes radio waves, microwaves, infrared, (visible) light, ultraviolet, X-rays, and gamma rays.
  • Man-made technologies that produce electromagnetic radiation include radio and TV transmitters, radar, MRI scanners, microwave ovens, computer screens, mobile phones, all types of lights and lamps, electric blankets, electric bar heaters, lasers and x-ray machines.
  • At the quantum scale of electromagnetism, electromagnetic radiation is described in terms of photons rather than waves. Photons are elementary particles responsible for all electromagnetic phenomena.
  • The term quantum refers to the smallest quantity into which something can be divided. A quantum of a thing is indivisible into smaller units so they have no sub-structure.  A photon is a quantum of electromagnetic radiation.
  • A single photon with a wavelength corresponding with gamma rays might carry 100,000 times the energy of a single photon of visible light.
Hertz (Hz)

The hertz (symbol: Hz) is a unit used to measure the frequency of electromagnetic waves. It represents the number of wave-cycles per second.

  • One hertz is defined as one cycle per second.
  • Hertz measure the number of oscillations of the perpendicular electric and magnetic fields in electromagnetic radiation per second.
  • Frequency conversions:
  • 1 Hertz (Hz) = 1 cycle per second
  • 1 Kilohertz (kHz) = 1,000 (thousand) cycles per second
  • 1 Megahertz (MHz) = 1,000,000 (million) cycles per second
  • 1 Gigahertz (GHz) = 1,000,000,000 (billion) cycles per second
  • 1 Terahertz (THz) = 1,000,000,000,000 (trillion )cycles per second
Oscillation

An oscillation is a periodic motion that repeats itself in a regular cycle. An oscillating movement is always around an equilibrium point or mean value. It is also known as a periodic motion.

  • The term oscillation denotes something that moves in one direction, then moving back in a repeating pattern.
  • An electromagnetic wave describes an oscillatory motion as the electric field and then the magnetic field take turns to increase to their maximum values and then drop back to zero.
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.

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