Electromagnetic Spectrum

Electromagnetic Spectrum

The spectrum of visible light is 7 colours (red - violet). This spectrum actually extends beyond both red and violet (invisible). This wider spectrum is known as the electromagnetic spectrum.

Electromagnetic waves are transverse waves and they travel at the speed of light through a vacuum.

Electromagnetic Spectrum, figure 1

Some types of radiation have enough energy to remove electrons from atoms. This is known as ionisation. Ionising radiation can damage cells, leading to cancer. If the dose is high enough, they can kill cells.

Electromagnetic Spectrum, figure 2

Gamma, X-rays and Ultraviolet are ionising radiation.

You need to be familiar with the uses and risks of each type of radiation.


Electromagnetic Spectrum, figure 3


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Visible Light:

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All objects emit and absorb IR radiation. The higher the temperature of an object, the more infrared radiation it emits in a given time.

A body (object) at a constant temperature is emitting the same amount of radiation that it is absorbing.

Electromagnetic Spectrum, figure 1

The amount of radiation entering/leaving a body will determine what happens to it’s temperature.

Electromagnetic Spectrum, figure 2

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Dark, matt colours are best at absorbing and emitting IR radiation.

Light, shiny surfaces are more likely to reflect IR radiation.

A ‘black body’ absorbs all the radiation that it receives, it does not reflect or transmit any radiation. A ‘black body’ is also the best possible emitter of radiation.

An object at a constant temperature emits a range of wavelengths of radiation. The peak intensity emitted depends on the temperature. As the temperature increases, the intensity at every wavelength increases AND the peak intensity will be at a shorter wavelength.

Electromagnetic Spectrum, figure 4

Non-ionising Radiation

Non-ionising radiation can not cause cancer, although there are other risks.

Visible Light

Electromagnetic Spectrum, figure 1


  1. Seeing!
  2. Cameras
  3. Telescopes


  1. Blindness

Ionising Radiation


Ultraviolet is produced by very hot objects. Most of the UV radiation we are exposed to comes from the sun. UV is absorbed by the skin.


  1. Vitamin D is produced by the skin when it absorbs radiation
  2. UV causes the body to release chemicals that can improve your mood
  3. Skin that absorbs UV will tan (become a darker colour). Some people see this as a benefit of UV exposure, while others think it is not worth the risks.


  1. Sunburn – Skin can burn from UV exposure
  2. Skin Cancer – Exposure to UV over a long period of time increases the risk of skin cancer.


X rays will pass through soft tissue but not through more dense material like bones. This can be used to develop images of bones or to view the inside of luggage at an airport.

Electromagnetic Spectrum, figure 1Electromagnetic Spectrum, figure 2

Exposure to X-rays increases the risk of cancer. For a patient, the benefit of having an X-ray to get an accurate diagnosis outweighs the very small increased risk of cancer. For medical staff who work with X-ray machines, precautions are necessary to prevent high levels of exposure:


  1. Only use when necessary
  2. Target at specific body parts
  3. Operate machine from separate room or behind lead lined screen

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Gamma Rays

Gamma Rays are released from the nucleus of radioactive materials.

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To use a scan from a gamma camera, the patient swallows a gamma tracer or has it injected into the bloodstream. The camera detects the gamma radiation emitted from the body, producing an image of the body parts containing the tracer.

Electromagnetic Spectrum, figure 5

As ionising radiation is dangerous to cells, it can be used to kill cancerous cells. The radiation is targeted at the tumour, but some damage to healthy cells is inevitable.


Every activity has some level of risk associated with it (eg crossing the road or drinking a glass of water). A person will do an activity if they believe the benefits outweigh the risks.

This can be complicated based on the way we think about risks. For example, a person may perceive the risk of skin cancer from sunbeds as quite low risk because it is invisible (can’t see yourself developing cancer) and long term (will happen in a few years, not straight away). This may mean that, although the risk is serious, the person is willing to take that risk for the short term benefit of a tan.

Required Practical – Infrared Radiation

To investigate the radiation emitted from different types of surface, a Leslie cube can be used. The Leslie cube is filled with hot water and then the temperature is measured at each side (shiny/dull/white/black) over a period of time. The temperature is measured using an infrared thermometer and so it is important to keep the thermometer the same distance from each side.

Electromagnetic Spectrum, figure 1

The results of the experiment should confirm that dark/matt surfaces emit the most IR radiation.

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Microwave radiation can be used to heat food:

  1. Microwave radiation is absorbed by water molecules in the food
  2. Water molecules heat up, turning to steam
  3. Hot water/steam heats the food
  4. Internal heating (therefore heats up faster than a normal oven)

Electromagnetic Spectrum, figure 1

Microwave radiation is also used for mobile phone communications. This is a different frequency of microwave radiation and so it can not be used to cook food! However, some heating of the brain does occur when using a mobile phone. There are no short term risks from this heating but the long term risks of mobile phones are unknown as they have only been in widespread use for the last 20-25 years.

Electromagnetic Spectrum, figure 2


Radiowaves are used for astronomy and communicating signals from one location to another.

Information can be carried by a radio wave by modulating a carrier wave. The input signal is an alternating current. This is used to vary the amplitude or frequency of a carrier wave (this is called modulation). Amplitude modulation is known as AM, these signals can be transmitted across further distances but are lower quality. Frequency modulation is known as FM, this is higher quality but can not travel as far.

Electromagnetic Spectrum, figure 1

The radio waves are absorbed by an aerial producing an alternating current in the receiver. The original signal is then separated from the carrier wave.

Electromagnetic Spectrum, figure 2

Which of these types of radiation is ionising? Ultraviolet, Microwaves, Visible light, Infra red
Which of these types of radiation is non-ionising? Radiowaves, Gamma rays, X rays, Ultraviolet
Which type of electromagnetic radiation has the shortest waves? Gamma Rays, Radio waves, Infra red, Ultraviolet
Which type of electromagnetic radiation has the longest waves? Radio waves, Gamma Rays, Infra red, Ultraviolet
Which type of radiation is used by mobile phones?