EM spectrum in Cosmology (SQA National 5 Physics): Revision Note

EM spectrum in cosmology

• Astronomers use telescopes to obtain information about astronomical objects

• Optical telescopes detect wavelengths of light from the visible part of the electromagnetic spectrum

• Telescopes that look at other parts of the electromagnetic spectrum are known as non-optical telescopes, including

  • radio telescopes

  • infrared (IR) telescopes

  • ultraviolet (UV) telescopes

  • X-ray telescopes

Different types of telescopes

• Being able to collect radiation from all parts of the electromagnetic spectrum opens up a whole world of new information for astronomers

  • For example, different areas of an astronomical object known as the Crab Nebula are found to emit strongly at all wavelengths

  • In particular, radio waves, X-rays and gamma rays all appear to originate from its centre, whilst the infrared, visible and ultraviolet wavelengths appear to come from the gas and dust that surrounds it

• Note: images of astronomical objects are often given 'false colour' to help us visualise wavelengths the human eye cannot see

Observing the Crab Nebula at different wavelengths

Ground-based telescopes

• The atmosphere can have a significant impact on the quality of astronomical observations made using ground-based telescopes

  • The reflection of light from moisture in the atmosphere can cause light pollution

  • Air currents can cause atmospheric distortion (this is what makes stars appear to 'twinkle')

• The atmosphere absorbs certain wavelengths of electromagnetic waves

• This means that not all electromagnetic radiation coming from space reaches the Earth’s surface

• As a result, ground-based telescopes are able to observe:

  • all visible wavelengths (although there is often some distortion)

  • very narrow ranges of infrared wavelengths

  • most microwave & radio wavelengths

Absorption of wavelengths by the atmosphere

Space-based telescopes

• Above the atmosphere, space-based telescopes can detect all wavelengths, making it possible to clearly observe:

  • gamma rays, X-rays & ultraviolet rays

  • all infrared wavelengths

• Some benefits of space-based telescopes are:

  • They lead to the discovery of objects not detectable by visible light

  • More information and data can be collected

  • Different EM waves can give different types of information about astronomical objects

  • They can produce much more detailed and magnified images

  • They produce clearer images, which are unaffected by distortion from the atmosphere and light pollution

• Some downsides of space-based telescopes are:

  • They are much harder to repair

  • They cannot be made too large since they need to fit into a rocket to be launched

  • They are much more expensive

Ground & space-based telescopes