7.19P Describe how methods of observing the Universe have changed over…

The idea

Observation has moved from naked-eye viewing, through optical telescopes on the ground, to instruments that detect other parts of the electromagnetic spectrum and to telescopes placed in space. Ground-based optical telescopes grew larger to gather more light and resolve finer detail, and astronomers began photographing and later electronically recording images. Radio telescopes revealed sources invisible to the eye. Some telescopes orbit above the atmosphere because the atmosphere absorbs much of the infrared, ultraviolet, x-ray and gamma radiation, and because moving air blurs images. Placing a telescope in space therefore lets it detect radiation that never reaches the ground and avoids atmospheric distortion.

What earns the marks

• Traces a change from naked-eye observation to the use of telescopes and then to instruments detecting non-visible radiation.
• Explains that the atmosphere absorbs certain wavelengths, so space telescopes detect infrared, ultraviolet, x-ray or gamma radiation that ground telescopes cannot.
• Explains that the atmosphere blurs and distorts images, so a telescope above it produces sharper images.
• Recognises that ground-based telescopes can still be large and useful for visible and radio observation.

Common mistakes and how to put them right

• Claims that space telescopes are used only because they are closer to the stars.

  Put it right by focusing on absorption of radiation and atmospheric blurring.

• Assumes the atmosphere blocks all radiation equally.

  Put it right: State that it transmits visible light and radio waves well but absorbs much infrared, ultraviolet, x-ray and gamma radiation.

• Says ground telescopes cannot detect anything useful.

  Put it right: Recognise their value for visible and radio astronomy.

• Treats the change over time as a single invention rather than a progression of methods.

Exam tips

• Look for the option that names a specific wavelength absorbed by the atmosphere rather than a vague statement about clearer views.
• Check whether an option confuses distance with the real reasons: absorption and blurring.
• Eliminate options claiming the atmosphere blocks visible light, since ground-based optical telescopes rely on it.