HOME PAGE * SEARCH * UK KS3 level Science Quizzes for students aged ~13-14

UK GCSE level Biology * Chemistry * Physics age ~14-16 * Advanced Level Chemistry age ~16-18

School Physics Notes: Reflection of visible light 7. The reflecting telescope

GCSE level Physics exam revision notes on light and reflection

Reflected light: 7. Diagram describing and explaining how a reflecting telescope works and its use in astronomy

[Author © Dr Phil Brown PhD: Doc Brown's physics exam revision notes suitable for students of UK IGCSE & GCSE level physics courses, ~ US grades 9-10 physics [light- page updated April 20th 2026 *]

 email doc brown: comments? query?  *  [privacy & cookies policies & disclaimer]  *  ]SEARCH]

[KEY POINTS and learning objectives for this page, after initial notes]

INDEX of physics notes: Light reflection and uses of plane and curved mirrors

7. How a reflecting telescope works - use in astronomy

A reflecting telescope uses a concave mirror

A relatively large concave mirror collects as much light as possible from distant astronomical object e.g. a star, so a reflecting telescope has found extensive use in astronomy.

The collected light is reflected by a small plane mirror at ~45o into an eyepiece or camera.

By means of a magnifying lens in the eyepiece tube you can produce a clear focussed and greatly magnified image of the star or any other distant object.

This type of reflecting telescope is also known as the Newtonian telescope (or Newtonian reflector) because it was invented by the English scientist Sir Isaac Newton. He used a concave primary mirror and a flat diagonal secondary mirror and his first designed reflector was completed in 1668.

Technically, the Newtonian reflecting telescope is made of a primary mirror or objective mirror, usually parabolic in shape, and a smaller flat (plane) secondary mirror. The primary mirror collects light from the pointed region of the sky and the secondary mirror redirects the light out of the optical axis at a right angle so it can be viewed with a magnifying eyepiece.

INDEX notes: Light reflection and uses of plane & curved mirrors

Key points on reflection and mirrors: Use of reflecting telescope in astronomy

Information sources for Doc Brown's key points: IGCSE-GCSE physics are based on textbooks & syllabus-specifications for students taking the UK AQA, Edexcel, OCR 21st Century Science, OCR Gateway science suite, WJEC, CCEA and CIE GCSE physics 9-1 level science examinations

A concise and exam-focused set of summary revision notes on reflecting telescopes and their use in astronomy, tailored to the GCSE/IGCSE Physics specifications across WJEC, CCEA, CIE, AQA, Edexcel, and OCR.

How Does a Reflecting Telescope Work?

Core Principle

  • Uses mirrors (not lenses) to collect and focus light.
  • The primary mirror is curved (usually parabolic) and reflects incoming light to a focal point.
  • A secondary mirror redirects the focused light to an eyepiece or detector.

Optical Path

  1. Light enters the telescope tube.
  2. Hits the primary concave mirror at the base.
  3. Reflected to the secondary mirror.
  4. Directed to the eyepiece or camera for viewing or recording.

Types of Reflecting Telescopes

  • Newtonian: Secondary mirror reflects light sideways to the eyepiece.
  • Cassegrain: Secondary mirror reflects light back through a hole in the primary mirror.

Uses of reflecting telescopes in Astronomy

  • Observing distant celestial objects: Stars, galaxies, nebulae.
  • Collecting faint light: Large mirrors gather more light than lenses.
  • Imaging: Used with CCDs or cameras for detailed astronomical photographs.
  • Spectroscopy: Analysing light to determine composition and movement of stars.

Advantages of Reflecting Telescopes

Feature Benefit
No chromatic aberration Mirrors reflect all wavelengths equally.
Larger apertures possible Mirrors can be supported from behind.
Compact design Shorter tubes than refracting telescopes.

Common Misconceptions about the structure and use of reflecting telescopes in astronomy

  • Reflecting telescopes use lenses
    → Correction: They use mirrors, not lenses.
  • Magnification is the most important factor
    → Correction: Light-gathering power and resolution are more critical.
  • All telescopes show upright images
    → Correction: Astronomical telescopes often show inverted images.
  • Bigger magnification means better image
    → Correction: High magnification can reduce image brightness and clarity.

Exam Tips on the structure and use of reflecting telescopes in astronomy

  • Be able to label diagrams of reflecting telescopes.
  • Know the optical path and function of each component.
  • Understand why mirrors are preferred over lenses in large telescopes.
  • Link telescope design to real-world applications (e.g. Hubble Space Telescope).
  • Practice calculating magnification:
    Magnification = (Focal length of primary mirror) / (Focal length of eyepiece lens)
  • For a refracting telescope using a convex glass lens:
  • Magnification = (Focal length of objective lens) / (Focal length of eyepiece lens)

Typical Exam Board specification on the structure and use of reflecting telescopes in astronomy

Focus Areas
Telescope structure, image formation, practical applications.
Types of telescopes, advantages of reflectors, magnification.
Detailed ray diagrams and telescope comparisons.
Emphasis on astronomical observation and telescope design.
In-depth coverage of optical systems and telescope function.

Keywords, phrases and learning objectives for the reflection of visible light rays - How a reflecting telescope works and use in astronomy

Know that the Newtonian reflector telescope was invented by the famous scientist Sir Isaac Newton

Know how a reflecting telescope works using a parabolic concave collecting mirror.

The image is viewed through the magnifying eyepiece lens.

Appreciate its important use in astronomy to study planets, stars and galaxies.

WHAT NEXT?

TOP of page

INDEX of physics notes on light reflection and mirrors

INDEX of all notes on waves, radiation, astronomy etc.

Revision notes on diagram explaining how a reflecting telescope works based on the syllabus-specifications for students taking IGCSE/GCSE level physics examinations, summary revision notes and key points on diagram explaining how a reflecting telescope works for students taking the AQA igcse/gcse physics notes on diagram explaining how a reflecting telescope works, Edexcel gcse physics notes on diagram explaining how a reflecting telescope works,  OCR 21st century GCSE physics notes on diagram explaining how a reflecting telescope works, OCR gateway GCSE physics notes on diagram explaining how a reflecting telescope works, WJEC gcse physics notes on diagram explaining how a reflecting telescope works, CCEA gcse physics notes on diagram explaining how a reflecting telescope works for students taking CIE Cambridge igcse physics, exam revision notes on diagram explaining how a reflecting telescope works, useful for US grade 9-10 physics courses, importance of describing how a reflecting telescope is used in astronomy in GCSE level physics, What you need to know about describing how a reflecting telescope is used in astronomy for GCSE level physics, Explaining the use of describing how a reflecting telescope is used in astronomy knowledge in GCSE level physics, Examples of describing how a reflecting telescope is used in astronomy explained when studying GCSE level physics, What is significant about describing how a reflecting telescope is used in astronomy, describing the theory of describing how a reflecting telescope is used in astronomy when studying GCSE level physics, revision notes for describing how a reflecting telescope is used in astronomy in exams, online exam help for describing how a reflecting telescope is used in astronomy, revision notes about describing how a reflecting telescope is used in astronomy, what do I need to learn about describing how a reflecting telescope is used in astronomy for by GCSE physics exam? help to understand the describing how a reflecting telescope is used in astronomy topic in preparation for GCSE physics exam question, how to prepare for questions involving describing how a reflecting telescope is used in astronomy in a GCSE physics examination?

SITEMAP Website content © Dr Phil Brown 2000+. All copyrights reserved on Doc Brown's physics revision notes, images, quizzes, worksheets etc. Copying of website material is NOT permitted. Exam revision summaries and references to GCSE science course specifications are unofficial.

INDEX notes: Light reflection and uses of plane & curved mirrors

TOP OF PAGE