A-Level · Physics · AQA · Mark scheme decoded
AQA A-Level Physics: Cassegrain Reflectors and Aberrations in Telescopes — mark scheme explained
The short answer
In the realm of astrophysics, telescopes play a crucial role in observing distant celestial objects. One of the most advanced designs is the Cassegrain reflector, which uses a parabolic concave primary mirror and a convex secondary mirror to focus light.
The question
Draw a ray diagram showing the path of light rays through a Cassegrain reflector, from the primary mirror to the eyepiece.
[Paraphrased for study — not reproduced from any exam paper.]
Mark scheme, decoded
How the examiner actually awards the marks on this topic.
Gradora's own decode of the marking approach — not the exam board's published mark scheme.
How marks are awarded
For questions involving ray diagrams, ensure that all rays are correctly drawn and labeled. For explanations, provide clear and concise answers, including relevant scientific principles and terminology. When comparing telescopes, highlight both advantages and disadvantages of each type.
What the command words demand
- Explain
- Provide a detailed account of the topic, including reasons and causes.
- Describe
- Give a detailed account or picture of something without necessarily providing reasons.
- Draw
- Create a diagram or sketch to illustrate the concept.
- Compare
- Identify similarities and differences between two or more items.
- State
- Clearly identify key points without extensive explanation.
Model answer
A full-mark response to the question above, worked through step by step.
Timing: Allocate about 5-7 minutes for a 4-mark question to allow time for careful drawing and detailed explanation.
- 1. Draw a large parabolic concave primary mirror at the bottom of your diagram.1 mark
- 2. Draw a smaller convex secondary mirror above and in front of the primary mirror.1 mark
- 3. Draw light rays entering the telescope from the top, striking the primary mirror.0 marks
- 4. Show the reflected rays converging, intercepted by the convex secondary mirror before they reach the primary focus.1 mark
- 5. Draw the convex secondary mirror reflecting these still-converging rays back through a hole in the primary mirror to a focus behind it.1 mark
- 6. Show the final path of the rays to the eyepiece or camera.0 marks
Final answer: A correctly drawn ray diagram showing the path of light rays from the primary mirror, through the secondary mirror, and to the eyepiece.
Work through every step correctly and you earn all 4 marks.
Another worked example
Explain why reflecting telescopes do not suffer from chromatic aberration.
- 1. State that reflecting telescopes use mirrors instead of lenses.1 mark
- 2. Explain that mirrors reflect all wavelengths of light equally, without causing different wavelengths to focus at different points.1 mark
- 3. Conclude that this eliminates the issue of color fringing seen in refracting telescopes.1 mark
Final answer: Reflecting telescopes do not suffer from chromatic aberration because they use mirrors instead of lenses, which reflect all wavelengths of light equally and do not cause different wavelengths to focus at different points.
Work through every step correctly and you earn all 3 marks.
Common mistakes
Confusing the primary and secondary mirrors in a Cassegrain reflector.
Why it happens: Students may mix up which mirror is concave and which is convex, leading to incorrect ray diagrams and explanations.
Fix: Reinforce that the primary mirror is parabolic and concave, while the secondary mirror is convex.
Forgetting to mention chromatic aberration when comparing reflectors and refractors.
Why it happens: Students may focus on other advantages of reflectors and overlook this key point.
Fix: Emphasize that reflecting telescopes do not suffer from chromatic aberration, which is a significant advantage over refractors.
Incorrectly stating that spherical mirrors can correct spherical aberration.
Why it happens: Students may confuse the shape of the mirror with its ability to correct aberrations.
Fix: Clarify that only parabolic mirrors, not spherical ones, can correct spherical aberration by focusing all light rays to a single point.
Failing to explain why reflectors are more compact than refractors.
Why it happens: Students may know that Cassegrain reflectors are compact but not understand the reason behind it.
Fix: Explain that the design of reflecting telescopes, particularly the Cassegrain arrangement, allows for a shorter tube length by reflecting light back through a hole in the primary mirror.
Not mentioning the need for periodic realignment and cleaning of reflectors.
Why it happens: Students may focus on the advantages of reflectors and overlook their maintenance requirements.
Fix: Highlight that reflecting telescopes require periodic realignment and cleaning of the mirrors to maintain optimal performance.
Confusing achromatic lenses with parabolic mirrors in terms of correcting aberrations.
Why it happens: Students may mix up the methods used to correct chromatic and spherical aberrations.
Fix: Clarify that achromatic lenses are used in refractors to reduce chromatic aberration, while parabolic mirrors are used in reflectors to correct spherical aberration.
Where the marks go
The question types you’ll meet on this topic and the marks each one carries — so you know what to expect and where to focus.
| Question type | What you’re asked to do | Marks |
|---|---|---|
| Cassegrain Ray Diagram | Draw and label light rays through a Cassegrain reflector from primary mirror to eyepiece. | 4 |
| Explain Chromatic Aberration | Explain why mirrors in reflecting telescopes avoid chromatic aberration seen in refracting telescopes. | 3 |
| Compare Telescope Types | Explain why a Cassegrain reflector performs better than an equivalent refracting telescope. | 4 |
| Spherical Aberration Correction | Define spherical aberration and describe how telescopes correct it. | 4 |
| Total across these question types | 15 | |
Question types and mark tariffs are Gradora’s guidance based on how this topic is typically examined — not the board’s official paper structure.