A-Level · Physics · AQA · Mark scheme decoded
AQA A-Level Physics: Image Formation and Angular Magnification in a Telescope — mark scheme explained
The short answer
In this section, we will explore the principles of image formation and angular magnification in a telescope when it is used in normal adjustment. Understanding these concepts is crucial for grasping how telescopes work and how they enhance our ability to observe distant objects.
The question
A telescope has an objective lens with a focal length of 120 cm and an eyepiece with a focal length of 5 cm. Calculate the angular magnification of the telescope. [Paraphrased for study — not reproduced from any exam paper.]
Mark scheme, decoded
What each mark is really for — in plain English — and the wording trap that loses it.
- S1
Identify the given values: f o = 120 cm, f e = 5 cm.
- S2
Use the formula for angular magnification: M = f o / f e .
- S3
Substitute the values into the formula: M = 120 cm / 5 cm .
- S4
Calculate the result: M = 24 .
Model answer
Worked through, with each step tagged to the mark it earns.
- S1
Identify the given values: f o = 120 cm, f e = 5 cm.
- S2
Use the formula for angular magnification: M = f o / f e .
- S3
Substitute the values into the formula: M = 120 cm / 5 cm .
- S4
Calculate the result: M = 24 .
Final answer: The angular magnification of the telescope is 24.
Common mistakes
- Confusing the focal lengths of the objective lens and the eyepiece — Always label the focal lengths clearly: f o for the objective lens and f e for the eyepiece.
- Using the wrong formula for angular magnification — Memorize and practice using the correct formula: M = f o / f e .
- Forgetting to convert angles to radians when necessary — Always check if the problem requires angles in degrees or radians and convert accordingly.
- Not understanding the concept of normal adjustment — Review the definition and importance of normal adjustment in telescopes.
- Incorrectly drawing ray diagrams — Practice drawing ray diagrams step-by-step and ensure that the rays from the object intersect at the correct points.
- Misinterpreting the angle subtended by the image — Clearly identify and label all angles in the problem, especially θ image and θ object .
Where the marks go
- Full worked solution (all marking points)3 marks