AQA · A-Level · Physics
Course version: 7408 · Version from Sept 2016
Ray diagram to show path of rays through the telescope up to the eyepiece.
Full specification wording
Reflecting telescopes (A-level only): Ray diagram to show path of rays through the telescope up to the eyepiece.
A reflecting telescope ray diagram traces incoming parallel rays from a distant object through the objective mirrors, stopping just before the eyepiece.
Checked against the AQA 7408 specification (opens in a new tab) on .
- Point 2 of 3
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
A reflecting telescope ray diagram traces incoming parallel rays from a distant object through the objective mirrors, stopping just before the eyepiece. For a Cassegrain arrangement, parallel rays arrive at the concave primary mirror and reflect towards the principal focus. A convex secondary mirror intercepts this converging beam, redirecting it through a hole in the primary mirror to form a real image. For a Newtonian reflector, a small plane mirror redirects the converging rays to a focal point outside the side of the tube. The diagram must show non-axial parallel rays entering the telescope, reflecting off the primary, then the secondary, and crossing to form the real intermediate image at the focal plane, strictly up to the eyepiece.
What earns the marks
- Incoming rays from a distant object are drawn parallel to each other but at a slight angle to the principal axis.
- The concave primary mirror reflects the parallel rays so they converge towards its principal focus.
- A secondary mirror (convex for Cassegrain, plane for Newtonian) intercepts the converging beam before it reaches the primary focus.
- The secondary mirror redirects the rays to cross and form a real intermediate image (e.g., through a gap in the primary mirror for a Cassegrain).
- Ray directions are marked with arrows, and the path is drawn strictly up to the formation of the intermediate image, without including the eyepiece.
Common mistakes and how to put them right
Drawing rays passing through the eyepiece.
Put it right by stopping the ray diagram exactly at the real intermediate image, as required by the specification.
Drawing incoming rays parallel to the principal axis.
Put it right by drawing non-axial parallel rays to show the formation of an off-axis image point.
Omitting the secondary mirror or drawing rays passing straight through the primary mirror.
Put it right by showing reflection at the primary and redirection by the secondary.
Rays crossing before hitting the secondary mirror.
Put it right by placing the secondary mirror inside the focal length of the primary mirror so it intercepts the converging beam.
Exam tips
- Use a ruler and draw thin, continuous lines so each ray path is unambiguous, ensuring arrows indicate the direction of light.
- Remember to stop your diagram at the focal plane where the real image forms; do not draw the eyepiece lens or the emergent rays.
Try the lesson
The start of the Reflecting telescopes (A-level only): Ray diagram to show path of rays through the telescope up to the eyepiece. lesson, made from checked teaching notes for this specification point.
Have a go first
When drawing a ray diagram for a Cassegrain telescope, which path correctly represents the light rays after they reflect from the primary mirror?
Listen
A reflecting telescope ray diagram traces incoming parallel rays from a distant object through the objective mirrors, stopping just before the eyepiece. For a Cassegrain arrangement, parallel rays arrive at the concave primary mirror and reflect towards the principal focus. A convex secondary mirror intercepts this converging beam, redirecting it through a hole in the primary mirror to form a real image. For a Newtonian reflector, a small plane mirror redirects the converging rays to a focal point outside the side of the tube. The diagram must show non-axial parallel rays entering the telescope, reflecting off the primary, then the secondary, and crossing to form the real intermediate image at the focal plane, strictly up to the eyepiece. A strong answer earns marks for these points. Incoming rays from a distant object are drawn parallel to each other but at a slight angle to the principal axis. The concave primary mirror reflects the parallel rays so they converge towards its principal focus. A secondary mirror (convex for Cassegrain, plane for Newtonian) intercepts the converging beam before it reaches the primary focus. The secondary mirror redirects the rays to cross and form a real intermediate image (e.g., through a gap in the primary mirror for a Cassegrain). A common slip: Drawing rays passing through the eyepiece. Instead, stopping the ray diagram exactly at the real intermediate image, as required by the specification. Exam tip: Use a ruler and draw thin, continuous lines so each ray path is unambiguous, ensuring arrows indicate the direction of light.
Key words
Say what each word means before you turn the card.
Reflection at curved and plane surfaces
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Formation of a real intermediate image
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Optical ray construction and labelling
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See every definition
- Reflection at curved and plane surfaces
- Incoming parallel rays reflect off the concave primary mirror and are redirected by a secondary mirror, convex for Cassegrain or plane for Newtonian.
- Formation of a real intermediate image
- The secondary mirror redirects the rays to cross and form a real intermediate image at the focal plane, strictly up to the eyepiece.
- Optical ray construction and labelling
- Ray directions are marked with arrows, and the path is drawn strictly up to the formation of the intermediate image, without including the eyepiece.
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