What sets how much a telescope can see and magnify, and how fine the detail is?
The light grasp and aperture of a telescope, the magnification formula using the focal lengths of objective and eyepiece, the field of view, and the resolution of a telescope.
A focused answer to Edexcel GCSE Astronomy statements 11.19 to 11.23, covering how light grasp depends on the square of the objective diameter, the aperture and field of view, the magnification formula (focal length of objective over focal length of eyepiece), and how resolution depends on aperture and wavelength.
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What this dot point is asking
Edexcel statements 11.19 to 11.23 want you to understand that light grasp is proportional to the area of the objective (and so to the square of its diameter), how aperture relates to the objective diameter, that field of view is the circle of sky seen through the eyepiece, how resolution depends on aperture and wavelength, and to use the magnification formula .
Light grasp and aperture
This square law is the key calculation: to compare two telescopes, square the ratio of their diameters. More light grasp means fainter objects become visible and images are brighter, which is the main reason research telescopes have huge mirrors. Confusing light grasp with the diameter itself (forgetting to square) is the classic error here.
Field of view
Field of view is a practical trade-off: high magnification shows fine detail on a small area, while a wide field is better for large objects or for finding things. It is measured as an angle (degrees or arcminutes, units of arc that recur in resolution and parallax). The eyepiece choice sets both the magnification and the field of view.
Magnification
This is the most-used formula in the topic. To raise magnification you fit a shorter-focal-length eyepiece (or use a longer objective). The formula is given on the data sheet, so the marks are for correct substitution and units: convert both focal lengths to the same unit, then divide. Remember magnification alone does not reveal faint objects, that is light grasp; nor does it improve fine detail beyond the resolution limit.
Resolution
Resolution is why aperture matters twice over: a big objective gathers more light and resolves finer detail. The wavelength dependence explains why radio telescopes must be enormous to match the detail an optical telescope achieves, and why observing in the optical or ultraviolet gives sharper images than radio for the same size. Resolution is measured in arc units (arcseconds), linking to the small angles used in parallax (Topic 13).
How Edexcel examines this
This is telescopic Paper 2 content and a dense source of calculation marks. The magnification calculation uses from the data sheet: substitute the focal lengths in the same units and divide (no units in the answer). The light grasp calculation squares the ratio of objective diameters, so doubling the diameter gives four times the light; expect a "how many times more light" question. Field of view and resolution are tested by recall: resolution improves with aperture and worsens at longer wavelengths, and field of view is the circle of sky in arc units. A common explanation asks why a larger objective helps (more light grasp for fainter objects, plus finer resolution). Synoptic links run to why research and radio telescopes are large (Topic 13) and to arc units in parallax (Topic 13). The biggest errors are forgetting to square the diameter and inverting the magnification formula, so secure both.
Try this
Q1. State the magnification formula for a telescope. [1 mark]
- Cue. Magnification = focal length of the objective / focal length of the eyepiece ().
Q2. State how the light grasp of a telescope depends on the diameter of its objective. [1 mark]
- Cue. It is proportional to the square of the diameter (the area).
Exam-style practice questions
Practice questions written in the style of Pearson Edexcel exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Edexcel 1AS0 20223 marksA telescope has an objective of focal length 1200 mm and an eyepiece of focal length 25 mm. Calculate the magnification of the telescope.Show worked answer →
The magnification is the focal length of the objective divided by the focal length of the eyepiece: (1 mark). Substituting gives (1 mark). This evaluates to (so the image appears 48 times larger; magnification has no units) (1 mark). Markers reward the correct formula, substitution of the two focal lengths in the same units, and the answer of 48. Using a shorter-focal-length eyepiece would increase the magnification.
Edexcel 1AS0 20214 marksTelescope A has an objective diameter of 100 mm and telescope B has an objective diameter of 300 mm. Calculate how many times more light telescope B gathers than telescope A, and explain why a larger objective is an advantage.Show worked answer →
Light grasp is proportional to the area of the objective, and therefore to the square of its diameter, so the ratio is (2 marks). This gives , so telescope B gathers 9 times as much light as telescope A (1 mark). A larger objective is an advantage because gathering more light lets the telescope see fainter objects and produce brighter images, and a larger aperture also improves the resolution (finer detail) (1 mark). Markers reward squaring the diameter ratio to get 9, and explaining that more light grasp means fainter objects are visible (with better resolution). Light grasp depends on the square of the diameter, not the diameter itself.
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