5.A.1% Lenses & Optical Telescopes

Oct 26, 2026

A practice worksheet on optical instruments and telescope performance, focusing on reflecting and refracting systems. Tasks include completing ray diagrams for a Cassegrain telescope, locating focal points, and tracing rays through eyepiece lenses. Additional questions compare reflecting vs refracting advantages, relate radio and optical resolving power, and explain chromatic aberration and its reduction in Cassegrain designs. Several problems compute angular magnification and resolving power using lens focal lengths, object angles, wavelength, and aperture diameter.

LanguageEnglish

CategoryExam

FormatPDF

Pages29

Reading Time1 h 13 min

AccessFree to download

Lenses and optical telescopes

Cassegrain reflecting telescope ray diagrams

Reflecting vs refracting advantages

Radio telescope comparisons and resolution

Chromatic aberration and its effects

Angular magnification and resolving power calculations

Lens imaging: real and virtual formation

1. The diagram shows the concave mirror of a Cassegrain reflecting telescope, together with the eyepiece lens. Complete the diagram of the telescope and mark on it the focal point of the concave mirror. Draw a ray diagram for two rays from a star, parallel to the principal axis, passing through the telescope and emerging from the eyepiece lens State, with reasons, two optical advantages which the reflecting telescope normally has over a refracting telescope. | | | --- | | | | | (Total 6 marks) 2. (a) State two similarities between a radio telescope and an optical reflecting telescope. | | | --- | | | | | (2) (b) The dish of a radio telescope has holes of diameter 20 mm spaced close together in its reflecting surface in order to reduce the weight of the dish. Explain why the performance of this telescope will be far more satisfactory when receiving signals of frequency 7.5 × 108 Hz than when receiving signals of frequency 1.5 × 1010 Hz. | | | --- | | | | | (3) (c) Explain why the resolving power of a single dish radio telescope is normally much less than that of a normal optical telescope. | | | --- | | | | | (2) (Total 7 marks) 3. (a) A telescope is made from two converging lenses of focal lengths 2.50 m and 0.020 m. (i) Show, with the aid of a labelled diagram, how the lenses would be placed for normal adjustment. Show, on the diagram, the principal focus of each lens. | | | --- | | | | | | | | | (ii) The telescope is used to observe a planet which subtends an angle of 5.0 × 10–5 radat the objective. Calculate the angle subtended at the eye by the final image. | | | --- | | | | | (4) (b) The diagram (not drawn to scale) shows an incomplete Cassegrain reflecting telescope. F 1 is the principal focus of the concave mirror. (i) Add to the diagram the second necessary mirror, M. (ii) Complete the path of the two rays through the telescope when it is in normal adjustment. Show your reasoning in drawing these rays, either on the diagram or below. Label the principal focus, F2, of the eye lens, and the position of C, the centre of curvature of M. | | | --- | | | (6) (c) (i) State what is meant by chromatic aberration and explain the effect it would have on the image in an uncorrected refracting telescope. | | | --- | | | | | (ii) Explain why the Cassegrain telescope would be almost free of chromatic aberration. | | | --- | | | | | (5)(Total 15 marks) 4. A converging lens of focal length 100mm is used to produce a magnified image of a small object. The magnification is × 2. Draw rays on the diagrams to show how the images are produced. The principal foci are each labelled F. The diagrams need not be drawn to scale. (a) real image formation (2) (b) virtual image formation (2)(Total 4 marks) 5. (a) An astronomical telescope consists of two thin converging lenses, as shown. The focal lengths ofthe objective lens and the eyepiece lens are fandf, respectively. The telescope is used to view the Moon and the separation of the lenses is such that the telescope is in normal adjustment. objective eyepiece (i) Draw rays to show how a magnified image is formed by the telescope. Your ray diagram should show the paths through the telescope of two parallel non-axial rays from a point on the Moon to the observer‟s eye. Indicate the position of the principal focus for each lens. (ii) An observer‟s unaided eye has a resolving power of 120 seconds of arc. If the angular magnification of a telescope is 24, determine the angular separation of two points on the Moon, which the same observer can just resolve with the aid of the telescope. The angular magnification of the instrument is given by angle subtended attheeyeby theimage M = angle subtended at theeyeby theobject | | | --- | | | (7) (b) A parallel beam of monochromatic light, incident on a converging lens and parallel to the axis, fills the lens aperture completely. After tra

FAQs

What must be shown in the Cassegrain telescope ray diagram for the concave mirror and eyepiece lens?

The diagram should trace two rays from a star parallel to the principal axis through the telescope and emerging from the eyepiece lens, and it must mark the focal point of the concave mirror.

Why does a reflecting telescope normally have optical advantages over a refracting telescope?

The worksheet asks for two reasons and expects justification based on how reflecting optics avoid issues that refractors typically suffer from, particularly wavelength-dependent effects that create image errors.

Why is the resolving power of a single-dish radio telescope normally much less than that of a typical optical telescope?

The resolving power depends on effective aperture and operating wavelength; radio telescopes use much longer wavelengths and therefore achieve poorer angular resolution than optical telescopes for comparable aperture limitations.

Oct 26, 2026

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